research paper for biofuel

Loading metrics

Open Access

Essays articulate a specific perspective on a topic of broad interest to scientists.

See all article types »

The potential of biofuels from first to fourth generation

Contributed equally to this work with: Philipp Cavelius, Selina Engelhart-Straub

Roles Conceptualization, Data curation, Investigation, Validation, Visualization, Writing – original draft, Writing – review & editing

Affiliation Werner Siemens-Chair of Synthetic Biotechnology, TUM School of Natural Sciences, Technical University of Munich (TUM), Garching, Germany

Roles Conceptualization, Data curation, Funding acquisition, Project administration, Supervision, Writing – original draft, Writing – review & editing

Roles Conceptualization, Data curation, Supervision, Writing – review & editing

Affiliation Chair of Technical Chemistry II, TUM School of Natural Sciences, Technical University of Munich (TUM), Garching, Germany

Roles Conceptualization, Data curation, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected] (DA); [email protected] (TB)

• Philipp Cavelius, 
• Selina Engelhart-Straub, 
• Norbert Mehlmer, 
• Johannes Lercher, 
• Dania Awad, 
• Thomas Brück

Published: March 30, 2023

• https://doi.org/10.1371/journal.pbio.3002063
• Reader Comments

The steady increase in human population and a rising standard of living heighten global demand for energy. Fossil fuels account for more than three-quarters of energy production, releasing enormous amounts of carbon dioxide (CO 2 ) that drive climate change effects as well as contributing to severe air pollution in many countries. Hence, drastic reduction of CO 2 emissions, especially from fossil fuels, is essential to tackle anthropogenic climate change. To reduce CO 2 emissions and to cope with the ever-growing demand for energy, it is essential to develop renewable energy sources, of which biofuels will form an important contribution. In this Essay, liquid biofuels from first to fourth generation are discussed in detail alongside their industrial development and policy implications, with a focus on the transport sector as a complementary solution to other environmentally friendly technologies, such as electric cars.

Citation: Cavelius P, Engelhart-Straub S, Mehlmer N, Lercher J, Awad D, Brück T (2023) The potential of biofuels from first to fourth generation. PLoS Biol 21(3): e3002063. https://doi.org/10.1371/journal.pbio.3002063

Copyright: © 2023 Cavelius et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by the German Federal Ministry of Education and Research (BMBF) (031B0853A to NM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Abbreviations: EEA, European Environment Agency; EIC, European Innovation Council; GHG, greenhouse gas; GMO, genetically modified organism; ILUC, indirect land use change; IPCC, Intergovernmental Panel on Climate Change; IRENA, International Renewable Energy Agency; RED, Renewable Energy Directive

Introduction

For decades, global energy demand is on the rise due to economic growth and a rapidly growing world population. Additionally, the standard of living is increasing worldwide, in most cases correlating with increased energy consumption, as energy is needed in almost every aspect of our lives, including land, water, and air transport as well as in agriculture, commercial, industrial, and domestic sectors [ 1 ]. To date, fossil fuels account for around 80% of the world’s energy demand [ 2 ], despite being a major instigator for global warming, representing roughly 89% of total greenhouse gas (GHG) emissions in 2020 [ 3 ]. Additionally, fossil fuels are predicted to deplete with the steadily increasing energy demands. As petroleum demand is constantly on the rise, estimations predict a shortage by 2070 to 2080 [ 4 ]. To that end, distinct biofuel types such as liquid and biogas should be methodologically and strategically developed as a preventive measure against predicted energy shortages, all while reducing the anthropogenic climate impact and preserving the environment.

Currently, biofuels are categorized as first to fourth generation, depending on feedstock and/or biosynthetic platform (i.e., genetic engineering). In this Essay, we present comparative advantages and disadvantages among these categories, as well as fossil sources. Furthermore, the development of biofuel technologies hinges on the socioeconomic and political landscape, which can greatly benefit from policy recommendations by respective regulatory bodies. At present, the European Union has the most stringent biofuel legislation and the most ambitious climate impact goals. Hence, we focus on EU-centered development with respect to current biofuel technology platforms at various stages of industrial deployment, the legislative framework implemented in the EU, as well as policy recommendations that would accelerate academic breakthroughs toward industrial implementation. Although, our recommendations are EU-centric, many are also applicable on a global level.

The four generations of biofuels

One alternative to fossil fuels are biofuels, which originate from organic matter and therefore can be regrown and are termed renewable. Biofuels emit less GHGs and are in general more eco-friendly (non-toxic, sulfur-free, biodegradable) than their fossil fuel predecessors [ 5 ]. Biofuels contribute to the achievement of Sustainable Development Goals 7 (affordable and clean energy) and 13 (climate action) of the United Nations [ 6 ]. Global demand for biofuels is set to grow by 41 to 53 billion liters, or 28%, over 2021 to 2026 [ 7 ]. Typically, one can find four main types of biofuel discussed in the context of fermentation: biogas, bioethanol, biobutanol, and biodiesel. The physiochemical properties of these biofuels are compared to fossil-based fuels in Table 1 .

• PPT PowerPoint slide
• PNG larger image
• TIFF original image

https://doi.org/10.1371/journal.pbio.3002063.t001

Biogas formation is a fairly simple process that has been utilized for several decades. It includes four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Mixed microorganisms consortia and waste streams are combined in a sealed fermentation system in the absence of oxygen. During the biogas production process, microorganisms hydrolyze waste materials into sugars, peptides and amino acids, fatty acids, and to some part into acetate and hydrogen. Afterwards, acidogenic bacteria convert those intermediate products into organic acids, mainly constituting acetic acid. In addition, they produce carbon dioxide and hydrogen. In the third step, acetogenesis, acetate is formed from hydrogen and carbon dioxide produced in the previous stage. Lastly, methanogenesis follows, creating methane from the products of acetogenesis and acidogenesis [ 8 ]. These gases can then be transformed into hydrogen and/or electricity, or can be stored as biomethane in existing geological reservoirs [ 9 ]. Since the Ukraine crisis began, the resulting lack of fossil fuel availability in the EU has led to biogas being politically pushed as a substitute to natural gas [ 10 ].

Compared to gas (biogas/hydrogen), liquid fuels offer higher energy density and simplified transport and storage. This renders them more compatible with current engine and turbine technologies [ 11 ]. Most engines and turbines are designed and built for the use of liquid fuels, which makes liquid biofuels an easy drop-in solution without the need for modifying present engine technologies or infrastructure [ 5 , 12 ]. These gaseous fuels pose a significant safety hazard as they ignite at lower energies and are flammable over a range of concentrations, hydrogen to higher extent, requiring high level of safety procedures [ 13 ]. The low boiling point and high octane number of bioethanol allow blending with gasoline to a certain extent. The added benefits include a more complete combustion and reduced tailpipe emissions, boosting the engine performance and reducing CO 2 emissions. It is, however, inapt for blending with diesel. Diesel engines require hydrocarbons of higher chain length and low autoignition temperature. However, biodiesel, being of similar chemical constitution, can be blended with fossil-based diesel and hence constitutes a major energy-dense liquid biofuel. A third increasingly attractive biofuel is biobutanol, which holds high promise as it displays superior properties to bioethanol such as higher energy density (25% more energy than ethanol) and usually lower water content due to increased hydrophobicity. Biobutanol is less volatile and possesses less corrosive properties, making it easier and safer to use and store [ 11 , 14 – 19 ]. More importantly, it can be blended with both gasoline, fossil-based or biodiesel at any ratio without the need of new engine technologies and might even allow complete substitution of gasoline, while the use of ethanol is only possible as additive [ 11 , 18 ].

While the classification of biofuel technologies somewhat varies in the literature, products can generally be classed as first to fourth generation, depending on the type of feedstock and conversion process that was applied ( Fig 1 ) [ 5 ].

https://doi.org/10.1371/journal.pbio.3002063.g001

First-generation biofuels

Biofuels of the first generation are mainly divided into bioethanol and biodiesel. Bioethanol production of the first generation is based on microbial fermentation of edible feedstocks, rich in starch and sucrose, such as wheat, corn, and sugarcane in Europe, North America, and South America, respectively. Commercial strains include but are not limited to Saccharomyces cerevisiae , S . stipites , and S . pombe . Bioethanol production is not limited to first-generation biofuels; depending on the feedstock and production strain, bioethanol can also be categorized as second and third generation [ 32 – 35 ]. Biodiesel is mainly obtained from food-grade rapeseed, soy, or palm oil sourced from Europe, South America, and Asia, respectively. In contrast to bioethanol, it is only partially biosynthesized as its production includes chemically catalyzed steps such as transesterification of the lipids with alcohols. Enzymatic catalysis currently only exists on a lab scale [ 36 , 37 ]. Although biobutanol production is also possible by sugar fermentation from sugar cane, corn, wheat, and other food crops, it is limited by lower productivity and yields, product inhibition, and high costs [ 11 , 16 , 18 , 38 ].

During the global food demand crisis in 2007/2008, crops used for biofuel became more important to be used as food, giving rise to the “food versus fuel” debate that persists to date. Additionally, an increased demand for crops (e.g., corn) for fuel production yielded an increased market price for those foods [ 5 ]. Models predict that massive agricultural areas would be needed for fuel production and still could supply only limited amounts of fuel compared to the overall demand. It is estimated that more than two times the globally available area of arable land would needed to meet the global market demand for biodiesel when produced from rapeseed oil [ 39 ]. Furthermore, increased market values of palm oil and other biofuel cultures prompted extended deforestation of tropical rainforests for biofuel crop plantations, which releases more CO 2 than the emission saved by those biofuels. In 2008, Fargione and colleagues estimated that it would take 319 years to repay the biofuel carbon debt resulting from clearing of tropical rainforest in Brazil and subsequent conversion to soybean plantations [ 40 ].

Second-generation biofuels

As a result of the issues of the first generation, second-generation biofuels were developed, utilizing lignocellulosic biomass from agricultural and woodland residues as well as other waste streams (for example, from food industry like wheat bran, animal fats, or wastes of cooking and frying oil). Other non-food plants like the drought-resistant shrub or tree Jatropha curcas , which can also be grown in wastelands, might yet be a different promising source for second-generation biofuels [ 41 ]. Hence, second-generation biofuels circumvent the need for agricultural land use change and do not compete with food resources. However, often second-generation waste streams represent more complex feedstocks than sugarcane or palm oil, potentially containing compounds able to reduce fermentation efficiency, such as lignin. Therefore, application of additional pretreatment steps are common, increasing process time and costs [ 5 , 42 , 43 ].

For the most part, biofuels of the first and in the vast majority of the second generation are commercially produced, around 4% and 96% in 2019, respectively [ 44 ]. One example is the commercially available sunliquid from Clariant, which is a cellulosic ethanol from currently underutilized agricultural residues, such as straw. The first commercial ethanol plant in Romania started production in 2022, with plans to convert 250,000 tons of locally sourced agricultural residues to 50,000 tons of ethanol per year. After enzyme production, which hydrolyses cellulose and hemicellulose to sugar monomers, optimized microorganisms are used in fermentation to produce ethanol. These microorganisms can utilize various carbon sources like glucose and xylose, ensuring higher yields and enabling high efficiency and flexibility in waste valorization as more building blocks of waste streams can be converted to product [ 45 ]. Alongside ethanol producers, the production of second-generation biodiesel is possible from microbial lipids produced by organisms, such as Cutaneotrichosporon oleaginosus , a yeast capable of producing up to 90% (w/w) lipids per biomass in a fermentation process, which can be grown on residue streams (e.g., wheat bran hydrolysate medium) [ 46 – 49 ]. Second-generation biodiesel can also be sourced from waste oils via catalytic cracking and hydrogenation. Drawbacks of this process include incomplete conversion and coke formation, which leads to the deactivation of the catalyst. [ 50 , 51 ]. Biobutanol production on lignocellulose biomass and other waste streams is most commonly based on Clostridia fermentation, as it is one of the oldest and best-established fermentative processes for butanol production. Many Clostridia are natural butanol producers and possess the ability to metabolize a variety of different substrates. However, similar to its first-generation predecessor, the process is limited by low butanol titers and product inhibition [ 11 , 16 , 18 , 38 ]. Typically, butanol is produced via ABE fermentation, which results in solvents in ratio of 3 parts acetone, 6 parts butanol, and 1 part ethanol, and butanol refinement is not an energetically favorable solution. Other drawbacks also include cell toxicity at low concentration [ 52 , 53 ]. To that end, cell-free isobutanol biosynthesis using a designed artificial metabolic pathway has been developed [ 54 ]. At present, this approach remains costly for commercialization.

Various carbonaceous compounds can be transformed to syngas by gasification. Commonly, it is a gaseous waste stream from industrial processes such as steel manufacture, in which fossil fuels are burned in the process. Syngas is a mixture mainly consisting of carbon monoxide (CO), CO 2 , and hydrogen. It can be derived from biomass, including lignocellulosic compounds, coal, animal or municipal solid waste, and industrial CO-rich gases. This gas can be metabolized by strictly anaerobic, methanogenic archaea as well as by acetogenic bacterial genera such as Acetobacterium or Clostridium , often used in syntrophic fermentations. The process is mostly focused on biosynthesis of organic acids and alcohol compounds such as acetate, ethanol, and butanol [ 55 – 57 ]. Advantages of syngas fermentation compared to other second-generation approaches are high feedstock flexibility as well as high rates of energy and carbon capture. Complicated pretreatments of second-generation feedstocks can be replaced with gasification, using all components of the biomass, including lignin and other recalcitrant compounds [ 58 ]. LanzaTech developed a process converting feedstocks including industrial waste streams to fuel and chemicals utilizing bacteria. They estimate a total product capacity of 600,000 metric tons as well as 1,000,000 metric tons of captured carbon per year, for all their plants combined [ 59 ]. Since 2022, a demonstration plant in Japan has turned municipal solid waste to ethanol, with a production target of 20 tons of ethanol per day [ 60 ].

More than half of the biologically stored carbon is bound in marine biomass, especially macroalgae and seagrass. Detached seagrass material is seasonally washed on beaches and shore lines; due to low biological degradation and herbivore consumption, an excess of it accumulates as waste. Estimations of up to 40 million tons of dry seagrass biomass, which can be used for biofuel production, are given. Through enzymatic hydrolysis, the carbohydrate content of the seagrass can be used in a fermentation medium for microorganisms, additionally offering low nitrogen and phosphorus content, which is typically required for lipid production [ 61 ].

Despite the highly favorable ability to valorize waste streams, second-generation biofuels by themselves will not be sufficient to supply energy for the current worldwide demand. As is the case for food crops with first-generation biofuels, biomass used in these processes is available in limited amounts. Therefore, second-generation biofuels must be combined with other technologies to ensure sufficient provision of fuels. This prompted research on third-generation biofuels. However, scientific estimations predict second-generation biofuels could supply up to 30% of the world’s transportation energy [ 5 ].

Third-generation biofuels

Third-generation biofuels are mainly derived from microalgae and cyanobacteria biomass, which can be used to naturally generate alcohols and lipids to transform into biodiesel or any other high energy fuel product. Algae exhibit 2- to 4-fold higher photosynthesis rates than terrestrial plants, resulting in faster biomass formation [ 62 ]. Algae do not require arable land or fresh water for cultivation. Many cultures can be grown using waste water, brackish or salt water, which is cost efficient and circumvents competition with agricultural activity [ 63 , 64 ]. Most importantly, efficient algae cultivation requires a direct CO 2 supply, which can be derived from industrial emitters or by atmospheric carbon capture. In conventional cultivation systems, around 70% of supplied CO 2 is used for photosynthesis and therefore biomass production [ 65 ]. Hence, algae biofuels potentially could have a negative carbon footprint as they directly bind the GHG in their biomass. One of the most prominent third-generation processes is the production of biodiesel or other energy density biofuels, such as biokerosene, using oleaginous microalgae [ 66 , 67 ].

One of the most economically critical and versatile operations in algal biofuel production is algae cultivation. Algal bioreactors ( Fig 2 ) are independent of location and climate, therefore can be operated almost irrespective of these factors. For low price, high volume products, such as biofuels, algae are commonly cultivated in open ponds. Open pond reactors are significantly cheaper in their construction and operation but have drawbacks like high loss of water through evaporation and lack of temperature control, which lowers biomass productivity. The alternative, preferred for high price, low volume products, such as cosmetic ingredients, is a closed photobioreactor, where process parameters can be precisely controlled, which often leads to higher productivity [ 63 , 68 ]. These bioreactors also enable a three-dimensional mode of cultivation, significantly increasing the productivity per area. In contrast to second-generation biofuels, the third-generation processes completely decouple biofuel production from the need for agricultural land. Additionally, algal-based oil production is likely greater than that in higher plants, as lipids mainly accumulate in specific parts of the plant (e.g., in rape seeds), while in algae, each cell can contain high amount of lipids, making the process more mass efficient. One bottleneck in production is harvesting, as the low size and density of the microalgal cells combined with the sensitivity of the cells to changes in pH render it challenging. [ 66 ]. Furthermore, downstream processing for algal biofuels is commonly more energy intensive than other biofuel productions [ 63 , 69 ]. Araújo and colleagues mapped 447 algae and cyanobacteria Spirulina production units in 2021 in the EU [ 70 ]. Most of these companies directed their biomass to the production of food, feed, and related uses; commercial application of biofuels only had a very small share. Further technological developments in upscaling and reduction of production costs are necessary for commercialization.

This image showcases the open algae cultivation systems located at Technical University of Munich, Ottobrunn.

https://doi.org/10.1371/journal.pbio.3002063.g002

Fourth-generation biofuels

The latest biofuel generation, termed fourth-generation biofuels, encompasses the use of genetic engineering to increase desired traits of organisms used in biofuel production. This applies to a variety of traits from utilizing multiple types of sugars (e.g., pentoses and hexoses), to higher lipid synthesis or increased photosynthesis and carbon fixation. For model organisms, such as Escherichia coli and Saccharomyces cerevisiae , a wide variety of tools for genetically engineering the regulation of endogenous pathways or inserting new pathways are reported. Unfortunately, for most native producers of biofuels, the genetic engineering toolbox is far more limited.

Currently, two different approaches have been adopted: engineering of pathways in native producers (optimizing growth rates, utilization of different carbon sources, directing the metabolic flux toward biofuel production and increased production titers) and reconstruction of pathways identified in natural producers in more genetically accessible model organisms. A wide variety of microorganisms can be used as heterologous hosts for the production of biofuels, including bacteria, yeast, and algae. Their metabolic versatility enables the use of various substrates to produce a wide range of biofuels. For example, butanol pathway genes from Clostridia were introduced into E . coli , Pseudomonas putida , and Bacillus subtilis strains [ 14 , 16 , 19 ]. While the introduction of heterologous genes is well established, a major challenge is the disruption of competing metabolic fluxes. Another obstacle for high product titers can be toxicity of large amounts of product on the cell. To enable increased accumulation of biofuels, the cellular stress response can be modified through genetic engineering, for example, with cell membrane modifications. Through the overexpression of certain membrane transporters, biofuel molecules can be secreted into the medium thereby circumventing accumulation as well as toxicity while simultaneously simplifying product recovery. In E . coli , membrane transporters have been used successfully to excrete n-alkanes, such as n-octane [ 71 , 72 ]. However, the overexpression of transporters is challenging as it modifies the membrane composition, creating a metabolic burden as well as potentially overloading the cellular import and export, thereby disabling the cells ability to regulate its internal environment/homeostasis [ 71 ].

Genetically modified algae can offer higher product yields and a variety of other improvements compared to wild-type algae. In order to enhance photosynthetic efficiency, the antennae systems of algae capable of absorbing a broader range of the light spectrum could be transferred to more suitable production organisms [ 44 , 73 ]. With respect to genetic engineering, CRIPSR/Cas9 is a frequently used tool, as it offers a simple design with efficient transfection and targeted gene disruption [ 74 ].

In fourth-generation biofuel processes that focus on genetically optimized cyanobacteria, the production of ethanol, as well as other fuel products such as butanol, isobutanol, and modified fatty acids have been realized successfully [ 75 , 76 ]. While 1-butanol production reached titers of 300 mg/L, bioethanol titers of up to 5.5 g/L were reported [ 77 – 79 ].

For the efficient optimization of native producers, systems biology can offer many insights. The availability of whole-genome sequences is essential, as this information allows for the annotation of genes to their respective function and reconstruction of the innate metabolic pathways, which can subsequently be modified. Recent advances have been made in the field of genome sequencing allowing for a more rapid and cost-efficient collection of data [ 19 ], while the gene expression patterns in different growth environments can be analyzed by transcriptomics and protein products identified by proteomics.

With genetic engineering tools, the quantity and quality of biofuels can be controlled and increased but will need political acceptance and support to be widely adopted [ 5 ]. There is a controversial debate around genetic engineering in agriculture and medicine, especially in Europe; therefore, similar concerns can be anticipated surrounding the use in biofuel production. A European-based study came to the conclusion that genetically engineered algae for biofuel production would be accepted by the majority of consumers, when the safety of the systems can be guaranteed [ 80 ]. However, with proper containment methods and carefully selected locations, such risks could be drastically minimized. Therefore, closed production systems with high security standards are expected to be built [ 80 ]. Additional biocontainment methods can be directly based on genetic changes inside the production cells such as auxotrophies or kill switches, significantly decreasing the risk of genetically modified organism (GMO) escape [ 44 , 81 ].

One alternative to targeted genetic engineering is random mutagenesis, which can be described as accelerated evolution. Microorganisms and products generated by this approach are not subjected to GMO regulations. Furthermore, this technique can be performed with little knowledge about the production organism and production pathway. Random mutagenesis can be achieved by a variety of methods such as UV light, chemical agents, or fast neutron irradiation. For the first time, the latter was applied on C . oleaginosus , resulting in mutants with elevated lipid titers suitable for biodiesel applications. It is noteworthy that biodiesel from prominent oleaginous yeast platforms, such as Yarrowia lipolytica , C . oleaginosus , Rhodosporidium toruloides , and Lipomyces starkeyi , are compliant with international biodiesel standards, including US ASTM D6751 and EU standard EN 14214 [ 82 , 83 ].

A new, more experimental approach to fourth-generation biofuels is the production of electrobiofuels. These are based on the approach to establish new-to-nature hybrid systems, which are able to use renewable electricity and carbon sources directly for the production of commodity chemicals and biofuels, thereby enabling the conversion of solar energy into storable liquid fuel. Such a process could combine the higher photon efficiency of modern photovoltaic systems (compared to photosynthesis) with the sustainability of biofuel production, increasing overall process effectiveness [ 84 ].

Economics of biofuels in transportation

Apart from reducing GHG emissions and air pollution, biofuel industries can contribute to energy security on a local and national scale, as it is not reliant on local reservoirs of fossil oil. Additionally, the creation of new employment and economic growth, especially in rural locations, should positively impact the social environment as well. However, to fully exploit all the positive traits of biofuels, further research and investments are necessary, as the production of biofuels requires more processing steps compared with the conventional methods of drilling into the ground to obtain crude oil, followed by refining. Therefore, at present, biofuels commonly exceed fossil fuel production costs. Furthermore, raw materials for biofuel production do not compare to crude oil in energy density, requiring far greater amounts of biomass for the same energy output compared to fossil sources. The infrastructure required for the sector of biofuel production has to be extensively developed as well. One example is the primary energy needed to run the process, which should be obtained through sustainable operations. Candidates for that include solar and wind energy among others. Thus, by reducing the overall production cost and increasing process efficiency, biofuels could become more competitive to fossil fuels. Furthermore, by-products of biofuel production should be efficiently utilized in a circular economy, which could increase cost efficiency of such processes.

Transportation is one of the most socioeconomically sensitive sectors for the use of liquid biofuels ( Fig 3 ). It contributes about 17% of global CO 2 emissions [ 85 , 86 ], and so far, sustainable solutions are not fully developed. Due to their limitations, current technologies for biofuels are not likely to completely replace fossil fuels in their entirety but can offer new routes for waste stream valorization in a circular economy and contribute significantly to minimize our dependency on fossil fuels one step at a time. A complementary approach to this goal is electric cars, which have zero tailpipe emissions, although CO 2 emissions are associated with the production of the car and the source of the electricity. Essential in electric vehicle batteries are metals like lithium, cobalt, nickel, and manganese. The demand for these metals is surging, while at the same time toxic waste electronics are accumulating all over the world. Traditional recycling/extraction methods require high temperatures and strong acids. This is a high energy process involving toxic chemicals. One alternative is bioleaching or biomining, which employs microbes such as Acidithiobacillus ferrooxidans that can bind and recover metals, bypassing the need for high temperatures and toxic chemicals [ 87 – 90 ]. This emerging technology offers an eco-friendly approach to recycling but still requires extensive research and development. Additionally, a new infrastructure must be put into place, supporting millions of electric cars at the same time. To that point, a combination of synthetic and biofuels in synergy with electric cars might be an optimal solution for the years to come, partially substituting fossil fuels, thereby drastically reducing CO 2 output of transportation.

The transport sector, specifically, results in 17% of emissions. Adapted from Ritchie and colleagues (2020), Carbon Leadership Forum 2020 [ 85 , 86 ].

https://doi.org/10.1371/journal.pbio.3002063.g003

EU policy recommendations

In order to promote the use of clean and sustainable energy at the industrial, retail, and consumer level, a cohesive framework of policies is imperative. The European Commission and European Environment Agency (EEA) have cooperated with the International Renewable Energy Agency (IRENA) and the Intergovernmental Panel on Climate Change (IPCC) in leading the efforts for clean energy transition through a number of directives and legislations since the 1990s [ 91 – 94 ]. These efforts manifest as a commitment by EU countries to lower GHG emissions and increase the use of renewable energy. Most notable is the Renewable Energy Directive (RED), which came into force in 2009. Through this directive, EU countries set targets for renewable energy consumption, including a subtarget mandating 10% of energy used in transport to be produced from renewables. It is noteworthy that the deployment of renewable energy has continuously grown, exceeding 22% in 2020 [ 92 ]. The legislation also mandates GHG reduction targets for fuel suppliers, requiring a reduction in GHG intensity of the fuel mix by 6% in 2020 [ 92 ]. In 2018, the commission revised the legislative proposal and the European Parliament and the EU Council proposed amendments as RED II. Therewith, the EU aims to increase the share of renewable energy to 32% and in transport to at least 14%, including a minimum share of 3.5% of advanced biofuels (second- and third-generation biofuels). The latter streamlines waste residues, such as agricultural waste (e.g., straw), and also encompasses renewable electricity in road and rail transport [ 95 ].

At present, the industrial biofuel production is dominated by first- and second-generation processes, respectively. Nevertheless, RED II and indirect land use change (ILUC) proposals have initiated the gradual shift toward second- and third-generation processes, which are associated with significant changes in feedstock supply and logistics, as well as technology deployment (e.g., market penetration of advanced biofuels). ILUC qualifies first-generation biofuels based on the unintended consequences of releasing carbon emissions as a result of land use changes [ 96 , 97 ]. While technical process development for third- and fourth-generation biofuels is advancing rapidly in academic and start-up settings, large-scale industrial implementation remains lagging. This indicates a profound gap in transferring technologies from a pilot scale (TRL 5) to an industrial scale (TRL 8). To that end, clear and implementable criteria remain to be addressed by legislators for industrial technology transition toward advanced biofuels with a notable climate impact. Table 2 summarizes our policy recommendations aimed at advancing biofuels implementation as well as their respective expected results and acting entity.

https://doi.org/10.1371/journal.pbio.3002063.t002

First and foremost, legislators need to create stable policies and regulatory frameworks based on measurable cradle-to-cradle sustainability performance indicators. In the past, one of the greatest barriers for industry to adopt new biofuel technologies, at least in the EU, was the constantly changing regulatory and provisions framework, which ultimately led to waves of market and company consolidation for first-generation fuels such as crop-based biodiesel, corn and sugar beet-based bioethanol, and, more recently, corn-based biogas products. Therefore, it is of the utmost importance that policy makers provide clearly formulated, long-term stable policies, provisions, and regulatory frameworks to allow industrial transition to advanced biofuel technologies with clear climate impact.

With respect to sustainability, measurable criteria can be categorized as agriculture biomass, forest biomass with respect to biodiversity, and carbon stocks and emissions. Biofuel ILUC factors could be included in the biannual reports of fuel suppliers and EU countries. Accordingly, biofuel produced from palm oil and soy should carry a high ILUC factor and phasing out these feedstocks could be achieved by encouraging the diversification of feedstock. Reports estimate that 130,000 to 210,000 hectares of deforestation, which has detrimental effect on biodiversity and soil quality, could be avoided by limiting the demand of EU countries for palm oil biofuels [ 98 ]. Land requirement and fresh water use, carbon trading, and carbon offsets should also be factored in upcoming legislations. The criteria should also include GHG emissions that take the levels of methane, nitric oxides, and sulfur oxides into account in addition to levels of CO 2 . Legislation criteria should also take into consideration end-use performance, whereby industry sector, energy efficiency, and socioeconomic impact could represent qualifying measures. Risk determination and possible exceptions could be evaluated for specific industries, such as security and electricity. With respect to energy efficiency, it should be considered that distinct biofuels differ in their output. For example, ethanol yields 25% more energy than that invested in its production, while biodiesel yields 93% more [ 99 ]. To that end, performance-based renewable energy policies are needed. Finally, a reliable system that verifies compliance and reporting is eminent to putting these proposals into practice. In that respect, a mass balance system that observes the global carbon inventory and defines optimal distribution of energy profiles (first to fourth generation) and mixtures (e.g., E10 petrol/ethanol) to ensure minimal climate impact is in order. This system could integrate a range of parameters, including flexible distribution channels, demand management, storage, and price signals in real time [ 97 , 100 ]. Independent auditing services could further ensure compliance, which could also be extended to trading partners of the EU countries at a later stage.

As the implementation of industrial biofuel production sites are associated with immense capital investments, it is crucial to shed light on the financial aspect linked to these policies, primarily, multilevel incentives schemes, investment risk reduction, and infrastructure and logistics. On an EU level, specific funding mechanisms such as European Innovation Council (EIC) pathfinder, EIC Transition, and EIC Accelerator that aim to enable and accelerate the scaling trajectory of new technologies toward market entry already exist. While this is an initial step toward implementing new biofuel technologies, these measures do not translate into national actions and legislation on a member state level, which impedes the regional mobilization of capital, leading to a slow uptake and implementation of new technologies. Hence, a significant step toward rapid technology adoption and implementation would be the regional implementation of funding and capital mobilization as already practiced on the EU level.

An integral element in promoting advanced biofuels could be incentivizing biofuel processes that show favorable sets of sustainability parameters and end-use performance by a higher cost of CO 2 certificates, which realistically should be in the order of 500 to 1,000 Euros/ton CO 2 . Consolidated long-term measures would also provide companies and investors with valuable tools to calculate return of investment and hence de-risk decision-making for iterative technology transition. To enable more efficient technology transfer from academia toward industrial technology deployment, additional factors need to be considered. To that end, academic projects should receive sequential, stage-gated extended funding periods of 4 to 8 years that commonly go beyond a single governmental administration period. This would allow ideas to be developed toward a proof of concept stage, where they can be translated to spin-outs or industry partners. Governments should incentivize start-up formation derived from academic units using focused funding measures, such as the EXIST funding program in Germany [ 101 ]. As technology development from proof of concept (TRL 2 to 4) in academic settings to pilot plant level often requires time periods exceeding 5 to 7 years, synergistic midterm private funding resources also have to be mobilized. To that end, technology familiarity, better understanding of time frames for solid technology development, and proper risk assessment are essential for private capital investors. In order to motivate private capital in the EU to accept development risks and extended time frames for return of investment in biofuel start-up companies, governments could implement tax write-offs for spent risk capital. This legislatively guided de-risking of capital investment into new technologies is already implemented in the United States of America and the United Kingdom, as well as in other, less compliance-driven, financial markets. Hence, the EU has to rapidly implement such legislative tax reliefs to secure innovation on the biofuels and other innovation and sustainability-driven sectors for added economic value and a vibrant job sector.

Capital is also short at the infrastructure and logistics level. Investments are required to construct dedicated pilot plants that allow industrial scale validation and optimization of new technologies, independent of any large-scale industrial partner. In that respect, multiple regionally decentralized pilot plants could provide dedicated instrumental parks that house state of the art fermentation and downstream processing equipment. In the case of gas fermentation, these parks could be associated with significant security measures and demand special regulatory approval and regular inspection. Accordingly, construction and operation by large national research organizations, such as Fraunhofer institutions in Germany, or private–public partnerships is recommended. Governmentally driven funding actions that enable access and use of these pilot plant facilities by innovators in the biofuels sector could further accelerate industrial deployment and market entry. In parallel to technology market readiness, the implementation of biofuels in industrial processes requires a secured feedstock supply.

Contrary to Nordic countries that are the forefront of advanced biofuel processes development, most industrialized countries in the EU with a high population density do not have sufficient land or biomass availability for large-scale biofuel production [ 100 ]. Hence, the location and feedstock supply require strategic positioning. Two routes for biofuels production are viable in the EU: a large production plant located in a region with abundant, long-term feedstock/biomass supply or secured trade routes; or a network of smaller, decentralized production facilities. In the latter case, a farm-integrated production facility with secured access to local residue streams can be envisioned. To optimize the economics of the production facilities, its location should be leveraged with maximal carbon credits in order to meet fuel market prices. To make an informed decision on the location and mode of production, a global carbon inventory map would be extremely beneficial. While we have a good overview of regional carbon emissions, there is little information on correlative carbon storage, which is mostly limited to terrestrial biomass. To that end, other carbon storage mechanisms should be considered, such as existing geological carbon (CO 2 ) capture activities and marine biomass. Considering that 68% of the world population is projected to live in urban areas by 2050, it is sensible to consider urban waste streams, such as sewage sludge and food waste, as yet underutilized biomass feedstocks for biofuel production processes [ 102 ]. More generally, a map of the carbon flux resolved on a country-specific level would enable a more informed decision on the selection of process feedstock (biomass residues/CO 2 ) and trading partners that could secure operation of large-scale production facilities for third- and fourth-generation biofuels. Currently, the major trading partners of the EU are Argentina, Brazil, USA, Indonesia, and Malaysia [ 97 ]. These trading practices do not ensure level field sustainability over the long term. To that end, future trading legislation should consider balanced trade between the global North and global South to ensure long-term beneficial socioeconomic impact on the stability and sustainability of feedstock and biofuel production.

Conclusions

In this Essay, we laid out the reasoning for biofuel production as immediate and long-term measures to limit and eliminate energy and mobility-related GHG emissions. In that regard, biofuels will not be the only solution but an essential building block in a network with other physical (i.e., wind power, photovoltaic systems [ 103 – 105 ]) and chemical technologies (i.e., Sabatier process, Power to X [ 106 , 107 ]) that together can provide carbon neutral or even carbon negative energy and mobility solutions. In regard to transportation, biofuels should act in synergy with other technologies, such as electrified vehicles. In addition to biofuel manufacturing, similar processes could also be implemented in other applications. Here, algal and yeast oil can be transformed into building materials such as carbon fibers and cement additives. Via these routes, atmospheric CO 2 can be absorbed from the environment and stored for very long periods of time. Such technologies could complement materials derived from fossil fuels or that generate large amounts of CO 2 during the manufacturing process (e.g., steel, aluminum and concrete) [ 108 ].

We are convinced that, in the last decades, mankind has been generally too hesitant to adopt climate-centered technologies, which has put the world on a perilous pathway toward catastrophic climate change [ 109 – 111 ]. The destructive outcomes of this scenario have been documented in the scientific literature and are subject to numerous high level reports [ 112 – 117 ].

As time for action is already overdue, it is essential to act now by implementing the tools and technologies we have at hand at the present time. It is our opinion, that the only path to enable climate effective energy security and mobility is to deploy available technologies at a global scale right now. The global implementation of large-scale production infrastructure for sustainable (bio)technologies to kick-start production of renewable energy carriers and sustainable commodities is imperative in this timely development scenario. Once production with a base process has commenced, these processes can be iteratively refined or modulated at scale to evolve toward the next technology generation. This approach demands close, long-term academic and industry partnerships.

This fundamental transition toward sustainable bio-based technologies will require long-sighted, fact-driven legislative guidance and immense capital investments across the private and governmental sectors. However, it will be the only route to limit climate change effects and provide a livelihood for future societies.

With respect to governments, this means that neither ideology nor demagogically driven decision-making will protect any society from the effects of climate change. There are just no simple answers to complex, global problems. What is needed are global governmental alliances that make technocratically oriented long-sighted decisions, aiming for definitively set climate-centered outcomes even if the communication of the measures that have to be taken may not be popular on first sight.

Even outside the scientific communities, people are ready to accept change of the status quo in order to curb climate change effects and transition to a sustainable society. The question remains if the global political elites are ready to communicate and implement this change. Time is running out to maintain the global ecosystems as we know it.

Acknowledgments

The authors dedicate this manuscript to Dr. Christian Patermann (former EU Program Director Biotechnology, Agriculture, and Food) and Dr. Günther von Au (Chairman of the Board of Directors of Clariant AG), each being outstanding political and industrial visionaries, influencers, and decision-makers in the field of sustainable (bio)technologies and the bioeconomy, respectively.

• View Article
• Google Scholar
• PubMed/NCBI
• 7. IEA. Renewables 2021. Paris: IEA; 2021.
• 59. LanzaTech. Annual Report 2021. 2021. Available from: https://lanzatech.com/wp-content/uploads/2022/08/LanzaTech_2021_Annual_Report.pdf .
• 93. European Environment Agency. 19.01.2023. Available from: https://www.eea.europa.eu/ .

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• Review Article
• Published: 25 June 2021

Microbial production of advanced biofuels

• Jay Keasling 1 , 2 , 3 , 4 , 5 , 6 ,
• Hector Garcia Martin   ORCID: orcid.org/0000-0002-4556-9685 1 , 4 , 7 , 8 , 9 ,
• Taek Soon Lee 1 , 4 ,
• Aindrila Mukhopadhyay   ORCID: orcid.org/0000-0002-6513-7425 1 , 4 , 9 ,
• Steven W. Singer 1 , 4 &
• Eric Sundstrom 4 , 10  

Nature Reviews Microbiology volume  19 ,  pages 701–715 ( 2021 ) Cite this article

12k Accesses

139 Citations

37 Altmetric

Metrics details

• Applied microbiology
• Metabolic engineering

Concerns over climate change have necessitated a rethinking of our transportation infrastructure. One possible alternative to carbon-polluting fossil fuels is biofuels produced by engineered microorganisms that use a renewable carbon source. Two biofuels, ethanol and biodiesel, have made inroads in displacing petroleum-based fuels, but their uptake has been limited by the amounts that can be used in conventional engines and by their cost. Advanced biofuels that mimic petroleum-based fuels are not limited by the amounts that can be used in existing transportation infrastructure but have had limited uptake due to costs. In this Review, we discuss engineering metabolic pathways to produce advanced biofuels, challenges with substrate and product toxicity with regard to host microorganisms and methods to engineer tolerance, and the use of functional genomics and machine learning approaches to produce advanced biofuels and prospects for reducing their costs.

This is a preview of subscription content, access via your institution

Access options

Access Nature and 54 other Nature Portfolio journals

Get Nature+, our best-value online-access subscription

24,99 € / 30 days

cancel any time

Subscribe to this journal

Receive 12 print issues and online access

195,33 € per year

only 16,28 € per issue

Buy this article

• Purchase on SpringerLink
• Instant access to full article PDF

Prices may be subject to local taxes which are calculated during checkout

Similar content being viewed by others

WLTC and real-driving emissions for an autochthonous biofuel from wine-industry waste

Physiological limitations and opportunities in microbial metabolic engineering

Towards carbon-neutral and clean propulsion in heavy-duty transportation with hydroformylated Fischer–Tropsch fuels

US Energy Information Administration. International energy outlook 2019: with projections to 2050 (EIA, 2019).

US Environmental Protection Agency. Sources of greenhouse gas emissions. EPA https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions (2020).

Kircher, M. Sustainability of biofuels and renewable chemicals production from biomass. Curr. Opin. Chem. Biol. 29 , 26–31 (2015).

Article   CAS   PubMed   Google Scholar  

Hughes, S. R. & Jones, M. A. in Green Energy to Sustainability: Strategies for Global Industries (eds Vertès, A. A., Qureshi, N., Blaschek, H. P. & Yukawa, H.) 137–156 (Wiley, 2020).

Liu, Y. et al. Biofuels for a sustainable future. Cell 184 , 1636–1647 (2021).

Field, J. L. et al. Robust paths to net greenhouse gas mitigation and negative emissions via advanced biofuels. Proc. Natl Acad. Sci. USA 117 , 21968–21977 (2020).

Article   CAS   PubMed   PubMed Central   Google Scholar  

Baral, N. et al. Techno-economic analysis and life-cycle greenhouse gas mitigation cost of five routes to bio-jet fuel blendstocks. Energy Environ. Sci. 12 , 807–824 (2018).

Article   Google Scholar  

Hannon, J. R. et al. Technoeconomic and life-cycle analysis of single-step catalytic conversion of wet ethanol into fungible fuel blendstocks. Proc. Natl Acad. Sci. USA 117 , 12576–12583 (2020).

Baral, N. R. et al. Approaches for more efficient biological conversion of lignocellulosic feedstocks to biofuels and bioproducts. ACS Sustain. Chem. Eng. 7 , 9062–9079 (2019).

Article   CAS   Google Scholar  

Yang, M., Baral, N. R., Anastasopoulou, A., Breunig, H. M. & Scown, C. D. Cost and life-cycle greenhouse gas implications of integrating biogas upgrading and carbon capture technologies in cellulosic biorefineries. Environ. Sci. Technol. 54 , 12810–12819 (2020).

Langholtz, M. H., Stokes, B. J. & Eaton, L. M. Billion-ton report: advancing domestic resources for a thriving bioeconomy, volume 1: economic availability of feedstocks (Oak Ridge National Laboratory, 2016).

Global Bioenergy Association. Global Bioenergy Statistics 2019 . http://www.worldbioenergy.org/uploads/191129%20WBA%20GBS%202019_HQ.pdf (Global Bioenergy Association, 2019).

Pattrick, C. A. et al. Proteomic profiling, transcription factor modeling, and genomics of evolved tolerant strains elucidate mechanisms of vanillin toxicity in Escherichia coli. mSystems 4 , e00163-19 (2019).

Article   PubMed   PubMed Central   Google Scholar  

Carroll, A. & Somerville, C. Cellulosic biofuels. Annu. Rev. Plant Biol. 60 , 165–182 (2009).

Blanch, H. W., Simmons, B. A. & Klein-Marcuschamer, D. Biomass deconstruction to sugars. Biotechnol. J. 6 , 1086–1102 (2011).

Dale, B. E. & Ong, R. G. Energy, wealth, and human development: why and how biomass pretreatment research must improve. Biotechnol. Prog. 28 , 893–898 (2012).

Klein-Marcuschamer, D., Oleskowicz-Popiel, P., Simmons, B. A. & Blanch, H. W. The challenge of enzyme cost in the production of lignocellulosic biofuels. Biotechnol. Bioeng. 109 , 1083–1087 (2012).

Lee, S. Y., Kim, H. M. & Cheon, S. Metabolic engineering for the production of hydrocarbon fuels. Curr. Opin. Biotechnol. 33 , 15–22 (2015). A comprehensive review of metabolic engineering for biofuel production .

Adom, F., Dunn, J. B., Han, J. & Sather, N. Life-cycle fossil energy consumption and greenhouse gas emissions of bioderived chemicals and their conventional counterparts. Environ. Sci. Technol. 48 , 14624–14631 (2014).

Biddy, M. J. et al. The techno-economic basis for coproduct manufacturing to enable hydrocarbon fuel production from lignocellulosic biomass. ACS Sustain. Chem. Eng. 4 , 3196–3211 (2016).

Brooks, K. P. et al. in Biofuels for Aviation: Feedstocks, Technology and Implementation (ed. Chuck, C.) 109–150 (Academic, 2016).

George, K. W., Alonso-Gutierrez, J., Keasling, J. D. & Lee, T. S. Isoprenoid drugs, biofuels, and chemicals-artemisinin, farnesene, and beyond. Adv. Biochem. Eng. Biotechnol. 148 , 355–389 (2015).

CAS   PubMed   Google Scholar  

Li, M. et al. Recent advances of metabolic engineering strategies in natural isoprenoid production using cell factories. Nat. Prod. Rep. 37 , 80–99 (2020).

Rodríguez-Concepción, M. Plant isoprenoids: a general overview. Methods Mol. Biol. 1153 , 1–5 (2014).

Article   PubMed   CAS   Google Scholar  

Gao, Y., Honzatko, R. B. & Peters, R. J. Terpenoid synthase structures: a so far incomplete view of complex catalysis. Nat. Prod. Rep. 29 , 1153–1175 (2012).

Peralta-Yahya, P. P. et al. Identification and microbial production of a terpene-based advanced biofuel. Nat. Commun. 2 , 483 (2011).

Harrison, K. W. & Harvey, B. G. Renewable high density fuels containing tricyclic sesquiterpanes and alkyl diamondoids. Sustain. Energy Fuels 1 , 467–473 (2017).

Zebec, Z. et al. Towards synthesis of monoterpenes and derivatives using synthetic biology. Curr. Opin. Chem. Biol. 34 , 37–43 (2016).

George, K. W. et al. Correlation analysis of targeted proteins and metabolites to assess and engineer microbial isopentenol production. Biotechnol. Bioeng. 111 , 1648–1658 (2014).

Meadows, A. L. et al. Rewriting yeast central carbon metabolism for industrial isoprenoid production. Nature 537 , 694–697 (2016). A good demonstration of microbial engineering for biofuel-producing yeast, especially at industrial scale .

Kang, A. et al. Isopentenyl diphosphate (IPP)-bypass mevalonate pathways for isopentenol production. Metab. Eng. 34 , 25–35 (2016).

Kang, A. et al. Optimization of the IPP-bypass mevalonate pathway and fed-batch fermentation for the production of isoprenol in Escherichia coli. Metab. Eng. 56 , 85–96 (2019).

Lennen, R. M. & Pfleger, B. F. Engineering Escherichia coli to synthesize free fatty acids. Trends Biotechnol. 30 , 659–667 (2012).

Budin, I. et al. Viscous control of cellular respiration by membrane lipid composition. Science 362 , 1186–1189 (2018).

Marella, E. R., Holkenbrink, C., Siewers, V. & Borodina, I. Engineering microbial fatty acid metabolism for biofuels and biochemicals. Curr. Opin. Biotechnol. 50 , 39–46 (2018).

Qiao, K. et al. Engineering lipid overproduction in the oleaginous yeast Yarrowia lipolytica . Metab. Eng. 29 , 56–65 (2015).

Steen, E. J. et al. Microbial production of fatty-acid-derived fuels and chemicals from plant biomass. Nature 463 , 559–562 (2010).

Zhou, Y. J. et al. Production of fatty acid-derived oleochemicals and biofuels by synthetic yeast cell factories. Nat. Commun. 7 , 11709 (2016).

Krivoruchko, A., Zhang, Y., Siewers, V., Chen, Y. & Nielsen, J. Microbial acetyl-CoA metabolism and metabolic engineering. Metab. Eng. 28 , 28–42 (2015).

Qiao, K., Wasylenko, T. M., Zhou, K., Xu, P. & Stephanopoulos, G. Lipid production in Yarrowia lipolytica is maximized by engineering cytosolic redox metabolism. Nat. Biotechnol. 35 , 173–177 (2017).

Zhu, Z. et al. Expanding the product portfolio of fungal type I fatty acid synthases. Nat. Chem. Biol. 13 , 360–362 (2017).

Schirmer, A., Rude, M. A., Li, X., Popova, E. & del Cardayre, S. B. Microbial biosynthesis of alkanes. Science 329 , 559–562 (2010).

Rude, M. A. et al. Terminal olefin (1-alkene) biosynthesis by a novel p450 fatty acid decarboxylase from Jeotgalicoccus species. Appl. Environ. Microbiol. 77 , 1718–1727 (2011).

Youngquist, J. T. et al. Production of medium chain length fatty alcohols from glucose in Escherichia coli . Metab. Eng. 20 , 177–186 (2013).

Goh, E.-B., Baidoo, E. E. K., Keasling, J. D. & Beller, H. R. Engineering of bacterial methyl ketone synthesis for biofuels. Appl. Environ. Microbiol. 78 , 70–80 (2012).

Javidpour, P. et al. Investigation of proposed ladderane biosynthetic genes from anammox bacteria by heterologous expression in E. coli . PLoS ONE 11 , e0151087 (2016).

Article   PubMed   PubMed Central   CAS   Google Scholar  

Czerwiec, Q. et al. Optimization of cyclopropane fatty acids production in Yarrowia lipolytica . Yeast 36 , 143–151 (2019).

Rabinovitch-Deere, C. A., Oliver, J. W. K., Rodriguez, G. M. & Atsumi, S. Synthetic biology and metabolic engineering approaches to produce biofuels. Chem. Rev. 113 , 4611–4632 (2013).

Bond-Watts, B. B., Bellerose, R. J. & Chang, M. C. Y. Enzyme mechanism as a kinetic control element for designing synthetic biofuel pathways. Nat. Chem. Biol. 7 , 222–227 (2011).

Atsumi, S., Hanai, T. & Liao, J. C. Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels. Nature 451 , 86–89 (2008).

Sherkhanov, S. et al. Isobutanol production freed from biological limits using synthetic biochemistry. Nat. Commun. 11 , 4292 (2020). A good demonstration of the cell-free system for biofuel (isobutanol) production with high TRY .

Rodriguez, G. M., Tashiro, Y. & Atsumi, S. Expanding ester biosynthesis in Escherichia coli . Nat. Chem. Biol. 10 , 259–265 (2014).

Lee, J.-W. & Trinh, C. T. Microbial biosynthesis of lactate esters. Biotechnol. Biofuels 12 , 226 (2019).

Yuzawa, S., Keasling, J. D. & Katz, L. Insights into polyketide biosynthesis gained from repurposing antibiotic-producing polyketide synthases to produce fuels and chemicals. J. Antibiot. 69 , 494–499 (2016).

Cai, W. & Zhang, W. Engineering modular polyketide synthases for production of biofuels and industrial chemicals. Curr. Opin. Biotechnol. 50 , 32–38 (2018).

Liu, Q. et al. Engineering an iterative polyketide pathway in Escherichia coli results in single-form alkene and alkane overproduction. Metab. Eng. 28 , 82–90 (2015).

Poust, S. et al. Divergent mechanistic routes for the formation of gem-dimethyl groups in the biosynthesis of complex polyketides. Angew. Chem. Int. Ed. 54 , 2370–2373 (2015).

Srirangan, K. et al. Engineering Escherichia coli for Microbial Production of Butanone. Appl. Environ. Microbiol. 82 , 2574–2584 (2016).

Yuzawa, S. et al. Short-chain ketone production by engineered polyketide synthases in Streptomyces albus . Nat. Commun. 9 , 4569 (2018).

Zargar, A. et al. Leveraging microbial biosynthetic pathways for the generation of “drop-in” biofuels. Curr. Opin. Biotechnol. 45 , 156–163 (2017).

Smith, K. M., Cho, K.-M. & Liao, J. C. Engineering Corynebacterium glutamicum for isobutanol production. Appl. Microbiol. Biotechnol. 87 , 1045–1055 (2010).

Lin, P. P. et al. Consolidated bioprocessing of cellulose to isobutanol using Clostridium thermocellum . Metab. Eng. 31 , 44–52 (2015).

Yan, Q. & Pfleger, B. F. Revisiting metabolic engineering strategies for microbial synthesis of oleochemicals. Metab. Eng. 58 , 35–46 (2020).

Hanko, E. K. R. et al. Engineering β-oxidation in Yarrowia lipolytica for methyl ketone production. Metab. Eng. 48 , 52–62 (2018).

Kim, H. M., Chae, T. U., Choi, S. Y., Kim, W. J. & Lee, S. Y. Engineering of an oleaginous bacterium for the production of fatty acids and fuels. Nat. Chem. Biol. 15 , 721–729 (2019).

Sasaki, Y. et al. Engineering Corynebacterium glutamicum to produce the biogasoline isopentenol from plant biomass hydrolysates. Biotechnol. Biofuels 12 , 41 (2019).

Yaegashi, J. et al. Rhodosporidium toruloides : a new platform organism for conversion of lignocellulose into terpene biofuels and bioproducts. Biotechnol. Biofuels 10 , 241 (2017).

Sundstrom, E. et al. Demonstrating a separation-free process coupling ionic liquid pretreatment, saccharification, and fermentation with Rhodosporidium toruloides to produce advanced biofuels. Green Chem. 20 , 2870–2879 (2018).

Zhuang, X. et al. Monoterpene production by the carotenogenic yeast Rhodosporidium toruloides . Microb. Cell Fact. 18 , 54 (2019).

Miao, R., Xie, H. & Lindblad, P. Enhancement of photosynthetic isobutanol production in engineered cells of Synechocystis PCC 6803. Biotechnol. Biofuels 11 , 267 (2018).

Nguyen, A. D., Kim, D. & Lee, E. Y. Unlocking the biosynthesis of sesquiterpenoids from methane via the methylerythritol phosphate pathway in methanotrophic bacteria, using α-humulene as a model compound. Metab. Eng. 61 , 69–78 (2020).

Krieg, T., Sydow, A., Faust, S., Huth, I. & Holtmann, D. CO 2 to terpenes: autotrophic and electroautotrophic α-humulene production with Cupriavidus necato r. Angew. Chem. Int. Ed. 57 , 1879–1882 (2018).

Grenz, S. et al. Exploiting Hydrogenophaga pseudoflava for aerobic syngas-based production of chemicals. Metab. Eng. 55 , 220–230 (2019).

Mukhopadhyay, A. Tolerance engineering in bacteria for the production of advanced biofuels and chemicals. Trends Microbiol. 23 , 498–508 (2015).

Niu, F.-X., He, X., Wu, Y.-Q. & Liu, J.-Z. Enhancing production of pinene in Escherichia coli by using a combination of tolerance, evolution, and modular co-culture engineering. Front. Microbiol. 9 , 1623 (2018).

Mendez-Perez, D. et al. Production of jet fuel precursor monoterpenoids from engineered Escherichia coli . Biotechnol. Bioeng. 114 , 1703–1712 (2017).

Chong, H. et al. Enhancing E. coli isobutanol tolerance through engineering its global transcription factor cAMP receptor protein (CRP). Biotechnol. Bioeng. 111 , 700–708 (2014).

Mukhopadhyay, A., Hillson, N. J. & Keasling, J. D. in Microbial Stress Tolerance for Biofuels (ed. Liu, Z. L.) 209–238 (Springer, 2012).

Park, J. I. et al. A thermophilic ionic liquid-tolerant cellulase cocktail for the production of cellulosic biofuels. PLoS ONE 7 , e37010 (2012).

Yu, C., Simmons, B. A., Singer, S. W., Thelen, M. P. & VanderGheynst, J. S. Ionic liquid-tolerant microorganisms and microbial communities for lignocellulose conversion to bioproducts. Appl. Microbiol. Biotechnol. 100 , 10237–10249 (2016).

Thorwall, S., Schwartz, C., Chartron, J. W. & Wheeldon, I. Stress-tolerant non-conventional microbes enable next-generation chemical biosynthesis. Nat. Chem. Biol. 16 , 113–121 (2020).

Sandoval, N. R. & Papoutsakis, E. T. Engineering membrane and cell-wall programs for tolerance to toxic chemicals: beyond solo genes. Curr. Opin. Microbiol. 33 , 56–66 (2016).

Basler, G., Thompson, M., Tullman-Ercek, D. & Keasling, J. A Pseudomonas putida efflux pump acts on short-chain alcohols. Biotechnol. Biofuels 11 , 136 (2018).

Chen, B., Ling, H. & Chang, M. W. Transporter engineering for improved tolerance against alkane biofuels in Saccharomyces cerevisiae . Biotechnol. Biofuels 6 , 21 (2013).

Chen, Y. et al. Reverse engineering of fatty acid-tolerant Escherichia coli identifies design strategies for robust microbial cell factories. Metab. Eng. 61 , 120–130 (2020).

Otoupal, P. B. & Chatterjee, A. CRISPR gene perturbations provide insights for improving bacterial biofuel tolerance. Front. Bioeng. Biotechnol. 6 , 122 (2018).

Kurgan, G. et al. Bioprospecting of native efflux pumps to enhance furfural tolerance in ethanologenic Escherichia coli . Appl. Environ. Microbiol. 85 , e02985-18 (2019).

Song, H.-S. et al. Increase in furfural tolerance by combinatorial overexpression of NAD salvage pathway enzymes in engineered isobutanol-producing E. coli . Bioresour. Technol. 245 , 1430–1435 (2017).

Frederix, M. et al. Development of an E. coli strain for one-pot biofuel production from ionic liquid pretreated cellulose and switchgrass. Green Chem. 18 , 4189–4197 (2016).

Eng, T. et al. Restoration of biofuel production levels and increased tolerance under ionic liquid stress is enabled by a mutation in the essential Escherichia coli gene cydC. Microb. Cell Fact. 17 , 159 (2018).

Ruegg, T. L. et al. Jungle Express is a versatile repressor system for tight transcriptional control. Nat. Commun. 9 , 3617 (2018).

Wang, S. et al. NaCl enhances Escherichia coli growth and isoprenol production in the presence of imidazolium-based ionic liquids. Bioresour. Technol. Rep. 6 , 1–5 (2019).

Nikel, P. I. & de Lorenzo, V. Pseudomonas putida as a functional chassis for industrial biocatalysis: from native biochemistry to trans-metabolism. Metab. Eng. 50 , 142–155 (2018).

Yang, S. et al. Zymomonas mobilis as a model system for production of biofuels and biochemicals. Microb. Biotechnol. 9 , 699–717 (2016).

Stella, R. G., Wiechert, J., Noack, S. & Frunzke, J. Evolutionary engineering of Corynebacterium glutamicum . Biotechnol. J. 14 , e1800444 (2019).

Castro, A. R., Rocha, I., Alves, M. M. & Pereira, M. A. Rhodococcus opacus B4: a promising bacterium for production of biofuels and biobased chemicals. AMB. Express 6 , 35 (2016).

Thompson, M. G. et al. Fatty acid and alcohol metabolism in Pseudomonas putida : functional analysis using random barcode transposon sequencing. Appl. Environ. Microbiol. 86 , e01665-20 (2020).

Sandberg, T. E., Salazar, M. J., Weng, L. L., Palsson B. O. & Feist, A. M. The emergence of adaptive laboratory evolution as an efficient tool for biological discovery and industrial biotechnology. Metab. Eng. 56 , 1–16 (2019).

Lim, H. G. et al. Generation of ionic liquid tolerant Pseudomonas putida KT2440 strains via adaptive laboratory evolution. Green Chem. 22 , 5677–5690 (2020).

Price, M. N. et al. Mutant phenotypes for thousands of bacterial genes of unknown function. Nature 557 , 503–509 (2018). A study showing the power of the RB-TnSeq approach in elucidating gene function in a vast number of microorganisms, which is not only valuable for identifying gene targets for strain engineering but is also broadly useful for improving GSMMs .

Li, W.-J. et al. Unraveling 1,4-butanediol metabolism in Pseudomonas putida KT2440. Front. Microbiol. 11 , 382 (2020).

Phaneuf, P. V., Gosting, D., Palsson, B. O. & Feist, A. M. ALEdb 1.0: a database of mutations from adaptive laboratory evolution experimentation. Nucleic Acids Res. 47 , D1164–D1171 (2019).

Article   PubMed   Google Scholar  

Zhang, F., Carothers, J. M. & Keasling, J. D. Design of a dynamic sensor-regulator system for production of chemicals and fuels derived from fatty acids. Nat. Biotechnol. 30 , 354–359 (2012).

DeLoache, W. C., Russ, Z. N. & Dueber, J. E. Towards repurposing the yeast peroxisome for compartmentalizing heterologous metabolic pathways. Nat. Commun. 7 , 11152 (2016).

Hu, T. et al. Engineering chimeric diterpene synthases and isoprenoid biosynthetic pathways enables high-level production of miltiradiene in yeast. Metab. Eng. 60 , 87–96 (2020).

McCloskey, D. et al. Adaptation to the coupling of glycolysis to toxic methylglyoxal production in tpiA deletion strains of Escherichia coli requires synchronized and counterintuitive genetic changes. Metab. Eng. 48 , 82–93 (2018).

Gubellini, F. et al. Physiological response to membrane protein overexpression in E. coli . Mol. Cell. Proteom. 10 , M111.007930 (2011).

Baumgarten, T., Ytterberg, A. J., Zubarev, R. A. & de Gier, J.-W. Optimizing recombinant protein production in the Escherichia coli periplasm alleviates stress. Appl. Environ. Microbiol. 84 , e00270-18 (2018).

Boyarskiy, S., Davis López, S., Kong, N. & Tullman-Ercek, D. Transcriptional feedback regulation of efflux protein expression for increased tolerance to and production of n -butanol. Metab. Eng. 33 , 130–137 (2016).

Henard, C. A., Freed, E. F. & Guarnieri, M. T. Phosphoketolase pathway engineering for carbon-efficient biocatalysis. Curr. Opin. Biotechnol. 36 , 183–188 (2015).

Lin, P. P. et al. Construction and evolution of an Escherichia coli strain relying on nonoxidative glycolysis for sugar catabolism. Proc. Natl Acad. Sci. USA 115 , 3538–3546 (2018). A very detailed and extensive demonstration of engineering non-oxidative glycolysis .

Bogorad, I. W., Lin, T.-S. & Liao, J. C. Synthetic non-oxidative glycolysis enables complete carbon conservation. Nature 502 , 693–697 (2013).

Fleige, C., Kroll, J. & Steinbüchel, A. Establishment of an alternative phosphoketolase-dependent pathway for fructose catabolism in Ralstonia eutropha H16. Appl. Microbiol. Biotechnol. 91 , 769–776 (2011).

Chubukov, V., Mukhopadhyay, A., Petzold, C. J., Keasling, J. D. & Martín, H. G. Synthetic and systems biology for microbial production of commodity chemicals. NPJ Syst. Biol. Appl. 2 , 16009 (2016).

Tian, T., Kang, J. W., Kang, A. & Lee, T. S. Redirecting metabolic flux via combinatorial multiplex CRISPRi-mediated repression for isopentenol production in Escherichia coli . ACS Synth. Biol. 8 , 391–402 (2019).

George, K. W. et al. Metabolic engineering for the high-yield production of isoprenoid-based C 5 alcohols in E. coli . Sci. Rep. 5 , 11128 (2015).

Strucko, T. et al. Laboratory evolution reveals regulatory and metabolic trade-offs of glycerol utilization in Saccharomyces cerevisiae . Metab. Eng. 47 , 73–82 (2018).

Caspeta, L. et al. Biofuels. Altered sterol composition renders yeast thermotolerant. Science 346 , 75–78 (2014).

Mohamed, E. T. et al. Generation of a platform strain for ionic liquid tolerance using adaptive laboratory evolution. Microb. Cell Fact. 16 , 204 (2017).

Lennen, R. M. et al. Adaptive laboratory evolution reveals general and specific chemical tolerance mechanisms and enhances biochemical production. Preprint at bioRxiv https://doi.org/10.1101/634105 (2019).

Shepelin, D., Hansen, A. S. L., Lennen, R., Luo, H. & Herrgård, M. J. Selecting the best: evolutionary engineering of chemical production in microbes. Genes 9 , 249 (2018). An excellent review on growth coupling .

Article   PubMed Central   CAS   Google Scholar  

Fong, S. S. et al. In silico design and adaptive evolution of Escherichia coli for production of lactic acid. Biotechnol. Bioeng. 91 , 643–648 (2005).

Zhang, X., Jantama, K., Moore, J. C., Shanmugam, K. T. & Ingram, L. O. Production of L-alanine by metabolically engineered Escherichia coli . Appl. Microbiol. Biotechnol. 77 , 355–366 (2007).

Shen, C. R. et al. Driving forces enable high-titer anaerobic 1-butanol synthesis in Escherichia coli . Appl. Environ. Microbiol. 77 , 2905–2915 (2011).

Machado, H. B., Dekishima, Y., Luo, H., Lan, E. I. & Liao, J. C. A selection platform for carbon chain elongation using the CoA-dependent pathway to produce linear higher alcohols. Metab. Eng. 14 , 504–511 (2012).

Reyes, L. H., Gomez, J. M. & Kao, K. C. Improving carotenoids production in yeast via adaptive laboratory evolution. Metab. Eng. 21 , 26–33 (2014).

Tai, Y.-S. et al. Engineering nonphosphorylative metabolism to generate lignocellulose-derived products. Nat. Chem. Biol. 12 , 247–253 (2016).

Hädicke, O. & Klamt, S. Computing complex metabolic intervention strategies using constrained minimal cut sets. Metab. Eng. 13 , 204–213 (2011).

Harder, B.-J., Bettenbrock, K. & Klamt, S. Model-based metabolic engineering enables high yield itaconic acid production by Escherichia coli . Metab. Eng. 38 , 29–37 (2016).

von Kamp, A. & Klamt, S. Growth-coupled overproduction is feasible for almost all metabolites in five major production organisms. Nat. Commun. 8 , 15956 (2017).

Banerjee, D. et al. Genome-scale metabolic rewiring improves titers rates and yields of the non-native product indigoidine at scale. Nat. Commun. 11 , 5385 (2020).

King, Z. A. et al. BiGG models: a platform for integrating, standardizing and sharing genome-scale models. Nucleic Acids Res. 44 , D515–D522 (2016).

Landon, S., Rees-Garbutt, J., Marucci, L. & Grierson, C. Genome-driven cell engineering review: in vivo and in silico metabolic and genome engineering. Essays Biochem. 63 , 267–284 (2019).

Yim, H. et al. Metabolic engineering of Escherichia coli for direct production of 1,4-butanediol. Nat. Chem. Biol. 7 , 445–452 (2011).

Ng, C. Y., Jung, M.-Y., Lee, J. & Oh, M.-K. Production of 2,3-butanediol in Saccharomyces cerevisiae by in silico aided metabolic engineering. Microb. Cell Fact. 11 , 68 (2012).

Izallalen, M. et al. Geobacter sulfurreducens strain engineered for increased rates of respiration. Metab. Eng. 10 , 267–275 (2008).

Fowler, Z. L., Gikandi, W. W. & Koffas, M. A. G. Increased malonyl coenzyme A biosynthesis by tuning the Escherichia coli metabolic network and its application to flavanone production. Appl. Environ. Microbiol. 75 , 5831–5839 (2009).

Chemler, J. A., Fowler, Z. L., McHugh, K. P. & Koffas, M. A. G. Improving NADPH availability for natural product biosynthesis in Escherichia coli by metabolic engineering. Metab. Eng. 12 , 96–104 (2010).

Asadollahi, M. A. et al. Enhancing sesquiterpene production in Saccharomyces cerevisiae through in silico driven metabolic engineering. Metab. Eng. 11 , 328–334 (2009).

Brochado, A. R. et al. Improved vanillin production in baker’s yeast through in silico design. Microb. Cell Fact. 9 , 84 (2010).

Choi, H. S., Lee, S. Y., Kim, T. Y. & Woo, H. M. In silico identification of gene amplification targets for improvement of lycopene production. Appl. Environ. Microbiol. 76 , 3097–3105 (2010).

Becker, J., Zelder, O., Häfner, S., Schröder, H. & Wittmann, C. From zero to hero–design-based systems metabolic engineering of Corynebacterium glutamicum for L-lysine production. Metab. Eng. 13 , 159–168 (2011).

Li, S., Huang, D., Li, Y., Wen, J. & Jia, X. Rational improvement of the engineered isobutanol-producing Bacillus subtilis by elementary mode analysis. Microb. Cell Fact. 11 , 101 (2012).

Xu, P., Ranganathan, S., Fowler, Z. L., Maranas, C. D. & Koffas, M. A. G. Genome-scale metabolic network modeling results in minimal interventions that cooperatively force carbon flux towards malonyl-CoA. Metab. Eng. 13 , 578–587 (2011).

Ranganathan, S. et al. An integrated computational and experimental study for overproducing fatty acids in Escherichia coli . Metab. Eng. 14 , 687–704 (2012). An illustrative example of using GSMMs to guide bioengineering .

Otero, J. M. et al. Industrial systems biology of Saccharomyces cerevisiae enables novel succinic acid cell factory. PLoS ONE 8 , e54144 (2013).

Ghosh, A. et al. 13 C metabolic flux analysis for systematic metabolic engineering of S. cerevisiae for overproduction of fatty acids. Front. Bioeng. Biotechnol. 4 , 76 (2016).

d’Espaux, L. et al. Engineering high-level production of fatty alcohols by Saccharomyces cerevisiae from lignocellulosic feedstocks. Metab. Eng. 42 , 115–125 (2017).

Lawson, C. E. et al. Machine learning for metabolic engineering: a review. Metab. Eng . 63 , 34–60. A good introduction to machine learning for the metabolic engineer .

Alonso-Gutierrez, J. et al. Principal component analysis of proteomics (PCAP) as a tool to direct metabolic engineering. Metab. Eng. 28 , 123–133 (2015).

Ohtake, T. et al. Metabolomics-driven approach to solving a CoA imbalance for improved 1-butanol production in Escherichia coli . Metab. Eng. 41 , 135–143 (2017).

Xu, P., Rizzoni, E. A., Sul, S.-Y. & Stephanopoulos, G. Improving metabolic pathway efficiency by statistical model-based multivariate regulatory metabolic engineering. ACS Synth. Biol. 6 , 148–158 (2017).

Zhou, Y. et al. MiYA, an efficient machine-learning workflow in conjunction with the YeastFab assembly strategy for combinatorial optimization of heterologous metabolic pathways in Saccharomyces cerevisiae . Metab. Eng. 47 , 294–302 (2018).

Costello, Z. & Martin, H. G. A machine learning approach to predict metabolic pathway dynamics from time-series multiomics data. NPJ Syst. Biol. Appl. 4 , 19 (2018).

Opgenorth, P. et al. Lessons from two design-build-test-learn cycles of dodecanol production in Escherichia coli aided by machine learning. ACS Synth. Biol. 8 , 1337–1351 (2019).

Jervis, A. J. et al. Machine learning of designed translational control allows predictive pathway optimization in Escherichia coli . ACS Synth. Biol. 8 , 127–136 (2019). An excellent application of machine learning to transcriptional control .

HamediRad, M. et al. Towards a fully automated algorithm driven platform for biosystems design. Nat. Commun. 10 , 5150 (2019). A fantastic example of the promise of combining machine learning, synthetic biology and automation .

Zhang, J. et al. Combining mechanistic and machine learning models for predictive engineering and optimization of tryptophan metabolism. Nat. Commun. 11 , 880 (2020).

CAS   Google Scholar  

Radivojević, T., Costello, Z., Workman, K. & Garcia Martin, H. A machine learning automated recommendation tool for synthetic biology. Nat. Commun. 11 , 4879 (2020).

Yadav, V. G., De Mey, M., Lim, C. G., Ajikumar, P. K. & Stephanopoulos, G. The future of metabolic engineering and synthetic biology: towards a systematic practice. Metab. Eng. 14 , 233–241 (2012).

Carbonell, P., Radivojevic, T. & García Martín, H. Opportunities at the intersection of synthetic biology, machine learning, and automation. ACS Synth. Biol. 8 , 1474–1477 (2019).

Dietrich, J. A., McKee, A. E. & Keasling, J. D. High-throughput metabolic engineering: advances in small-molecule screening and selection. Annu. Rev. Biochem. 79 , 563–590 (2010).

Crater, J. S. & Lievense, J. C. Scale-up of industrial microbial processes. FEMS Microbiol. Lett. 365 , fny138 (2018).

Article   CAS   PubMed Central   Google Scholar  

Davis, R. et al. Process design and economics for the conversion of lignocellulosic biomass to hydrocarbons: dilute-acid and enzymatic deconstruction of biomass to sugars and biological conversion of sugars to hydrocarbons (National Renewable Energy Laboratory, 2013).

Cruz Bournazou, M. N. et al. Online optimal experimental re-design in robotic parallel fed-batch cultivation facilities. Biotechnol. Bioeng. 114 , 610–619 (2017).

Tai, M., Ly, A., Leung, I. & Nayar, G. Efficient high-throughput biological process characterization: definitive screening design with the Ambr250 bioreactor system. Biotechnol. Prog. 31 , 1388–1395 (2015).

Wong, B. G., Mancuso, C. P., Kiriakov, S., Bashor, C. J. & Khalil, A. S. Precise, automated control of conditions for high-throughput growth of yeast and bacteria with eVOLVER. Nat. Biotechnol. 36 , 614–623 (2018).

Haringa, C. et al. Computational fluid dynamics simulation of an industrial P. chrysogenum fermentation with a coupled 9-pool metabolic model: towards rational scale-down and design optimization. Chem. Eng. Sci. 175 , 12–24 (2017).

Rugbjerg, P. & Sommer, M. O. A. Overcoming genetic heterogeneity in industrial fermentations. Nat. Biotechnol. 37 , 869–876 (2019).

Wehrs, M. et al. Investigation of Bar-seq as a method to study population dynamics of Saccharomyces cerevisiae deletion library during bioreactor cultivation. Microb. Cell Fact. 19 , 167 (2020).

Wang, O. & Coates, J. D. Biotechnological applications of microbial (per)chlorate reduction. Microorganisms 5 , 76 (2017).

Shaw, A. J. et al. Metabolic engineering of microbial competitive advantage for industrial fermentation processes. Science 353 , 583–586 (2016).

Dahl, R. H. et al. Engineering dynamic pathway regulation using stress-response promoters. Nat. Biotechnol. 31 , 1039–1046 (2013).

Dafoe, J. T. & Daugulis, A. J. In situ product removal in fermentation systems: improved process performance and rational extractant selection. Biotechnol. Lett. 36 , 443–460 (2014).

Xue, C. et al. Integrated butanol recovery for an advanced biofuel: current state and prospects. Appl. Microbiol. Biotechnol. 98 , 3463–3474 (2014).

Gaspar, D. Top ten blendstocks for turbocharged gasoline engines: bioblendstocks with potential to deliver the for highest engine efficiency (Pacific Northwest National Laboratory, 2019). A systematic analysis and down-election of petrol bioblendstock candidates based on fuel properties and engine performance .

Monroe, E. et al. Discovery of novel octane hyperboosting phenomenon in prenol biofuel/gasoline blends. Fuel 239 , 1143–1148 (2019).

Ignea, C. et al. Synthesis of 11-carbon terpenoids in yeast using protein and metabolic engineering. Nat. Chem. Biol. 14 , 1090–1098 (2018).

Huccetogullari, D., Luo, Z. W. & Lee, S. Y. Metabolic engineering of microorganisms for production of aromatic compounds. Microb. Cell Fact. 18 , 41 (2019).

Das, D. D., St. John, P. C., McEnally, C. S., Kim, S. & Pfefferle, L. D. Measuring and predicting sooting tendencies of oxygenates, alkanes, alkenes, cycloalkanes, and aromatics on a unified scale. Combust. Flame 190 , 349–364 (2018).

Huo, X. et al. Tailoring diesel bioblendstock from integrated catalytic upgrading of carboxylic acids: a “fuel property first” approach. Green Chem. 21 , 5813–5827 (2019).

Yang, M. et al. Accumulation of high-value bioproducts in planta can improve the economics of advanced biofuels. Proc. Natl Acad. Sci. USA 117 , 8639–8648 (2020).

Lin, C.-Y. & Eudes, A. Strategies for the production of biochemicals in bioenergy crops. Biotechnol. Biofuels 13 , 71 (2020).

Blombach, B. et al. Corynebacterium glutamicum tailored for efficient isobutanol production. Appl. Environ. Microbiol. 77 , 3300–3310 (2011).

Higashide, W., Li, Y., Yang, Y. & Liao, J. C. Metabolic engineering of Clostridium cellulolyticum for production of isobutanol from cellulose. Appl. Environ. Microbiol. 77 , 2727–2733 (2011).

Sawyer, R. F. Trends in auto emissions and gasoline composition. Environ. Health Perspect. 101 (Suppl. 6), 5–12 (1993).

Ghosh, P., Hickey, K. J. & Jaffe, S. B. Development of a detailed gasoline composition-based octane model. Ind. Eng. Chem. Res. 45 , 337–345 (2006).

Ghosh, P. & Jaffe, S. B. Detailed composition-based model for predicting the cetane number of diesel fuels. Ind. Eng. Chem. Res. 45 , 346–351 (2006).

ASTM International. ASTM D1655 — 20e1: standard specification for aviation turbine fuels (ASTM, 2020).

Download references

Acknowledgements

The authors thank C. Scown (Lawrence Berkeley National Laboratory) for helpful discussions on life cycle and technoeconomic analyses of biofuel production. This work was performed as part of the US Department of Energy (DOE) Joint BioEnergy Institute ( https://www.jbei.org ) supported by the DOE, Office of Science, Office of Biological and Environmental Research, and by the DOE, Energy Efficiency and Renewable Energy, Bioenergy Technologies Office, and as part of the Co-Optimization of Fuels & Engines project sponsored by the DOE, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office and Vehicle Technologies Office, under contract DEAC02-05CH11231 between the DOE and Lawrence Berkeley National Laboratory. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof. Neither the US Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of the manuscript, or allow others to do so, for US Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

Author information

Authors and affiliations.

Joint BioEnergy Institute, Emeryville, CA, USA

Jay Keasling, Hector Garcia Martin, Taek Soon Lee, Aindrila Mukhopadhyay & Steven W. Singer

Department of Chemical & Biomolecular Engineering, University of California, Berkeley, Berkeley, CA, USA

Jay Keasling

Department of Bioengineering, University of California, Berkeley, Berkeley, CA, USA

Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

Jay Keasling, Hector Garcia Martin, Taek Soon Lee, Aindrila Mukhopadhyay, Steven W. Singer & Eric Sundstrom

Center for Biosustainability, Danish Technical University, Lyngby, Denmark

Center for Synthetic Biochemistry, Institute for Synthetic Biology, Shenzhen Institute of Advanced Technology, Shenzhen, China

DOE Agile BioFoundry, Emeryville, CA, USA

Hector Garcia Martin

BCAM,Basque Center for Applied Mathematics, Bilbao, Spain

Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

Hector Garcia Martin & Aindrila Mukhopadhyay

Advanced Biofuels and Bioproducts Process Development Unit, Emeryville, CA, USA

Eric Sundstrom

You can also search for this author in PubMed   Google Scholar

Contributions

The authors contributed equally to all aspects of the article.

Corresponding author

Correspondence to Jay Keasling .

Ethics declarations

Competing interests.

J.K. has a financial interest in Amyris, Lygos, Demetrix, Napigen, Apertor Pharmaceuticals, Maple Bio, Ansa Biotechnologies, Berkeley Yeast and Zero Acre Farms. The other authors declare no competing interests.

Additional information

Peer review information.

Nature Reviews Microbiology thanks the anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

A standard measure of an engine or aviation fuel capability against compression.

An indicator of the combustion speed of diesel fuel and compression needed for ignition.

(ILs). A highly efficient set of reagents for the depolymerization and deconstruction of a range of feedstocks.

One-carbon microbial substrates, including CO 2 , CH 4 , CO, HCO 2 − and CH 3 OH.

A mixture of CO, CO 2 and H 2 .

Single fuel components that are blended with additional components to produce a finished fuel.

Modelling approach that uses a polynomial of up to grade 2 to predict the response.

Applied statistics techniques that deal with planning, conducting, analysing and interpreting controlled tests to evaluate the factors that control the experimental output under study.

Modelling approach that takes the input of various different models and has them ‘vote’ for a particular prediction.

The exhaust gas stream exiting a bioreactor.

Development of the genetic tools necessary to allow metabolic engineering of a previously unengineered microorganism.

The degree to which a fuel mixture generates black carbon soot when combusted.

Petrol containing 10% ethanol by volume.

Fuel viscosity at low temperature; poor cold flow can lead to gelling and compromise engine operability in cold weather conditions.

Rights and permissions

Reprints and permissions

About this article

Cite this article.

Keasling, J., Garcia Martin, H., Lee, T.S. et al. Microbial production of advanced biofuels. Nat Rev Microbiol 19 , 701–715 (2021). https://doi.org/10.1038/s41579-021-00577-w

Download citation

Accepted : 13 May 2021

Published : 25 June 2021

Issue Date : November 2021

DOI : https://doi.org/10.1038/s41579-021-00577-w

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

This article is cited by

Enhanced bioethanol production by evolved escherichia coli lge2-h in a microbial electrolysis cell system.

• Dongdong Chang
• Zhisheng Yu

Bioresources and Bioprocessing (2024)

Increased CO2 fixation enables high carbon-yield production of 3-hydroxypropionic acid in yeast

Nature Communications (2024)

RhizoNet segments plant roots to assess biomass and growth for enabling self-driving labs

• Zineb Sordo
• Peter Andeer
• Daniela Ushizima

Scientific Reports (2024)

Sustainable power generation from sewage with engineered microorganisms as electrocatalysts

• Zhengyu Bai

Nature Sustainability (2024)

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Sign up for the Nature Briefing: Translational Research newsletter — top stories in biotechnology, drug discovery and pharma.

• Forum article
• Open access
• Published: 18 May 2017

Biofuel research: perceptions of power and transition

• Lena Partzsch 1  

Energy, Sustainability and Society volume  7 , Article number:  14 ( 2017 ) Cite this article

14k Accesses

8 Citations

1 Altmetric

Metrics details

Whether biofuels represent a sustainable innovation, a creative alternative, or a gold rush, very much depends on our perception of power and change with regard to sustainability. This article provides an overview of existing understandings of power in the research on biofuels, including positive perceptions that often lead to more optimistic evaluations of biofuels. It exposes the diversity with which one can understand power through three ideal type concepts: “power with,” “power to,” and “power over”. Integrating these concepts in one power framework allows for examining how the three dimensions interrelate with each other and developing the contours of a power lens on biofuel governance and research. With the 2007–2008 food price crisis, critics re-politicized the governance of biofuels. Several farmer associations have completely turned against biofuels. The article argues that this rejection of biofuels is due to a limited perception of power as a coercion and manipulation (power over). While the current governance of biofuels basically reproduces systems and positions, we should start to more seriously and intensively ask questions of where, when, and how the governance of biofuels may also allow for “green” resistance (power to) and collective empowerment (power with).

Introduction

Whether biofuels represent a sustainable innovation, a creative alternative or a gold rush [ 1 ], very much depends on our perception of power and change with regard to sustainability. This leads to the challenge of how to conceptualize these understandings. I gather diverse perceptions of power and illustrate them for biofuel research. The aim is to initiate a broader, more comprehensive debate across ontological and epistemological differences in this field of research. To begin the discussion, I introduce key components of the debate by identifying different perceptions of power that are common to research on biofuels along three ideal type conceptions:

Power with means collective empowerment through convincing and learning with and from each other. It refers to processes of developing shared values, finding common ground, and generating collective strengths [ 2 ]. Based on this understanding of power, biofuels can potentially be a sustainable innovation that serves the common good (climate protection, energy security, regional development, etc.) (e.g., [ 3 , 4 ]).

Power to corresponds to the ability of agents “to get things done” [ 5 ]. While Pitkin [ 6 ] defines power to as non-relational, Barnett and Duvall [ 7 ] define power to as tied to social relations of constitution that define who the actors are, along with their capacities and practices. Footnote 1 Scholars, who take a perspective of power to, may highlight the agency of producing biofuels as a creative alternative in hitherto fossil fuel-dependent societies (e.g., [ 8 , 9 ]).

Power over describes the direct and indirect ability of powerful actors, structures, and discourses to influence the actions and even the thoughts of others. It is based on power concepts by Dahl [ 10 ], Bachrach and Baratz [ 11 ], and Lukes [ 12 ], among others. I also discuss concepts of discursive power under this category (e.g., [ 13 , 14 ]), while I am aware that these concepts partly fall under the category of power to [ 7 ]. From a perspective of power over, biofuels can be seen as a gold rush: While everybody expected sudden wealth in this new field, there are very few winners and many losers (e.g. [ 15 , 16 ]).

I chose this tripartite approach as a framework for my article, because it is most comprehensive and makes an extension of the power discussion on biofuels possible. At the same time, the framework allows for the discussion of the well-known grouping of the four “faces of power” under the category of power over [ 17 , 18 ]. I will argue that in the research on biofuels, the understandings of power as power with and power to tend to prevail, even when they are not made explicit. This means that scholars have overemphasized the potential of biofuels as a creative alternative to fossil fuels and sustainable innovation for rural development. Concepts of power over have only more recently been applied, specifically since research has started to explicitly issue power. This has, in particular, been used to explain why any process of governing biofuels (biofuel governance) did not lead to urgent sustainability transitions, and why the biofuel boom should rather be seen as a gold rush. Scholars have demonstrated that the development of biofuels markets benefitted large companies and conglomerates [ 19 ]. Critical and post-structuralist perspectives have helped to understand this development by exploring structures and discourses favoring them [ 20 ]. Scholars have used Foucault’s concepts to outline how scientific knowledge practices render the very essence of problems (and solutions) raised on the biofuel agenda [ 21 , 22 ].

This article involves first of all implicit and explicit understandings of power (how do biofuel researchers think and talk about power?). These understandings are expressed in empirical research, as I will demonstrate below, and they hence also allow for an illustration of the practice of biofuel governance (how is power exercised in and through biofuel governance?). This makes the article also relevant for political practice. We should understand, not only in theoretical but also in practical terms, how we effectuate or prevent changes towards a more sustainable supply of energy and transport fuel. As in analytical heuristics, it is not possible to offhand separate power with , power to , and power over in empirical research. These categories shine multiple lights on different aspects of the same empirical phenomena. In practice, these forms of power exercise are mostly interrelated. My less concern is to weigh and compare the pros and cons of each perspective, but rather to outline an agenda for a multidimensional analysis of all three mechanisms of power and their interrelations.

In order to get the full picture of how change happens, we should understand how different perspectives add on to each other (besides overlaps and contradictions). To do this, I will begin by describing each perspective in itself. Based on a survey on biofuel research, I will give references for each perspective. These references are only illustrative. Then, I will exemplify the interrelations between each of these perspectives with respect to biofuel research. I explain how power imbalances can affect processes of power with and power to . Again, scholars have demonstrated how large conglomerates have manipulated biofuel governance in their favor, and why therefore the biofuels boom should be considered as a gold rush. However, I argue that interrelations may also work the other way around, and this is particularly relevant to the main argument of this article. Biofuels as a creative alternative and a sustainable innovation may also provoke changes in existing relations of power over and contribute to address asymmetries and inequalities in agrifood and transport systems. We need a multidimensional power approach to explore these interrelations.

Biofuel: sustainable innovation (power with)

Research on biofuel governance and other studies in the field of sustainability are most often based on a positive perception of power in the sense of power with . Power with is a term that refers to processes of developing shared values, finding common ground, and generating collective strengths [ 2 ]. This conception does not necessarily refer to the diffusion of already existing (predefined) norms. Rather, power with implies learning processes that allow actors to question self-perceptions and to actively build up a new awareness of individuals or groups [ 23 , 24 ]. In this vein, with regard to biofuels, scholars have assumed that collective empowerment and solidarity are possible and that biofuel technologies as a “sustainable innovation” can pave the way to post-carbon societies [ 25 , 26 ].

Power with is often linked to Arendt’s definition of power [ 27 ]. Footnote 2 According to Arendt, power always refers to a group or to a collective of individuals:

Power corresponds to the human ability not just to act but to act in concert. Power is never the property of an individual; it belongs to a group and remains in existence only so long as the group keeps together. When we say of somebody that he is ‘in power’ we actually refer to his being empowered by a certain number of people to act in their name ([ 28 ]: 44). Footnote 3

Research on environmental leadership (e.g., [ 29 ]) in pioneer countries, such as Germany and France in the biofuel sector [ 3 , 30 ], most obviously reflects such an understanding of power. Leaders or pioneers are empowered to act in the name of others from this perspective (while they dominate others from a perspective of power over , see below). In this sense, (Young [ 31 ]: 285) defines leadership in the interest of common welfare:

Leadership (…) refers to the actions of individuals who endeavor to solve or circumvent the collective action problems that plague the efforts of parties seeking to reap joint gains in processes of institutional bargaining.

Leaders and pioneers do not enforce their own interests against or over others; rather they seek “to reap joint gains” of environmentalism. Environmental leadership studies, based on such an understanding of power, usually follow the discourse of Ecological Modernization that highlights flexible and cost-efficient problem solving. Ecological modernization outlines a win-win storyline of environmental protection that benefits green (biofuel) business as much as the environment [ 32 , 33 ]. From this perspective, those who are neither leaders nor pioneers are considered free-riders or laggards , rather than subordinates. Non-leaders also benefit, at least in the long run, from power (with), since biofuels are expected to tackle common problems, such as climate change, enhance energy security, and to contribute to regional development [ 3 , 34 ]. Policies promoting biofuels are hence per se seen to be desirable since, from this perspective, they serve everybody’s interest.

Scholars have extensively analyzed the emergence, diffusion, efficiency, and effectiveness of policies promoting biofuels, with the (at least implicit) aim to foster their adoption and implementation [ 30 , 35 ]. In this context, policy learning and experiments have been gaining momentum [ 9 , 26 ]. Deliberative processes, including third-party certification schemes, were initiated and observed with the aim to introduce sustainable biofuel production schemes that would integrate those formerly excluded stakeholders with new technology; in everyday practice, every actor in the field would then become a winner [ 4 , 36 ].

Scholars who share this perspective of power as power with do not think in dichotomies such as winners - losers or good-bad . Instead, they understand power (or similar concepts, such as leadership) as serving the common good (climate protection, energy security, and sustainability). As there are no subordinates from this power perspective, no imperative follows to empower or to resist. The empowerment of non-leaders is not an issue because scholars assume that, in principle, they are also interested in developing sustainable innovations and that they likewise benefit from respective leadership efforts.

Biofuel: creative alternative and “green” resistance (power to)

While power with pertains to collective empowerment and solidarity, power to refers to single actors and separate groups, such as farmers, co-operatives, and individual processors who were initially key players in pioneering biofuel regions [ 19 ]. Accordingly, biofuels are often seen as an opportunity to empower green ideas and values. Pitkin [ 6 ] emphasizes how power can be non-relational, since an actor may have the power to accomplish something all by him- or herself. This understanding of power is related to the development of an individual identity; self-confidence and consciousness raising [ 23 ]. It is here where Nussbaum’s and Sen’s [ 37 ] capability approach comes in, which defines power as “a capability to act upon one’s environment” [ 38 ]. For example, an individual farmer can simply start to produce and use biomass-based fuels without any permission or interference from another actor, such as the petrol industry. However, constructivist research has demonstrated how every actor or group is defined through socially constituted relations that, at least indirectly, shape the actions of individuals [ 7 ]: only a farmer who receives knowledge about alternative technologies may effectively implement them.

Power to can be linked to Parsons’ definition of power as the ability “to get things done” [ 5 ]. It highlights a productive agency, especially in the cases where actors’ goals are opposed or resisted. Biofuel research by small farmers and rural communities is often based on this perception of power [ 9 , 39 ]. Scholars highlight the potential of biofuels for rural development by providing new markets for agricultural production. They assume that through the introduction of radically new technologies in niches, farmers are able to empower themselves in an attempt of an “agro-ecological revolution” [ 8 ]. They highlight the self-empowering agency of hitherto marginalized people to become “energy sheiks” [ 40 ], based on biomass production.

Scholars, who take a perspective of power to , focus on the productive agency of the biofuel sector. They are interested in the empowerment of alternative ideas and values which, in the case of biofuels, allow for transforming fossil fuel-dependent societies. These alternative agents criticize the practices or the authority of the dominant, carbon-intense system and refuse to reproduce their own positions in this system. Their non-conformism is perceived to serve the common good as they develop alternative technologies required by everyone in a world beyond petrol. From a perspective of power to and in difference to a perspective of power with , there are only a limited number of transformational agents: not everybody in the field is assumed to be a “winner” in the first place; there are only a few “energy sheiks”. However, scholars see an imperative to act based on normatively prior “green” values, for example, climate protection and sustainability (and everybody benefits from the realization of these values).

Biofuel: gold rush (power over)

Scholars who explicitly issue power in the context of biofuels usually perceive power as asymmetric. Biofuel governance is seen as a zero-sum game which produces winners and losers. From this perspective, powerful actors, structures, and discourses in the field of biofuel governance influence the actions and even the thoughts of others. In the following, I will illustrate this perspective, further differentiating the “four faces” of power over (see Table  1 ): visible , hidden , invisible , and unconscious power [ 2 , 41 ]. (the fourth dimension does not understand power as a zero-sum game and can also be added to power to , see the first footnote.)

In the first dimension, agents exercise visible power when they directly influence political decision-makers based on their material and ideational resources [ 42 ]. What is visible is not the power as such, but rather its physical means such as lobbying activities, party financing, and armed force. (Dahl [ 10 ]: 201) defines: “A has power over B to the extent that he can get B to do something that B would not otherwise do” (emphasis added). Any kind of state force implementing objectives of sustainability by top downregulation means exercising direct power. Non-state actors may also play a role in this game. Coase [ 43 ] explains this for business firms. Also when Pilgrim and Harvey [ 44 ] demonstrate how NGO lobbying significantly affected biofuel policy changes and sustainability regulation in the UK and in Europe, they assume that NGOs enforce their ideas against others in an arena of obviously competing demands.

The second dimension of hidden power refers to power not obviously opposed by anyone. Bachrach and Baratz [ 11 ] speak of “two faces of power” emphasizing that some issues never even make it onto the political agenda and are dismissed before observable negotiations start. For a long time, the EU issued biofuels only in the context of climate change, completely neglecting aspects of competing food demands and land use change in the Global South [ 45 , 46 ]. Scholars demonstrating such hidden aspects apply this second dimension of power over to analyze biofuel governance.

The traditional conception of structural (hidden) power in international relations aims to address the coercion resulting from the capital mobility of transnational corporations. Threats to shift investments abroad do not even need to be voiced in order to influence policies in their favor [ 42 , 47 ]. More recent studies point to the fact that businesses also exercise structural power by self-regulation and public-private partnerships; these types of governance allow business actors to actively set rules, for example, for the “sustainable” production of biofuels at the expense of state actors [ 42 , 48 ]. In addition, as public authorities have faced challenges in facilitating the implementation of their sustainability criteria outside their jurisdictions, the EU has started to use these private schemes to verify compliance with sustainability criteria in biofuel production outside its own territory [ 49 , 50 ]. As a result, following this perspective, power in the global political economy has been diffused, leaving biofuel conglomerates with considerable power over others [ 51 ].

Further, scholars are increasingly focusing on power relations linked to latent conflicts of interest. In the third dimension, invisible power comes to play as a result of norms and ideas [ 41 ]. Research analyzes discourses, communication practices, cultural values and institutions, which all work to shape relevant thoughts and actions [ 12 ]. With regard to biofuels, Munro [ 22 ] has shown how, in the United States, a powerful coalition of agricultural interests manipulated the governance of biofuels by linking it to public concerns about climate change and energy security. In consequence, corn biofuel received political support, tax reductions, and subsidies. Likewise, Puttkammer and Grethe [ 52 ] have found a coalition of biofuel advocates to dominate the public discourse in Germany, while scientists who doubted the efficiency of biofuels could not make their voice heard. The discourse only shifted with the 2007–2008 food price crisis when scholars demystified the “ethanol bubble” [ 53 ] and outlined potentially devastating implications for global poverty and food security. Experts, NGOs, and business actors who have challenged the sustainability of biofuels on many fronts began to be heard [ 20 , 22 ].

For the most part, these discourse scholars blame other scholars who apply a perspective of power with for neglecting and postponing important questions of social justice linked to biofuel production [ 21 , 54 ]. Win-win rhetoric is demonstrated to manifest global power asymmetries rather than to contribute to more ecology and fairness [ 22 , 53 ]. From this perspective, pioneers and leaders, whose role Young [ 31 ] and Bernard and Prieur [ 30 ], among others, consider to be positive, only serve dominant interests and prevent a more fundamental social transformation to sustainability. With reference to the International Political Economy, most scholars deny a simple confrontation of biofuel proponents (or pioneers) and opponents (or laggards). In this vein, Levidow [ 55 ] outlines how the EU can continue “its global plunder of resources” because it pursues global leadership for sustainable biofuels. Silva-Castaneda [ 56 ] demonstrates how, in Indonesia, some NGOs decided to participate in the Roundtable on Sustainable Palm Oil (RSPO), a certification process initiated by the WWF, among others. The local NGOs managed to include important clauses regarding indigenous and land rights in the RSPO standard. In practice, however, auditors rarely recognize as valid evidence the forms of proof put forward by local communities, and global conglomerates could even use the standards to increase their primacy vis-à-vis local farmers [ 56 ]. These examples reveal power over within multi-stakeholder processes.

Studies demonstrate that the expansion of biofuels in countries of the Global South was only possible through the partial neglect (simplification) of their cultural and ecological diversity [ 57 ]. Nygren [ 58 ] illustrates how leading retailers, in negotiation with environmental organizations, have guided consumers’ expectations of certified Southern forest products by building images of Southern community forest producers as authentic and exotic others . She concludes that certification as a market-based form of governance has only had a limited impact on altering the unequal relationship characteristic of global networks of production and consumption.

With reference to Foucault [ 13 ] and Bourdieu [ 59 ], we can capture links between knowledge, power, and politics in a fourth dimension of power over [ 17 ]. Critical and (post-) structuralist approaches understand power in a way that everything is socially constructed. Scholars analyze the normative impact on (supposed) losers, such as farmers in the Global South, as well as on (supposed) winners, such as major agribusiness actors. All actors work to mainly reproduce systems and positions [ 60 ]. With regard to biofuels, several studies have highlighted the central role of knowledge and framing [ 15 , 16 , 21 ]. Drawing on Foucault, Kuchler and Linnér [ 21 ] have analyzed the discursive practices of the three major international organizations focused on food and agriculture, energy, and climate with regard to biofuels over the last 20 years: the UN Food and Agriculture Organization (FAO), the International Energy Agency (IEA), and the Intergovernmental Panel on Climate Change (IPCC). They found that, in contrast to pro and contra accounts, the arguments of all three organizations reflected a policy consensus based on the mainstream notion of industrial agricultural production, promoting the intensification and expansion of rural production. The biofuel discourse has further constituted a concatenation of the three issues of agricultural production, energy security, and climate change mitigation. When the discourse shifted with the 2007–2008 food price crisis, all the three major organizations adapted to this shift [ 21 ]. Instead of exercising power over by manipulating discourses on biofuels according to specific pro or contra interests, the organizations were found to rather reproduce hegemonic discourses and their own positions.

The gold rush metaphor is used a lot to describe the situation of biofuels from a power over perspective [ 1 ]. Biofuel production, like gold mining, is unprofitable for most farmers, just like it was for diggers and mine owners. Both biofuel production and gold mining can in addition have very negative environmental effects. While, however, people are made to believe that everyone can become abundantly wealthy (“energy sheiks”), only some few investors make large fortunes. Applying discursive approaches of power over , we can argue that even such investors and major businesses are subject to and not only conscious manipulators of discourses of agricultural intensification and economic growth. The analysis of power over helps to understand why change to more sustainable transport and agricultural systems does not happen. However, as I argue in this article, it falls short on explaining when and why there also sometimes is disruptive change and empowerment.

Power to change: interrelations between power with, power to, and power over

While the perspectives of power with and power to (over-) emphasize the potential for change with regard to biofuels, scholars with understandings of power over often exaggerate their negative impacts. The tripartite framework allows for the combining of different analytical perspectives and to examine their interrelations. While the three categories are first of all analytical heuristics, they also stand for different mechanisms of the exercise of power (see Fig.  1 ). Power over affects what is considered a “sustainable innovation” and “creative alternative”. Research has demonstrated this. However, I argue that it is also possible the other way round: there are situations in which power with and power to can address power imbalances and prevent a situation in which there are only a few winners and many losers as a result of biofuel governance.

Agent-based power

As shown in Fig.  1 , besides considering material and ideational sources of power, we also need to consider different mechanisms of power (over/to/with), since they lead to different results of power (leading to a new distribution of sources in a circular process, see the arrow at the bottom of Fig.  1 ). Biofuels per se are neither a sustainable innovation, a creative alternative nor a gold rush. The three metaphors exemplify three different results of power: the exercise of power over leads to a gold rush situation. So, if scholars only ask for power over , they will always find winners and losers. By contrast, if we ask for the exercise of power to , we may find that biofuels are creative alternative. Finally, the exercise of power with can be exemplified by a case of finding an agreement on sustainability criteria of biofuel production. To demonstrate overlaps, especially, in terms of the results of power, I used dashed lines in Fig.  1 .

When, in the field of biofuels, scholars explicitly issue power, they generally use concepts of power over to explain why governance and research in this field have a blind spot for power asymmetries [ 49 , 53 ]. Biofuel opponents may have accomplished a shift in the biofuel discourse after the 2007–2008 food price crisis [ 20 , 22 ]. However, overriding power asymmetries have prevented a structural change in both the energy/transport and the agricultural sectors. The trend is now definitely towards large companies and conglomerates [ 49 , 50 ].

However, the fact that biofuels have caused no structural change and have disadvantaged rather than empowered small farmers in the Global South, does not mean that a structural change is impossible. What I want to argue in this article is that exercising consensual forms of power (power with) as well as self-empowerment and resistance (power to) can also eclipse and overcome power asymmetries (power over). Empirical research on deliberative processes suggests that communication and common action never happen among equals and that they are never free from any form of power over [ 36 , 61 ]. Hence, we need to understand power with as a form of exercising power, which is strategic (bargaining) as well as communicative (arguing). A crucial part of this process is the orientation of agents involved in processes of biofuel governance. If actors are open to changing their positions and developing shared understandings, transitions to sustainability can follow from dialogues [ 61 , 62 ].

Following this perspective, even if small farmers in the Global South have fewer capabilities compared to conglomerates from the EU and the United States, this does not mean that they have no possibility to act independently from them. For example, sugar is costly to establish, and thus is economically most efficient at large plantation scales. However, Jatropha can more readily be produced through outgrower schemes as it is less capital intensive [ 9 , 49 ]. While currently almost all bio-ethanol is produced from grain or sugarcane and therefore competes with food purposes, other efficient and economically viable technologies for ethanol production are available [ 63 ]. The production of perennial energy crops, such as grasses and trees, and crop residues, such as straw, are seen to require fewer inputs and less prime land [ 64 ].

Under specific conditions, empowerment is possible; processes of power with and power to can have a (positive) impact on unwanted relations of power over . For example, processes of stakeholder dialogue and certification demonstrate that an agreement beyond the lowest common denominator is possible. In addition, they can weaken the perceived legitimacy of powerful actors that are producing biofuels unsustainably. The critical discourse on biomass certification has issued consumers’ accountability for harmful social and environmental effects in countries of production [ 55 , 65 ]. When the legitimacy of unconditional import as well as of private certification schemes was put into question [ 50 ], transnational conglomerates lost ideational and material resources on which their power over others was based. In the agrifood sector, we can clearly see that certification has become a new normative obligation [ 66 ].

We can observe various kinds of empowerment and resistance related to biofuels. While Nygren [ 58 ] argues that certification schemes reproduce (inferior) positions of southern producers as authentic and exotic others, she does not completely deny that certification had a positive impact on altering asymmetries in global networks of production and consumption. Silva-Castaneda’s [ 56 ] study discloses new ways in which local communities can legally prove their land rights, for instance, by video documentation to replace missing formal documents or destructed land marks.

Scholars have described movements, such as Via Campesina, in terms of exercising power over and opposing transnational agriculture corporations [ 67 ]. In terms of reducing and overcoming power asymmetries, however, what is most striking is the fact that small farmers within this movement exercise power to by doing healthy and sustainable agriculture independently of the major agribusinesses to which, from a power over perspective, they would only be subordinated. At the same time, when producing organically, small farmers do not reproduce the system of industrial agricultural production (and their inferior positions within that system). So, their way of farming can be considered as a creative alternative and as a way of resistance. Moreover, within this movement of Via Campesina, despite widely different internal cultures, farmers also exercise power with by (re-) constituting a new shared peasant identity. From a perspective of power with, we can argue that, in the long run, everybody, even from outside this movement, may benefit and share norms and values developed here such as sustainability in farming. The movement delegitimizes the acquisition of land by established conglomerates (“land grabbing”), whose ideational sources of power shrink in consequence. The visible result is a new, more equal, and just distribution of (power) resources through land reforms.

Conclusions

This article should not only encourage a debate on power issues with regard to biofuels, but moreover, develop the debate more comprehensively. When political power has been analyzed in the context of biofuels, this has happened so far through using confrontational or structuralist and discursive approaches that are based on an understanding of power over . Respective scholars have accused other researchers of neglecting “real power concentrations” in the biofuels industries. Often quite rightly: biofuel research has neglected the limits of win-win for a very long time. Scholars have taken sides and normatively inflated their own pro biofuel position, while they have dispatched their adversaries as laggards with regard to the future of transport and agriculture. Of course, not every (supposedly) sustainable innovation is necessarily good in the sense that it is completely uncontroversial (even if there is no visible opposition as in the case of biofuels for a long time). In this context, the question of power essentially addresses the re-politicization of decisions perceived to be urgent and without alternative. With the 2007–2008’s shift in discourse, critics re-politicized the governance of biofuels. Several farmer associations have completely turned against biofuels. I argue that this rejection of biofuels is due to a limited perception of power as power over .

Why does it make sense to complement such a perception of power over ? Why does a multidimensional power framework make more sense? Naming different perspectives, as done here, with one and the same term—“power”—means, first, to put them on one normative level. Gold rush (power over) is a term with strongly negative connotations, on the one hand, and leads to normatively inflating sustainable innovations (power with) and creative resistance (power to), on the other. This is often unjustified because the exercise of power with and power to are not per se more legitimate forms of achieving social change. For example, preventing greenhouse gas emissions “from above” can be quite legitimate.

Secondly, as illustrated in this article, all three conceptions of power are already used in research on biofuels (although sometimes only implicitly; this should change). My hope is that this article addresses diverse communities and overcomes boundaries between them with this multidimensional power approach (in particular, between those who still celebrate biofuels as a “sustainable innovation” and those scholars who completely condemn them because of related power asymmetries). Especially those whose research is (implicitly) based on understandings of power as power with and power to could take stronger reference to researchers taking a critical viewpoint on their studies (power over)—in particular, through showing how consensual forms of power exercise (power with) and resistance and empowerment (power to) not only reproduce power asymmetries but also help overcome them. If we look at the gold rush metaphor from a perspective of power to , we may see that there is a lot of entrepreneurship involved in the discovery of gold deposits. From the perspective of power with , we may also see that people in the field of gold mining as well as of biofuel production find common ground among diverse interests and organize with each other.

Third, convincing and learning (power with) as well as creative ability (power to) and coercion and manipulation (power over) do not completely capture concrete change processes. The analytical categories applied in this paper help to cluster the various understandings of power in biofuel research, but they also reflect different mechanisms of power in reality. Power with perspectives focus on the benefits of biofuels (sustainable innovation); power to focuses on how new actors develop alternatives to fossil (and nuclear)-based economies; power over points to the limits of change because of the dominance of specific actors, structures, and discourses. The common terminology allows that the three perspectives on power are not considered as mutually exclusive (different interpretations of the same phenomenon), but as supplementary (different aspects of a change process). It becomes possible to examine their interrelations and their supplementary potential. With this article, I hope to have given an impetus for further research in this direction. A comprehensive analysis of power in diverse parts of biofuel research and governance is definitely a prerequisite for more seriously and intensively exploring questions of where, when, and how the governance of biofuels may also allow for “green” resistance and collective empowerment.

If actors create (reproduce) discourses and structures, I call this power to . Most constructivist studies however deal with identifying dominant (hegemonic) structures and discourses over others that are unconsciously reproduced, i.e., power over .

Power with is not identical to Arendt’s understanding of power or its empirical operationalization hardly accomplishes Arendt’s demands. So deliberative theories of democracy build upon her understanding of power without finding it comprehensively implemented in reality [ 61 , 68 , 69 ]. In difference to deliberative processes, power with encompasses communicative as well as common action.

An example, to which Arendt refers in a footnote to her definition of power, is the student protests at Berkeley and elsewhere at the end of the 1960s. She contrasts the power of the students—“obviously the strongest power on every campus simply because of the students’ superior number” ([ 28 ]: 44)—to the violence of the university authorities. An individual student leader ‘in power’ would speak on behalf of the movement.

Balkema A, Pols AJ (2015) Biofuels: sustainable innovation or gold rush? Identifying responsibilities for biofuel innovations. In: Koops B-J, Oosterlaken I, Romijn H et al (eds) Responsible innovation 2: concepts, approaches, and applications. Springer, New York, pp 283–303

Google Scholar  

Partzsch L (2015) Kein Wandel ohne Macht–Nachhaltigkeitsforschung braucht ein mehrdimensionales Machtverständnis. GAIA 24(1):48–56

Article   Google Scholar  

Sordaa G, Banseb M, Kemfert C (2010) An overview of biofuel policies across the world. Energy Policy 38(11):6977–6988

Vermeulen S, Dufey A, Vorley B (2008) Biofuels: making tough choices. The International Institute for Environment and Development (IIED) (2)., pp 1–2

Parsons T (1963) On the concept of political power. Proc Am Philos Soc 107(3):232–262

Pitkin HF (1985) Wittgenstein and justice: on the significance of Ludwig Wittgenstein for social and political thought. University of California Press, Berkeley

Barnett M, Duvall R (2005) Power in global governance. In: Barnett MN, Duvall R (eds) Power in global governance. Cambridge University Press, Cambridge, pp 1–32

Altieri MA, Toledo VM (2011) The agroecological revolution in Latin America: rescuing nature, ensuring food sovereignty and empowering peasants. J Peasant Stud 38(3):587–612

van Eijcka J, Romijn H (2008) Prospects for Jatropha biofuels in Tanzania: an analysis with strategic niche management. Energy Policy 36(1):311–325

Dahl RA (1957) The concept of power. Behav Sci 2(3):201–215

Bachrach P, Baratz MS (1962) Two faces of power. Am Polit Sci Rev 4(56):947–952

Lukes S (1974) Power: a radical view. Macmillan, London

Book   Google Scholar  

Foucault M (1982) The subject and power. Crit Inq 8(4):777–795

Laclau E, Mouffe C (1985) Hegemony and socialist strategy: towards a radical democatic politics. Verso, London

Baka J (2014) What wastelands? A critique of biofuel policy discourse in South India. Geoforum 54(6):315–323

Hunsberger C (2010) The politics of Jatropha-based biofuels in Kenya: convergence and divergence among NGOs, donors, government officials and farmers. J Peasant Stud 37(4):939–962

Digeser P (1992) The fourth face of power. J Polit 54(4):977–1007

Haugaard M (2012) Rethinking the four dimensions of power: domination and empowerment. J Polit Power 5(1):33–54

Hunt SC, Sawin JL, Stair P (2006) Cultivating renewable alternatives to oil. In: Worldwatch Institute (ed) State of the world 2006: Special focus: India and China. Earthscan, London, pp 61–77

Sengers F, Raven R, van Venrooij A (2010) From riches to rags: biofuels, media discourses, and resistance to sustainable energy technologies. Energy Policy 38(9):5013–5027

Kuchler M, Linnér B-O (2012) Challenging the food vs. fuel dilemma: genealogical analysis of the biofuel discourse pursued by international organizations. Food Policy 37(5):581–588

Munro B (2015) The lost innocenece of ethanol: Power, knowledge, discourse, and the U.S. biofuel policy: Doctoral dissertation. Kansas State University, Kansas

Eyben R, Harris C, Pettit J (2006) Introduction: exploring power for change. IDS Bull 37(6):1–10

Gaard G (2010) Women, water, energy: an ecofeminist approach. In: Brown PG, Schmidt JJ (eds) Water ethics: foundational readings for students and professionals. Island Press, Washington, DC, pp 59–75

Balat M, Balat H (2009) Recent trends in global production and utilization of bio-ethanol fuel. Appl Energy 86(11):2273–2282

Nill J, Kemp R (2009) Evolutionary approaches for sustainable innovation policies: from niche to paradigm? Res Policy 38(4):668–680

Allen A (1998) Rethinking power. Hypatia 13(1):21–40

Arendt H (1970) On violence. Harcourt, Brace & World, New York

Skodvin T, Andresen S (2006) Leadership revisited. Glob Environ Polit 6(3):13–27

Bernard F, Prieur A (2007) Biofuel market and carbon modeling to analyse French biofuel policy. Energy Policy 35:5991–6002

Young OR (1991) Political leadership and regime formation: on the development of institutions in international society. Int Organ 45(3):281–308

Huber J (2000) Towards industrial ecology: sustainable development as a concept of ecological modernization. J Environ Policy Plan 2(4):269–285

Jänicke M (2008) Ecological modernisation: new perspectives. J Clean Prod 16(5):557–565

Bomb C, Deurwaarder E, Kaberger T (2007) Biofuels for transport in Europe: lessons from Germany and the UK. Energy Policy 35(4):2256–2267

Foxona T, Pearson P (2008) Overcoming barriers to innovation and diffusion of cleaner technologies: some features of a sustainable innovation policy regime. J Clean Prod 16(1):S148–S161

Longstaff H, Secko DM (2014) Assessing the quality of a deliberative democracy mini-public event about advanced biofuel production and development in Canada. Public Understanding of Science (online)., pp 1–10

Nussbaum M, Sen AK (1987) Internal criticism and Indian rationalist traditions. World Institute for Development Economics Research of the United Nations University, Helsinki

Scholtes F (2009) Analysing and explaining power a capability perspective, Bonn

Malik US, Ahmed M, Sombilla MA et al (2009) Biofuels production for smallholder producers in the Greater Mekong Sub-region. Appl Energy 86(S1):S58–S68

Rathke G-W, Diepenbrock W (2006) Mit Biomasse zum ‘Energie-Scheich’?: Stand, Ertragspotenziale und Perspektiven. Chancen für Landwirtschaft und angrenzende Industriezweige. In: DLG (ed) (Deutsche Landwirtschafts-Gesellschaft) Zukunftsstandort Deutschland: Strategien Für Die Landwirtschaft. DLG-Verlag, Frankfurt a.M

Gaventa J (2006) Finding the spaces for change: a power analysis. IDS Bull 37(6):23–33

Fuchs D (2005) Understanding business power in global governance. Nomos, Baden-Baden

Coase RH (1988) The firm, the market, and the law. University of Chicago Press, Chicago

Pilgrim S, Harvey M (2010) Battles over biofuels in Europe: NGOs and the politics of markets. Sociol Res Online 13(3):4

Stattman SL, Gupta A (2015) Negotiating authority in the global biofuel governance: Brazil and the EU in the WTO. Glob Environ Polit 15(1):41–59

Afionis S, Stringer LC (2012) European Union leadership in biofuels regulation: Europe as a normative power? J Clean Prod 32(1):114–123

Altvater E, Mahnkopf B (1999) Grenzen der Globalisierung: Ökonomie, Ökologie und Politik in der Weltgesellschaft, 4th edn. Westfälisches Dampfboot, Münster

Levin K, Cashore B, Koppell J (2009) Can non-state certification systems bolster state-centered efforts to promote sustainable development through the clean development mechanism? Wake Forest Law Review 44(1):777–798

Dauvergne P, Neville KJ (2010) Forests, food, and fuel in the tropics: the uneven social and ecological consequences of the emerging political economy of biofuels. J Peasant Stud 37(4):631–660

Ponte S, Daugbjerg C (2015) Biofuel sustainability and the formation of transnational hybrid governance. Environ Polit 24(1):96–114

Henriksen LF (2015) The global network of biofuel sustainability standards-setters. Environ Polit 24(1):115–137

Puttkammer J, Grethe H (2015) The public debate on biofuels in Germany: who drives the discourse? Ger J Agric Econ 64(4):263–273

Runge CF, Senauer B (2007) How biofuels could starve the poor. Foreign Aff 86(3):41–53

Horlings LG, Marsden TK (2011) Towards the real green revolution? Exploring the conceptual dimensions of a new ecological modernisation of agriculture that could ‘feed the world’. Glob Environ Chang 21(2):441–452

Levidow L (2013) EU criteria for sustainable biofuels: accounting for carbon, depoliticizing plunder. Geoforum 44(1):211–223

Silva-Castaneda L (2012) A forest of evidence: third-party certification and multiple forms of proof—a case study of oil palm plantations in Indonesia. Agric Hum Values 29(3):361–370

Selfa T, Bain C, Moreno R et al (2015) Interrogating social sustainability in the biofuels sector in Latin America: tensions between global standards and local experiences in Mexico, Brazil, and Colombia. Environmental Management

Nygren A (2015) Governance and images: representations of certified southern producers in high-quality design markets. Environ Values 24(3):391–412

Bourdieu P (1987) Sozialer Sinn: Kritik der theoretischen Vernunft. Suhrkamp, Frankfurt a.M

Guzzini S (1993) Structural power: the limits of neorealist power analysis. Int Organ 47(3):443–478

Dryzek JS (2000) Deliberative democracy and beyond: liberals, critics, contestations. Oxford University Press, New York

Prittwitz V (ed) (1996) Verhandeln und Argumentieren. Leske + Budrich, Opladen

Mussatto SI, Dragone G, Guimarães PM et al (2010) Technological trends, global market, and challenges of bio-ethanol production. Biotechnol Adv 28(6):817–830

Shortall O (2014) Rethinking bioenergy from an agricultural perspective: ethical issues raised by perennial energy crop and crop residue production for energy in the UK and Denmark: PhD thesis. University of Nottingham, Nottigham

Partzsch L (2011) The legitimacy of biofuel certification. Agric Hum Values 28(3):413–425

Kalfagianni A (2015) ‘Just food’: the normative obligations of private agrifood governance. Glob Environ Chang 31(1):174–186

Martínez-Torres ME, Rosset PM (2010) La Vía Campesina: the birth and evolution of a transnational social movement. J Peasant Stud 37(1):149–175

Habermas J (1998) Between facts and norms: contributions to a discourse theory of law and democracy. MIT Press, Cambridge

Young IM (2001) Activist challenges to deliberative democracy. Polit Theo 29(5):670–690

Partzsch L, Fuchs DA (2012) Philanthropy: power with in international relations. J Polit Power 5(3):359–376

Guzzini S (2007) The concept of power: a constructivist analysis. In: Berenskoetter F, Williams MJ (eds) Power in world politics. Routledge, New York, pp 23–42

Download references

Acknowledgements

The research received no funding.

Competing interests

The author declares that she has no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Author information

Authors and affiliations.

Sustainability Governance, Institute of Environmental Social Sciences and Geography, University of Freiburg, Tennenbacher Strasse 4, D-79106, Freiburg, Germany

Lena Partzsch

You can also search for this author in PubMed   Google Scholar

Corresponding author

Correspondence to Lena Partzsch .

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Reprints and permissions

About this article

Cite this article.

Partzsch, L. Biofuel research: perceptions of power and transition. Energ Sustain Soc 7 , 14 (2017). https://doi.org/10.1186/s13705-017-0116-1

Download citation

Received : 01 February 2017

Accepted : 04 May 2017

Published : 18 May 2017

DOI : https://doi.org/10.1186/s13705-017-0116-1

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

Energy, Sustainability and Society

ISSN: 2192-0567

• General enquiries: [email protected]

• Open access
• Published: 20 May 2019

Latest development in microalgae-biofuel production with nano-additives

• Nazia Hossain   ORCID: orcid.org/0000-0001-7925-0894 1 ,
• T. M. I. Mahlia 2 &
• R. Saidur 3 , 4  

Biotechnology for Biofuels volume  12 , Article number:  125 ( 2019 ) Cite this article

45k Accesses

151 Citations

2 Altmetric

Metrics details

Microalgae have been experimented as a potential feedstock for biofuel generation in current era owing to its’ rich energy content, inflated growth rate, inexpensive culture approaches, the notable capacity of CO 2 fixation, and O 2 addition to the environment. Currently, research is ongoing towards the advancement of microalgal-biofuel technologies. The nano-additive application has been appeared as a prominent innovation to meet this phenomenon.

The main objective of this study was to delineate the synergistic impact of microalgal biofuel integrated with nano-additive applications. Numerous nano-additives such as nano-fibres, nano-particles, nano-tubes, nano-sheets, nano-droplets, and other nano-structures’ applications have been reviewed in this study to facilitate microalgae growth to biofuel utilization. The present paper was intended to comprehensively review the nano-particles preparing techniques for microalgae cultivation and harvesting, biofuel extraction, and application of microalgae-biofuel nano-particles blends. Prospects of solid nano-additives and nano-fluid applications in the future on microalgae production, microalgae biomass conversion to biofuels as well as enhancement of biofuel combustion for revolutionary advancement in biofuel technology have been demonstrated elaborately by this review. This study also highlighted the potential biofuels from microalgae, numerous technologies, and conversion processes. Along with that, the study recounted suitability of potential microalgae candidates with an integrated design generating value-added co-products besides biofuel production.

Conclusions

Nano-additive applications at different stages from microalgae culture to end-product utilization presented strong possibility in mercantile approach as well as positive impact on the environment along with valuable co-products generation into the near future.

Biofuel has caught substantial attention worldwide nowadays as an alternative fuel due to its capability to adapt with gasoline for a maximum 85% blend without any engine modification. Subsequently, the suitability of various candidates for biofuel is being continuously quested by the researchers and environmentalists [ 1 , 2 , 3 , 4 , 5 ]. In this recent era, one of the most sophisticated technologies, nano-technology integration with bioenergy application by the nano-energy sector has brought a revolutionary impact on biofuel conversion processes and enhancement of engine performances. Nano-technology is defined as designing a device or material in nano-scale (10 −9 m). To accelerate the biofuel yield and improve the efficiency of biofuel utilization in petrol and diesel, nano-technology has been initiated via nano-additives such as nano-magnets, nano-crystals, nano-fibres, nano-droplets, and others [ 6 , 7 , 8 ]. Figure  1 presents the perspectives of nano-additives on microalgae cultivation to microalgal-biofuel implementation.

Nano-additive applications for the enhancement of microalgae cultivation to biofuel implementation

On this eve of the quest for suitable biomass for biofuel, the concept of microalgae cultivation appeared to the spotlight for biofuel manufacturing due to several positive perspectives such as (i) they do not clash with human or animal food chains, (ii) very rich with carbohydrate, protein, and oil content, (iii) can grow in aqueous media such as wastewater, freshwater, saline water, and assimilate nutrients from brackish water, salt water, or highly polluted water, (iv) demand low water, (v) sustain capability to grow whole year naturally with sunlight presence, (vi) can be cultivated in the waste dump area, sea, ponds, rivers, industrial, and municipal waste drainage, wet bare lands especially in cold regions, (vii) develop sustainable O 2 generation system, and (viii) diminish CO 2 by up taking it for photosynthesis respiration [ 9 , 10 , 11 , 12 , 13 , 14 ]. In addition, microalgae contain very short harvesting life cycle and yield nascent biomass that drives higher productivity of the desired biofuel. Interestingly, microalgae carry a prodigious amount of carbohydrates, protein and lipid, the sole components of biofuel conversion [ 13 , 15 , 16 , 17 ]. Nano-technology applications have been implemented to biofuel industries, since the existing controversial approaches of traditional microalgae culture-biofuel production contain a number of limitations such as inconsistent industrial-scale microalgae production, high microalgae production and harvesting cost, energy consumption for biofuel production from microalgae, and the increase of greenhouse gas intensity in environmental [ 18 ]. Nano-technology applications can be entailed in different stages from microalgae cultivation to microalgae-biofuel application in fuel engines due to durability, recyclability, adsorption efficiency, catalytic performance, stability, crystallinity, economical advantage, high storage capacity, excellent biofuel yield, and environment-friendly characteristics. According to the previous studies, nano-technology application enhanced microalgae cultivation, the maximum yield of numerous microalgae biofuels as well as microalgae-biofuel implications in petrol and diesel engines. Various nano-materials, e.g., nano-fibres, nano-particles, nano-tubes, nano-sheets, and other nano-structures, have been investigated as effective nano-catalysts in direct and indirect approaches in biofuel (e.g., bioethanol, biodiesel, biomethane, and others) yield enhancement [ 19 , 20 , 21 , 22 ]. For instance, magnetic nano-particles were used as a carrier for enzyme immobilization for bioethanol and biodiesel generation effectively. Owing to high coercivity and powerful paramagnetic characteristics, magnetic nano-particles were also preferred for methanogenesis to produce biomethane [ 21 ].

To authors’ best knowledge, no review study has been performed on numerous biofuel productions from microalgae integrated with the nano-additive application so far. The closest review with this study was conducted on the bioenergy production from lignocellulosic biomass (agricultural residues), industrial waste (sludge) as well as algae (microalgae and macroalgae) with the nano-scale optimization which has merely emphasized on the mechanism of nano-particles, biomass characteristics, and nano-particle application on biomass growth [ 23 ]. Compared with that, the current review contextualized the numerous biofuel productions from pure microalgae and optimization with nano-additive application on biomass growth to end-product application. Therefore, the major objectives of this review work are (i) to determine the array of the techniques and methods associated with nano-particles incorporation with microalgae culture as well as microalgal biofuel, (ii) to demonstrate divergent nano-additive applications on microalgae cultivation, biomass conversion to biofuels, and biofuel combustion, (iii) to identify the potential sources of microalgae, especially the carbohydrate, protein, and lipid-enriched microalgae types for biofuel production and determine the possible microalgae biofuels, biomass conversion technologies, and processes to biofuels, and (iv) to assess the future prospects of the process development planning along with integrated design of some other value-added products besides biofuel.

General perspective of microalgae

Microalgae are referred as photosynthetically driven single or multi-cellular living being, the habitat of moist environment either on the solid mud or float on various water types, e.g., fresh water, marine water, wastewater with the presence of sunlight, or artificial light. The scientific consensus is that through photosynthesis respiration, they convert CO 2 to O 2 and generate large amounts of cellular energy content embedded with sugar, protein, and lipid [ 24 , 25 , 26 , 27 ].

Nowadays, industrialization and urbanization threaten the existing ecosystem severely by dumping heavy metal waste containing phosphorus, nitrogen, sulfur and others as well as exhaling high amount CO 2 to the free air. Another knocking threat to the energy sector is rapid depletion of fossil fuel worldwide due to excessive energy uses [ 28 , 29 , 30 ]. With this circumstance, microalgae cultivation in the wastewater, unused fresh, and saline water, drainage is considered as suitable scientific solution for green energy due to some favorable aspects such as multi-functionality, genuine conversion competency biologically and flexibility with growth system, wastewater accumulation, CO 2 sequestration, and large amount of carbohydrate–lipid–protein content. To note, carbohydrate–lipid–protein are the main components to generate divergent biofuels (e.g., bio-oil, biodiesel, biobutanol, and others) and biogas (e.g., bio-hydrogen) [ 31 , 32 ]. The cellular components of microalgae are composed of huge fraction of lipid, protein, and carbohydrates resulting in the driving factors of biofuel production. Table  1 presents some well-known potential microalgae candidates for biofuels. These species were extensively researched in the laboratory and large-scale applications so far. Type and description of these species have been tabulated to present a detailed view of selected species, suitable growth conditions (such as water type and region for cultivation), availability, and cellular specifications. Table  2 represents prime microalgae cellular component composition of several well-known microalgae species for biofuel production [ 24 , 33 , 34 ].

Biofuels from microalgae

Numerous biofuels, e.g., bioethanol, biodiesel, bio-oil, biomethane, bio-hydrogen, and others, have been extracted from microalgae [ 46 , 47 ]. Nano-particles’ incorporation with microalgae cultivation (e.g., cell suspension, cell separation, and cell harvesting), biofuel conversion technologies, and biofuel application have amplified the overall yield in every stage [ 22 ]. According to the previous studies, a very small amount of colloidal hydrous iron(III) oxide particles boosted almost 100% microalgae cell suspension; magnetic particles incorporated with aluminum sulfate were very effective for cell separation from the mixed culture of Anabaena and Aphanizomenon microalgae species; silver nano-particles application on Chlamydomonas reinhardtii and Cyanothece 51142 microalgae harvesting increased 30% higher biomass productivity; and calcium-oxide nano-particles escalated the large-scale biodiesel conversion yield up to 91% via catalytic transesterification [ 18 , 22 , 48 ]. This study summarized the overall microalgae cultivation integrated with nano-particles until biofuel production in Fig.  2 . Different biofuels from microalgae and conversion processes are diagrammed in Table  3 .

Process flow diagram of carbon capture, water treatment and biofuels production from microalgae incorporated with nano-particles [ 18 , 25 , 49 , 50 , 51 , 52 , 53 , 54 ]

Preparing techniques of nano-additives for microalgae biofuel

Magnetic nano-particle (NP) powder has been enumerated to the microalgae cell suspension in the photobioreactor cultivation process to flocculate cells for uniform distribution of nutrients and light all over the reactor. Immunomagnetic detection and modification of microalgae cell by NPs are another well-practiced method for cell suspension enhancement. Nano-liquid has been injected to the cell culture for microalgae harvesting and bio-separation through this technique. Silver nano-materials have also been implemented on the photobioreactor surface coating for higher light accessibility [ 22 ]. Along with microalgae culture and harvesting, sphere nano-particles have been enacted during hydrolysis, lipid extraction, transesterification, and biofuel purification from microalgae via irradiation and ultra-sonication methods and much higher biofuel yield have been obtained. Another established method of nano-materials application includes enzymatic nano-catalyst, lipase carrier. The reactant diffusion rate enhancement by the NPs to the active side of lipase has been determined by Eq.  1 [ 55 ]:

where R df  = diffusion rate to the active sides of lipase and D  = diffusion path diameter of reactant to the access of lipase active side.

NPs for biofuel doping can be formulated by either physical or chemical methods. For instance, plasma-arcing, sol–gel method has been presented as chemical method and ball mill process (agitation rate: 450 rpm) was presented as a physical method for NPs’ preparation in the previous studies [ 56 , 57 , 58 ]. Subsequently, NPs were doped with microalgal biofuel (e.g., microalgae oil, biodiesel, bioethanol, and others) with different doses (e.g., 25 ppm, 50 ppm, 100 ppm, and others) via ultra-sonication processing by the presence of magnetic stirrer and implemented on compression ignition (CI), direct ignition (DI) engines without any engine modification. NPs are dispersed in a base fuel and smoothen potential agglomerate into nano-scale due to its’ larger surface area and surface energy [ 58 , 59 , 60 , 61 ]. The ultra-sonication method was conducted with various parameters such as frequency (e.g., 20 kHz, 40 kHz, and 45 kHz), power (e.g., 120 W and 220 W), and time (30 min and 60 min) [ 60 , 62 , 63 ]. Cationic surfactants, e.g., tetra methyl ammonium hydroxide, cetyl trimethyl ammonium bromide, have been incorporated on the nano-particle surface for a negative-charge envelope formation to resist NPs’ sedimentation [ 56 , 64 ]. After biofuel-NPs’ doping, the NP-blended biofuel was preserved under the static condition to stabilize for fuel purpose [ 59 ]. Several potential NPs–microalgae-biofuel blends are tabulated in Table  4 . The morphology and crystalline phases after NP-doping were analysed through a scanning electron microscope and X-ray diffraction, respectively [ 61 ]. Botryococcus braunii oil was doped with almost 50 nm sized titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ) incorporated with biodiesel (B20) of different doses for enhanced fuel efficiency in CI engine [ 60 ]. Caulerpa racemosa green algae biodiesel (B20) was doped with 50 nm sized zirconium dioxide (ZrO 2 ) by the different doses for CI engine [ 59 ].

Future applications of nano-additives for microalgae-biofuel

Nano-additive application on microalgae-biofuel enhancement has been categorized into several stages from raw material production to end-product implications. The stages are:

nano-additives for microalgae cultivation;

nano-additives for microalgae biomass conversion to biofuels;

nano-additives for microalgae-biofuel applications.

Nano-additives for microalgae cultivation

Improvement of the microalgae biomass productivity with the minimum area requirement is considered as the main purpose of nano-additive application in the microalgae culture. Nano-technology is being applied for enzyme immobilization, since nano-structures broaden the immobilization surface area causing high loading power of enzymes and stability of immobilized enzymes. Enzyme immobilization can be performed in different approaches such as electrospun nanofibers, covalently attached enzymes into nano-fibres, and enzyme aggregate coatings on nanofibers. The enzyme immobilization was investigated on various carbon nano-particles, e.g., graphene oxide (GO), multi-walled carbon nano-tubes (MWNTs), oxidized-MWNTs (O-MWNTs), and fullerene (C60). Among these nano-structures, O-MWNTs yielded the highest, and C60 yielded the lowest [ 21 , 22 ]. Nano-particles’ (NPs’) application was implemented on several microalgae species harvesting and yielded outstanding in each phase of the application. Application of nano-particles on microalgae harvesting claimed 20–30% microalgae production cost in large-scale application [ 22 ]. Table  5 presents the harvesting efficiency of several microalgae species cultivated with various nano-particles. Nano-particles also boosted the light conversion efficiency in photobioreactor (PBR) during the microalgae culture period. It is also worth mentioning that PBR is run by artificial light sources consuming additional energy and cost. However, during biomass growth, light sources do not reach in culture broth inadequately due to self-shading and biofilm formation on the PBR surface. To achieve desired illumination properties and photo-conversion efficiency in the PBR, various light-emitting diodes (LEDs) equipped with nano-materials fabrication are being implemented recently. Gallium aluminum arsenide (GAA)-fabricated LEDs have been experimented on laboratory scale algae culture so far. It was evident that the application of optical fibres in algal culture saves much energy, additional light cost, and increase efficiency [ 18 ]. Another latest development of nano-particle, integration of metallic nano-particles (MNPs) with localized surface plasmon resonance (LSPRs) amplifies the light scattering at certain wavelength [ 65 ]. An experimental study revealed that silver nano-particles’ (Ag-NPs’) suspension in plasmonic mini-PBRs backscatter blue light strongly. The blue light increased the photosynthetic efficiency significantly for green microalgae, Chlamydomonas reinhardtii, and blue–green microalgae, Cyanothece 51142, and 30% higher microalgae biomass have been obtained [ 48 ]. Nano-particles addition in microalgae cultivation can also improve the yield of the CO 2 absorption from the atmosphere and CO 2 sequestration that can boost the biomass growth. For instance, nano-bubbles in microalgae culture remained stable for a longer time. Nano-bubbles also floated algae biomass into the culture, ensured high mass transfer efficiency, and improved biomass density by sufficient accumulation of CO 2 , O 2 stripping, and minor buoyancy. Moreover, nano-bubbles suspended the biomass around airlift-loop bioreactor (ALB) and required less energy than micro-bubbles. Uniform nonporous membrane of ALB was also capable of producing 100 nm sized bubbles for this purpose [ 18 , 66 , 67 ]. The previous studies also delineated that nano-additives played a significant role in flocculation and separation process before biofuel production besides microalgae harvesting [ 22 ].

Nano-additives for microalgae biomass conversion to biofuels

Among microalgae biofuel, biodiesel has been appeared as the most popular and commercial biofuel in the mobile fuel market. For the case of biodiesel production, applications of acidic and basic nano-catalyst spheres can substitute the chemical compounds such as sodium methoxide by reacting with the free fatty acids and oils. Additional advantages of these nano-catalysts are recyclability and positive economical impact. Moreover, reactions can take place with low temperature and pressure as well as this approach reduces the contaminant release to the environment borne by sodium methoxide [ 6 ]. An industrial biodiesel study demonstrated that commercial CaO-NPs presented 91% biodiesel conversion efficiency during scaled-up catalytic transesterification [ 18 ]. Experimental study of microalgae cultivation with spherical nano-particles composed with sand (silica) and calcium compounds revealed that microalgae cellular growth increased drastically without harming harvesting as well as biofuel production from vegetable oil. The best way to address this issue was described as one of the major driving factors for commercial biofuel, biofuel production cost dropped effectively [ 6 , 7 , 8 , 68 ]. The experimental study mentioned that mesoporous silica nano-catalyst, Ti-loaded SBA-15 presented ten times higher free fatty acids (FFA) and water tolerance level than any other catalysts for biodiesel production from vegetable oil as well as this nano-catalyst performed three times better than other effective nano-catalysts titanium silicalite-1 (TS-1) and titanium dioxide silicate (TiO 2 -S) [ 69 ].

Moreover, Ti-loaded Santa Barbara Amorphous-15 (SBA-15) nano-catalyst application reduced the chemical (alkaline catalyst, NaOH) cost of transesterification process for biodiesel production by recycling the nano-catalyst as well as this process is more environment-friendly [ 6 , 69 ]. On the other hand, sulfate incorporated Ti-SBA-15 also performed as biocatalyst to convert vegetable oil to 100% esterified bio-lubricant. In consequences, this nano-particle can be expected to produce bio-lubricant from bio-oil of microalgae [ 70 ]. Other study showed that Niobia (N 2 O 5 ) incorporated with SBA-15 application on biodiesel production from biomass through esterification presented a significant scenario for microalgae-biodiesel yield [ 71 ]. Another study delineated that the enzyme extracted from Pseudomonas cepacia conjuncts with the nano-particles such as polyacrylonitrile (PAN) nanofibre, Fe 3 O 4, and nanoporous gold; silica nano-particles with lipase enzyme from Rhizopus miehei ; ferric silica and magnetic nano-particles with lipase from Burkholderia sp., polyacrylonitrile nano-fibre bound with lipase from Thermomyces lanuginosa has performed very effectively to produce biodiesel from various bio-oil by the transesterification process [ 7 ]. Furthermore, nano-magnetic biocatalyst of KF/CaO–Fe 3 O 4 , Li(lithium)-doped CaO, Fe 2 O 3 –CaO, sulfate (SO 4 − ) incorporated Zi (zirconium), sodium titanate and carbon-based nano-tubes and nano-particles reached up to 95% or above biodiesel yield from diverse types of biomass and biodegradable waste [ 7 , 72 ]. Besides enhancement of biodiesel-yield efficiency, a type of NP, zeolite (an alumina silicate mineral), has been used as commercial absorbent during the transesterification process. Zeolites absorbed the undesirable moisture content (4–6%) and produced pure glycerine as co-product besides biodiesel. Mesoporous nano-particles also presented a vital capability for continuous microalgal-biofuel generation from biomass without cell lysis. Zeolites also removed lipids from the microalgae cell membrane [ 18 , 73 ]. Table 6 presented the applications of nano-additives for biodiesel-yield enhancements during microalgae to biofuel conversion, suitable conversion processes, and efficiencies.

Nano-particles were efficiently capable to perform as immobilizing beds for valuable enzymes due to their large surface area-to-volume ratio. This capability of NPs broke down the long chains of complex sugar of microalgae, converted it to simple sugar, and consequently turned into bioethanol via the fermentation process. Due to the large surface area, the interaction between the surface of the nano-particles and fuel surrounded by them achieved adequate stability to overcome density variations. Nano-particles prepared by carbon nano-tubes doped with iron-oxide nano-particles presented excellent biocatalytic efficiency in a bioreactor, recyclable option enzyme applications, less capital cost as well as better enzyme loading for this purpose [ 6 , 7 , 74 ]. A catalytic study mentioned that mesoporous niobium oxide (N 2 O 5 ) application on complex sugar (sucrose) possessed both Lewis acid (LA) and Bronsted acid (BA) sites to convert fructose to 5-hydroxymethylfurfural (HMF) with the highest yield so far. The synergistic catalytic effect from a large amount of both LA and BA acid site quantities and surface areas played a positive impact on the reaction rate with a few times faster conversions [ 75 ]. Functionalised multiwall carbon-nano-tube (MWCNT) immobilization presented more than 55% initial activity of microalgal hydrolysis for Candida Antarctica. Nano-catalysts such as cobalt–molybdenum fabricated with Si/Al have been experimented on Botryococcus braunii and presented stable hydrocarbons. Another nano-catalyst, mobil composition of matter No. 41 (MCM-41), mesoporous material effectively reduced oxygenated fractions of bio-oil through catalytic pyrolysis [ 18 ].

Nano-catalysts can synthesize biomethane produced from microalgae from wastewater into pure hydrogen and carbon content. In a further step, this methane can produce biogas through anaerobic digestion. Biogas could be used as raw material of bio-fuelled electricity generation further. The elemental carbon can also be utilized as pure nano-graphite for the applications on batteries, aerospace, automobiles, and others [ 6 ]. The latest development conducted by quantum sphere on marine microalgae species evinced biogasification from wet microalgae biomass by metal nano-catalysts [ 18 ]. Besides that, nano-particles such as TiO 2 , CeO 2 were manifested to improve 10–11% of the biogas yield from wastewater treatment. Therefore, these nano-particles can be projected further for the biomethane production from microalgae grown in wastewater [ 7 ]. Apart from that, nano-substances with SiO 2 , nano-particles of platinum (Pt), nickel (Ni), cobalt (Co), and iron (Fe) can increase methane production from biomass up to 70%. Nano-fly ash and nano-bottom ash were proved to increase biomethane yield up to 3.5 times more. Nano-metal oxides, e.g., MgO, CaO, and SrO, nano-materials such as silica, single-walled nano-tubes of carbon-based materials, nano-clay, and nano-zero valence metal applications in biodiesel, bio-hydrogen, and biomethane production from microalgae and other biomass presented outstanding yield. These nano-particles can be projected for large-scale microalgal-biofuel production in the future to obtain revolutionary yield [ 7 , 8 ]. In addition, nano-hybrid catalysts are being commercialized as emulsion stabilizers in industrial applications. For instance, quaternary ammonium salts have been documented as an emulsifying surfactant for separation, extraction, isolation, and purification of biofuels. Carbon nano-tubes with silica fusion, SiO 2 –MgO nanohybrids have been performed as stabilizers on bio-oil in water emulsion due to its inherent hydrophobicity and resulted in full conversion in different emulsion phases [ 18 ].

Nano-additives for microalgae-biofuel applications

Solid nano-particles, nano-fluids, or nano-droplets with biofuel and fossil fuel were proved to improve the fuel lubricity, cetane number, burning rate, chemical reaction, catalytic performance, fire/flash point, heat and mass transfer efficiency and water co-solvency as well as decrease delay period [ 76 , 77 ]. That resulted in more complete and cleaner combustion of microalgae biofuel mixed with fossil fuel in compression ignition (CI), spark ignition (SI), and direct ignition (DI) engines. In line with that, nano-technology applications showed the capability of amplifying microalgal-biofuel combustion efficiency and reduced soot, NO x , smoke, HC, CO 2, and CO emission to the environment up to 72% [ 6 , 76 , 78 ]. Application of solid nano-additives such as alumina (Al 2 O 3 ), CERIA, carbon nano-tubes (CNT), Co 3 O 4 , ZrO 2 , La 2 O 3 , CeO 2 , SiO 2 , Ni 2 O, TiO 2 , ZnO, Fe 2 O 3 , CuO, Ce x Zr (1– x ) O 2 , and amide-doped MWCNTs-CeO 2 boosted the engine power, torque, and brake thermal performance of biodiesel (extracted from microalgae and other biomass) in CI and DI engines up to 11% [ 59 , 76 , 79 , 80 ]. The experimental study of nano-particles on DI engines demonstrated that nano-particles blended with biodiesel as well as diesel–biodiesel mixture performed outstanding. The effectiveness was higher compared to usual catalysts [ 61 , 81 ]. Another study presented that nano-particles of CeO 2 incorporated with an emulsion of biofuel with sol–gel combustion technology performed excellent mono-cylinder 4 stroke direct CI and DI engines without any hardware modification. Nano-particles addition with biofuels escalated the fuel calorific value, fastened evaporation rate, improved brake-specific fuel consumptions and thermal efficiency, reduced greenhouse gases (GHGs) such as CO, NO x , and smoke, and unburned HCs. Chemical reactions between CeO 2 and GHGs gases are presented in Rc. 2 , Rc. 3 , and Rc. 4 [ 82 ]:

In contrast, liquid nano-additive, nano-Al-droplet application (nano-suspension) on biofuel mixture has been manifested more efficient than even micro-suspension. Liquid nano-additives also presented outstanding performance by achieving better suspension than n -decane-based fuels. Nano-Al suspension with ethanol was strong enough for a longer period than other particles, because ethanol tended to form a gel around the nano-particles due to higher viscosity [ 74 ]. Nano-droplets coated a monolayer on the mechanical parts of the engine touched with liquid fuel and improved fuel efficiency [ 18 ]. In addition, NPs such as nano-Al, Al 2 O 3 , CuO, MgO, MnO, and ZnO incorporated with water–diesel–biodiesel (E10) emulsion and bioethanol performed remarkably. Among these NPs, Al 2 O 3 performed the best because of mandate disabling, consistent torque boosting, higher heat of combustion, super-high DTG max value, tiniest size of water droplets, the minimum value of brake-specific fuel consumption, and lowest values of Soot, NO x , CO, and HC [ 19 , 83 ].

Challenges and future prospects

Although nano-additive applications played significant role in microalgae cultivation, harvesting, conversion to biofuel and biofuel applications to enhance the efficiency, yet some challenges remained before the implementation of nano-additives for the mercantile approach. Most of the nano-additives from experimental research were not well-characterized in terms of particle size, shape, and size distribution as well as clustering [ 84 ]. Before large-scale application, well characterization of nano-particles and nano-fluids must be studied comprehensively. Appropriate nano-additive selection, preparation methods, and time for the selected application should be emphasized for optimum productivity. The effect of nano-catalyst implementation for microalgae-biofuel combustion quality, engine performance, and gas emission should be well studied and well-understood before implementation. In line with that, the availability of appropriate nano-additives with large amount might be a challenge for mass application though for laboratory scale, nano-additives are adequately available. Another constraint is cost-effectiveness of nano-catalysts for an industrial application which may hinder the commercial perspective, since many nano-catalysts are quite expensive.

Along with the potential microalgae determination and biofuel generation, integration of a plant design of value-added co-products will be the predominant advantage of the overall project with the economical aspect. This review encouraged biofuel research and development (R&D) sector worldwide to convert their unused, abandoned and wastewater sources, wet, and barren lands into microalgae farm as an eminent source of biofuel production. However, it should be highlighted that based on the existing research experiments, microalgae fuel production still stands at initial stage due to downward economic profile worldwide. Nano-additive applications on microalgal biofuel are yet confined into laboratory and pilot scale which can be counted as a significant limitation. Hence, it is strongly recommended to figure out large-scale process development with nano-additive applications for enhancement of microalgal growth, biofuel transformation processes, and fuel utilization in CI and DI engines. Nano-additive applications at different stages from microalgae culture to end-product utilization have a strong possibility to gain economical feasibility. Therefore, the detailed techno-economic analysis must be commanded to determine whether NP applications on microalgae biofuel are economically favorable or not, since economical issue is one of the most effective factors behind large-scale plant setup. Besides, these applications also have positive impact on the environment with value-added co-product generation into near further. Since the nano-additive utilization manifested itself environment-friendly, still a comprehensive life cycle assessment should be conducted to present the environmental positivities transparently. Besides all these factors, public safety, impact on flora and fauna, and the possibility of bio-hazards are also needed to be analysed extensively before commercialization.

Microalgae utilization for biofuel production is undoubtedly desirable all over the world. Though this approach is energy-efficient and environment-friendly, experts are still looking for an innovation that can boost the microalgae-biofuel yield from primary stage to end product as well as shift the whole process towards a cost-effective fuel solution. Hence, this review was emphasized on the synergistic effect of nano-additive-enhanced microalgal biofuel for mercantile approach and fuel-yield extension. Application of various forms of nano-additives in different phases on microalgae growth to biofuel demonstrated an excellent outcome that may project revolutionary improvement of commercial microalgae biofuel. However, the sustainability analysis of stepwise production rounds for microalgae biofuel still presented a bare need of further research and innovative concepts. These concepts may determine the most appropriate nano-additive for the desired type of biofuel in the context of economical aspect. Since nano-additive application on microalgae is quite new research concept, policy making and implementation of nano-additives will remain as the most vital issues for commercial output especially in developing countries. Therefore, managerial insights are needed to be emphasized further on proper policy, socio-economic impact, advantages and limitations for the overall system to attract the government and non-government fuel industries.

Enhancement of microalgae cultivation and harvesting by nano-bubbles and nano-particles application.

Identification of suitable microalgae species, possible biofuels from microalgae, latest conversion technologies, processes, and required equipments.

Excellent microalgal-biofuel yield by nano-droplet and nano-additives.

Complete and cleaner combustion in fuel engines by nano-emulsion and nano-stabilizers.

Availability of data and materials

Not applicable.

Abbreviations

acetone–butanol–ethanol

silver nano-particles

silver oxide

airlift-loop bioreactor

aluminum oxide/alumina

aluminum droplet

bronsted acid

calcium carbonate

cerium oxide

calcium oxide

calcium oxide nano-particles blends

compression ignition

carbon-dioxide

carbon mono oxide

cobalt oxide

carbon nano-tubes

copper oxide

cerium–zirconium oxide

chitosan/magnetic nano-particles

direct ignition

deep eutectic solvent

maximum derivative thermogravimetry

free fatty acids

ferric oxide

ferric chloride

gallium aluminum arsenide

greenhouse gases

graphene oxide

hydro-carbon

5-hydroxymethyl furfural

lanthanum oxide

light-emitting diodes

localized surface plasmon resonance

multiwall carbon nano-tubes

mobil composition of matter No 41

magnesium oxide

manganese oxide

metal nano-particles

niobia/niobium oxide

nickel oxide

nitrogen oxide

oxidized MWNTs

polyacrylonitrile

photobioreactor

poly dimethylammonium chloride

research and development

rhodium oxide

santa barbara amorphous-15

supercritical water gasification (SCWG)

surface-functionalized iron-oxide nano-particles

spark ignition

silicon dioxide

simultaneous saccharification and co-fermentation

separate hydrolysis and co-fermentation

separate hydrolysis and fermentation

simultaneous saccharification and fermentation

strontium oxide

titanium dioxide

titanium silicalite-1

titanium dioxide silicate

alumino silicate mineral

zirconium dioxide

Hossain N, Mahlia TMI, Zaini J, Saidur R. Techno-economics and sensitivity analysis of microalgae as commercial feedstock for bioethanol production. In: Environmental progress and sustainable energy. New Jersey: John Wiley & Sons; 2019. p. 1–14.

Google Scholar  

Silitonga AS, Mahlia TMI, Ong HC, Riayatsyah TMI, Kusumo F, Ibrahim H, et al. A comparative study of biodiesel production methods for Reutealis trisperma biodiesel. Energ Source Part A. 2017;39:2006–14.

Article   CAS   Google Scholar  

Iqbal M, Khan FU. Hybrid vibration and wind energy harvesting using combined piezoelectric and electromagnetic conversion for bridge health monitoring applications. Energy Convers Manag. 2018;172:611–8.

Article   Google Scholar  

Hossain N, Jalil R. Sugar and bioethanol production from oil palm trunk (OPT). Asia Pac J Energ Environ. 2015;2:89–92.

Hossain N, Zaini JH, Mahlia TMI. A review of bioethanol production from plant-based waste biomass by yeast fermentation. Int J Technol. 2017;8:5–18.

Trindade SC. Nanotech biofuels and fuel additives, biofuel’s engineering process technology. In: Bernardes MADS, editor. biofuel’s engineering process technology. New York: InTech; 2011.

Palaniappan K. An overview of applications of nanotechnology in biofuel production. World Appl Sci J. 2017;35:1305–11.

Nizami A, Rehan M. Towards nanotechnology-based biofuel industry. Biofuel Res J. 2018;18:798–9.

Laurens LML, Chen-Glasser M, McMillan JD. A perspective on renewable bioenergy from photosynthetic algae as feedstock for biofuels and bioproducts. Algal Res. 2017;24:261–4.

Collotta M, Champagne P, Mabee W, Tomasoni G. Wastewater and waste CO 2 for sustainable biofuels from microalgae. Algal Res. 2018;29:12–21.

Xu H, Lee U, Coleman AM, Wigmosta MS, Wang M. Assessment of algal biofuel resource potential in the United States with consideration of regional water stress. Algal Res. 2019;37:30–9.

Hossain N, Jalil R. Analyses of bio-energy properties from Malaysian local plants: sentang and sesendok. Asia Pac J Energy Environ. 2015;2:141–4.

Goh BHH, Ong HC, Cheah MY, Chen W-H, Yu KL, Mahlia TMI. Sustainability of direct biodiesel synthesis from microalgae biomass: a critical review. Renew Sust Energy Rev. 2019;107:59–74.

Hossain N, Zaini J, Mahlia TMI. Experimental investigation of energy properties for Stigonematales sp. microalgae as potential biofuel feedstock. Int J Sustain Eng. 2019;12:123–30.

Gumba RE, Saallah S, Misson M, Ongkudon CM, Anton A. Green biodiesel production: a review on feedstock, catalyst, monolithic reactor, and supercritical fluid technology. Biofuel Res J. 2016;3:431–47.

Cheng JJ, Timilsina GR. Status and barriers of advanced biofuel technologies: a review. Renew Energy. 2011;36:3541–9.

Schenk PM, Thomas-Hall SR, Stephens E, Marx UC, Mussgnug JH, Posten C, et al. Second generation biofuels: high-efficiency microalgae for biodiesel production. Bioenergy Res. 2008;1:20–43.

Pattarkine MV, Pattarkine VM. Nanotechnology for algal biofuels. In: Gordon R, Seckbach J, editors. The science of algal fuels. Berlin: Springer; 2012. p. 149–60.

Hasannuddin AK, Yahya WJ, Sarah S, Ithnin AM, Syahrullail S, Sidik NAC, et al. Nano-additives incorporated water in diesel emulsion fuel: fuel properties, performance and emission characteristics assessment. Energy Convers Manag. 2018;169:291–314.

Karthikeyan S, Prathima A. Microalgae biofuel with CeO 2 nano additives as an eco-friendly fuel for CI engine. Energy Source Part A. 2017;39:1332–8.

Kim J, Jia H, Wang P. Challenges in biocatalysis for enzyme-based biofuel cells. Biotechnol Adv. 2006;24:296–308.

Article   CAS   PubMed   Google Scholar  

Safarik I, Prochazkova G, Pospiskova K, Branyik T. Magnetically modified microalgae and their applications. Crit Rev Biotechnol. 2016;36:931–41.

Rahman KM, Melville L, Huq SMI, Khoda SK. Understanding bioenergy production and optimisation at the nanoscale—a review. J Exp Nanosci. 2016;11:762–75.

Becker W. Microalgae for human and animal nutrition. In: Richmond A, editor. Handbook of microalgal culture: biotechnology and applied phycology. New York: Blackwell Publishing Ltd: Academic; 2003. p. 312–51.

Chapter   Google Scholar  

Razzak SA, Hossain MM, Lucky RA, Bassi AS, Lasa HD. Integrated CO 2 capture, wastewater treatment and biofuel production by microalgae culturing—a review. Renew Sust Energ Rev. 2013;27:622–53.

Hu Q, Sommerfeld M, Jarvis E, Ghirardi M, Posewitz M, Seibert M, et al. Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances. Plant J. 2008;54:621–39.

Byreddy AR, Gupta A, Barrow CJ, Puri M. Comparison of cell disruption methods for improving lipid extraction from thraustochytrid strains. Mar Drugs. 2015;13:5111–27.

Article   CAS   PubMed   PubMed Central   Google Scholar  

Norhasyima RS, Mahlia TMI. Advances in CO 2 utilization technology: a patent landscape review. J CO2 Util. 2018;26:323–35.

Saïdane-Bchir F, El Falleh A, Ghabbarou E, Hamdi M. 3rd generation bioethanol production from microalgae isolated from slaughterhouse wastewater. Waste Biomass Valori. 2016;7:1041–6.

Sarkar B, Chakrabarti PP, Vijaykumar A, Kale V. Wastewater treatment in dairy industries—possibility of reuse. Desalination. 2006;195:141–52.

Harun R, Danquah MK. Influence of acid pre-treatment on microalgal biomass for bioethanol production. Process Biochem. 2011;46:304–9.

Pathak VV, Singh DP, Kothari R, Chopra AK. Phycoremediation of textile wastewater by unicellular microalga Chlorella pyrenoidosa . Cell Mol Biol (Noisy-le-Grand, France). 2014;60:35–40.

CAS   Google Scholar  

Umamaheswari J, Shanthakumar S. Efficacy of microalgae for industrial wastewater treatment: a review on operating conditions, treatment efficiency and biomass productivity. Rev Environ Sci Biotechnol. 2016;15:265–84.

Spolaore P, Joannis-Cassan C, Duran E, Isambert A. Commercial applications of microalgae. J Biosci Bioeng. 2006;101:87–96.

Li-Beisson Y, Peltier G. Third-generation biofuels: current and future research on microalgal lipid biotechnology. OCL. 2013;20:D606.

Andersen RA. Biology and systematics of heterokont and haptophyte algae. Am J Bot. 2004;91:1508–22.

Article   PubMed   Google Scholar  

Beetul K, Bibi SS, Taleb-Hossenkhan N, Bhagooli R, Puchooa D. An investigation of biodiesel production from microalgae found in Mauritian waters. Biofuel Res J. 2014;1:58–64.

Fasahati P, Liu JJ. Process simulation of bioethanol production from brown algae. 8th IFAC Symposium on Advanced Control of Chemical Processes; July 10–13, 2012; The International Federation of Automatic Control. Singapore: Furama Riverfront; 2012. p. 2012.

Hanh V, Minh N. Bioethanol production from marine algae biomass: prospect and troubles. J Viet Environ. 2012;3:25–9.

Chen CY, Zhao XQ, Yen HW, Ho SH, Cheng CL, Lee DJ, et al. Microalgae-based carbohydrates for biofuel production. Biochem Eng J. 2013;78:1–10.

Matos ÂP, Torres RCO, Morioka LRI, Moecke EHS, França KB, Sant’Anna ES. Growing Chlorella vulgaris in photobioreactor by continuous process using concentrated desalination: effect of dilution rate on biochemical composition. Int J Chem Eng. 2014;2014:1–6.

Özçimen D, Inan B. An overview of bioethanol production from algae. New York: Intech Open; 2015.

Book   Google Scholar  

Energy from Algae. Products, market, processes & strategies. Chennai: Clixoo Solutions Pvt Ltd.; 2013.

Singh S, Kate BN, Banerjee UC. Bioactive compounds from cyanobacteria and microalgae: an overview. Crit Rev Biotechnol. 2005;25:73–95.

Costard GS, Machado RR, Barbarino E, Martino RC, Lourenço SO. Chemical composition of five marine microalgae that occur on the Brazilian coast. Int J Fish Aquac. 2012;4:191–201.

Brennan L, Owende P. Biofuels from microalgae—a review of technologies for production, processing, and extractions of biofuels and co-products. Renew Sust Energ Rev. 2010;14:557–77.

Hossain N, Zaini J, Mahlia TMI, Azad AK. Elemental, morphological and thermal analysis of mixed microalgae species from drain water. Renew Energy. 2019;131:617–24.

Torkamani S, Wani SN, Tang YJ, Sureshkumar R. Plasmon-enhanced microalgal growth in mini photobioreactors. Appl Phys Lett. 2010;97:1–4.

Cheng H-H, Whang LM, Chan KC, Chung MC, Wu SH, Liu CP, et al. Biological butanol production from microalgae-based biodiesel residues by Clostridium acetobutylicum . Bioresour Technol. 2015;184:379–85.

Debowski M, Zielinski M, Krzemieniewski M. Microalgae—cultivation method. Pol J Nat Sci. 2012;27:151–64.

Mohanakrishna G, Sandipam S, Pant D. Bioprocesses for waste and wastewater remediation for sustainable energy. Bioremidiation and bioeconomy. Belgium: Academic; 2016. p. 537–65.

Posadas E, Bochon S, Coca M, García-González MC, García-Encina PA, Muñoz R. Microalgae-based agro-industrial wastewater treatment: a preliminary screening of biodegradability. J Appl Phycol. 2014;26:2335–45.

Rasouli Z, Valverde-Pérez B, D’Este M, De Francisci D, Angelidaki I. Nutrient recovery from industrial wastewater as single cell protein by a co-culture of green microalgae and methanotrophs. Biochem Eng J. 2018;134:129–35.

Cheah WY, Ling TC, Show PL, Juan JC, Chang J-S, Lee D-J. Cultivation in wastewaters for energy: a microalgae platform. Appl Energy. 2016;179:609–25.

Zhang TY, Hu HY, Wu YH, Zhuang LL, Xu XQ, Wang XX, et al. Promising solutions to solve the bottlenecks in the large-scale cultivation of microalgae for biomass/bioenergy production. Renew Sust Energy Rev. 2016;60:1602–14.

Ganesh D, Gowrishankar G, editors. Effect of nano-fuel additive on emission reduction in a biodiesel fuelled CI engine. In: International conference on electrical and control engineering; 2011. p. 16–18.

Sabarish R, Mohankumar D, Prem MJK, Manavalan S. Experimental study of nano additive with biodiesel and its blends for diesel engine. Int J Pure Appl Math. 2018;118:967–79.

Manibharathi S, Annadurai B, Chandraprakash R. Experimental investigation of CI engine performance by nano additive in biofuel. Int J Sci Eng Technol Res. 2014;3:3303–7.

Karthikeyan S, Prathima A. Analysis of emissions from use of an algae biofuel with nano-ZrO 2 . Energy Source Part A. 2017;39:473–9.

Karthikeyan S, Prathima A. Environmental effect of CI engine using microalgae methyl ester with doped nano additives. Transp Res D Transp Environ. 2017;50:385–96.

Prabu A. Nanoparticles as additive in biodiesel on the working characteristics of a DI diesel engine. Ain Shams Eng J. 2018;9:2343–9.

Jayanthi P, Srinivasa RM. Effects of nanoparticles additives on performance and emissions characteristics of a DI diesel engine fuelled with biodiesel. Int J Adv Eng Technol. 2016;9:689–95.

Devarajan Y, Munuswamy DB, Mahalingam A. Influence of nano-additive on performance and emission characteristics of a diesel engine running on neat neem oil biodiesel. Environ Sci Pollut Res Int. 2018;25:26167–72.

Ranaware AA, Satpute ST. Correlation between effects of cerium oxide nanoparticles and ferrofluid on the performance and emission characteristics of a CI engine. IOSR J Mech Civil Eng. 2002;2278:55–9.

Steele JM, Grady NK, Nordlander P, Halas NJ. Plasmon hybridization in complex nanostructures. In: Brongersma ML, Kik PG, editors. Surface plasmon nanophotonics. Dordrecht: Springer; 2007. p. 183–96.

Zimmerman WB, Hewakandamby BN, Tesař V, Bandulasena HCH, Omotowa OA. On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation. Food Bioprod Process. 2009;87:215–27.

Zimmerman WB, Tesař V, Bandulasena H. Towards energy efficient nanobubble generation with fluidic oscillation. Curr Opin Colloid Interface Sci. 2011;16:350–6.

Silitonga AS, Atabani AE, Mahlia TMI, Masjuki HH, Badruddin IA, Mekhilef S. A review on prospect of Jatropha curcas for biodiesel in Indonesia. Renew Sust Energy Rev. 2011;15:3733–56.

Chen S-Y, Mochizuki T, Abe Y, Toba M, Yoshimura Y. Ti-incorporated SBA-15 mesoporous silica as an efficient and robust Lewis solid acid catalyst for the production of high-quality biodiesel fuels. Appl Catal B Environ. 2014;148–149:344–56.

Sharma RV, Dalai AK. Synthesis of bio-lubricant from epoxy canola oil using sulfated Ti-SBA-15 catalyst. Appl Catal B Environ. 2013;142–143:604–14.

Silva Â, Wilson K, Lee AF, dos Santos VC, Cons Bacilla AC, Mantovani KM, et al. Nb 2 O 5 /SBA-15 catalyzed propanoic acid esterification. Appl Catal B Environ. 2017;205:498–504.

Liu G, Liao Y, Wu Y, Ma X. Synthesis gas production from microalgae gasification in the presence of Fe 2 O 3 oxygen carrier and CaO additive. Appl Energy. 2018;212:955–65.

Liu X, Piao X, Wang Y, Zhu S, He H. Calcium methoxide as a solid base catalyst for the transesterification of soybean oil to biodiesel with methanol. Fuel. 2008;87:1076–82.

Gan Y, Qiao L. Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles. Combust Flame. 2011;158:354–68.

Kreissl HT, Nakagawa K, Peng Y-K, Koito Y, Zheng J, Tsang SCE. Niobium oxides: correlation of acidity with structure and catalytic performance in sucrose conversion to 5-hydroxymethylfurfural. J Catal. 2016;338:329–39.

Sinha VK, Sharlin. A review approach on exhaust emission reduction by nanoparticle additives in diesel engine. Int J Innov Res Sci Eng Technol. 2017;6:15046–50.

Silitonga, Masjuki HH, Ong HC, Sebayang A, Dharma S, Kusumo F, et al. Evaluation of the engine performance and exhaust emissions of biodiesel–bioethanol–diesel blends using kernel-based extreme learning machine. Energy. 2018;159:1075–87.

Damanik N, Ong HC, Tong CW, Mahlia TMI, Silitonga AS. A review on the engine performance and exhaust emission characteristics of diesel engines fueled with biodiesel blends. Environ Sci Pollut Res Int. 2018;25:15307–25.

Karthikeyan S. Environmental effects of nano additive Co 3 O 4 in grape seed oil biofuel fuelled in CI engine. Res J Chem Environ. 2014;18:14–8.

Karthikeyan S, Prabhakaran TD. Emission analysis of Botryococcus braunii algal biofuel using Ni-Doped ZnO nano additives for IC engines. Energ Source Part A. 2018;40:1060–7.

Ong HC, Milano J, Silitonga AS, Hassan MH, Shamsuddin AH, Wang CT, et al. Biodiesel production from Calophyllum inophyllum -Ceiba pentandra oil mixture: optimization and characterization. J Clean Prod. 2019;219:183–98.

Dhinesh B, Annamalai M. A study on performance, combustion and emission behaviour of diesel engine powered by novel nano nerium oleander biofuel. J Clean Prod. 2018;196:74–83.

Jones M, Li CH, Afjeh A, Peterson G. Experimental study of combustion characteristics of nanoscale metal and metal oxide additives in biofuel (ethanol). Nano Res Lett. 2011;6:246.

Shaafi T, Sairam K, Gopinath A, Kumaresan G, Velraj R. Effect of dispersion of various nanoadditives on the performance and emission characteristics of a CI engine fuelled with diesel, biodiesel and blends—a review. Renew Sust Energ Rev. 2015;49:563–73.

Kamiński W, Tomczak E, Górak A. Biobutanol—production and purification methods. Ecol Chem Eng. 2011;18:31–7.

Phwan CK, Ong HC, Chen W-H, Ling TC, Ng EP, Show PL. Overview: comparison of pretreatment technologies and fermentation processes of bioethanol from microalgae. Energy Convers Manag. 2018;173:81–94.

Shokrkar H, Ebrahimi S, Zamani M. Bioethanol production from acidic and enzymatic hydrolysates of mixed microalgae culture. Fuel. 2017;200:380–6.

Tsukahara K, Sawayama S. Liquid fuel production using microalgae. J Jpn Petrol Inst. 2005;48:251–9.

Hossain N, Zaini J, Jalil R, Mahlia TMI. The efficacy of the period of saccharification on oil palm ( Elaeis guineensis) trunk sap hydrolysis. Int J Technol. 2018;9:652–62.

Ranjith Kumar R, Hanumantha Rao P, Arumugam M. Lipid extraction methods from microalgae: a comprehensive review. Front Energy Res. 2015;2:1–9.

Feltes MMC, de Oliveira D, Ninow JL, de Oliveira JV, editors. An overview of enzyme-catalyzed reactions and alternative feedstock for biodiesel production. New York: Federal University of Santa Catarina, Brazil; Intech; 2011.

Wang S, Zhu J, Dai L, Zhao X, Liu D, Du W. A novel process on lipid extraction from microalgae for biodiesel production. Energy. 2016;115:963–8.

Rahman MA, Aziz MA, Al-khulaidi RA, Sakib N, Islam M. Biodiesel production from microalgae Spirulina maxima by two step process: optimization of process variable. J Radiat Res Appl Sci. 2017;10:140–7.

Mohamadzadeh Shirazi H, Karimi-Sabet J, Ghotbi C. Biodiesel production from Spirulina microalgae feedstock using direct transesterification near supercritical methanol condition. Bioresour Technol. 2017;239:378–86.

Cheng J, Qiu Y, Huang R, Yang W, Zhou J, Cen K. Biodiesel production from wet microalgae by using graphene oxide as solid acid catalyst. Bioresour Technol. 2016;221:344–9.

Kings AJ, Raj RE, Miriam LRM, Visvanathan MA. Cultivation, extraction and optimization of biodiesel production from potential microalgae Euglena sanguinea using eco-friendly natural catalyst. Energy Convers Manag. 2017;141:224–35.

Pan Y, Alam MA, Wang Z, Huang D, Hu K, Chen H, et al. One-step production of biodiesel from wet and unbroken microalgae biomass using deep eutectic solvent. Bioresour Technol. 2017;238:157–63.

Ehimen EA, Sun ZF, Carrington CG, Birch EJ, Eaton-Rye JJ. Anaerobic digestion of microalgae residues resulting from the biodiesel production process. Appl Energy. 2011;88:3454–63.

Razon LF, Tan RR. Net energy analysis of the production of biodiesel and biogas from the microalgae: haematococcus pluvialis and nannochloropsis. Appl Energy. 2011;88:3507–14.

Silitonga AS, Masjuki HH, Ong HC, Mahlia TMI, Kusumo F. Optimization of extraction of lipid from Isochrysis galbana microalgae species for biodiesel synthesis. Energy Source Part A. 2017;39:1167–75.

Hu Z, Ma X, Li L, Wu J. The catalytic pyrolysis of microalgae to produce syngas. Energy Convers Manag. 2014;85:545–50.

Hong Y, Chen W, Luo X, Pang C, Lester E, Wu T. Microwave-enhanced pyrolysis of macroalgae and microalgae for syngas production. Bioresour Technol. 2017;237:47–56.

Azadi P, Brownbridgea GPE, Mosbacha S, Inderwildib OR, Krafta M. Production of Biorenewable hydrogen and syngas via algae gasification: a sensitivity analysis. In: The 6th international conference on applied energy—ICAE2014: energy procedia; 2014. p. 2767–2770.

Beneroso D, Bermudez JM, Arenillas A, Menendez JA. Microwave pyrolysis of microalgae for high syngas production. Bioresour Technol. 2013;144:240–6.

Freitas ACD, Guirardello R. Thermodynamic analysis of supercritical water gasification of microalgae biomass for hydrogen and syngas production. Chem Eng Trans. 2013;32:553–8.

Cui Y, Rashid N, Hu N, Rehman MSU, Han J-I. Electricity generation and microalgae cultivation in microbial fuel cell using microalgae-enriched anode and bio-cathode. Energy Convers Manag. 2014;79:674–80.

Khazraee ZM, Kariminia HR, Vosoughi M. Electricity generation, desalination and microalgae cultivation in a biocathode-microbial desalination cell. J Environ Chem Eng. 2017;5:843–8.

Costa C, Hadiyanto A. Bioelectricity production from microalgae-microbial fuel cell technology (MMFC). MATEC Web Conf. 2018;156:2–4.

Stucki S, Vogel F, Ludwig C, Haiduc AG, Brandenberger M. Catalytic gasification of algae in supercritical water for biofuel production and carbon capture. Energy Environ Sci. 2009;2:535.

Liu C-H, Chang C-Y, Liao Q, Zhu X, Liao C-F, Chang J-S. Biohydrogen production by a novel integration of dark fermentation and mixotrophic microalgae cultivation. Int J Hydrog Energy. 2013;38:15807–14.

Molino A, Nanna F, Ding Y, Bikson B, Braccio G. Biomethane production by anaerobic digestion of organic waste. Fuel. 2013;103:1003–9.

Gruber-Brunhumer MR, Jerney J, Zohar E, Nussbaumer M, Hieger C, Bromberger P. Associated effects of storage and mechanical pre-treatments of microalgae biomass on biomethane yields in anaerobic digestion. Biomass Bioenergy. 2016;93:259–68.

Visceral. Biophotolysis: green hydrogen fuel production: Daily Kos. 2008. http://www.dailykos.com/story/2008/6/27/543096/ -.

Carrillo-Reyes J, Buitron G. Biohydrogen and methane production via a two-step process using an acid pretreated native microalgae consortium. Bioresour Technol. 2016;221:324–30.

Tapia-Venegas E, Ramirez-Morales JE, Silva-Illanes F, Toledo-Alarcón J, Paillet F, Escudie R. Biohydrogen production by dark fermentation: scaling-up and technologies integration for a sustainable system. Rev Environ Sci Biotechnol. 2015;14:761–85.

Rashid N, Rehman MSU, Memon S, Rahman ZU, Lee K, Han JI. Current status, barriers and developments in biohydrogen production by microalgae. Renew Sustain Energ Rev. 2013;22:571–9.

Ma Z, Li C, Su H. Dark bio-hydrogen fermentation by an immobilized mixed culture of Bacillus cereus and Brevumdimonas naejangsanensis . Renew Energ. 2017;105:458–64.

Nagarajan D, Lee DJ, Kondo A, Chang JS. Recent insights into biohydrogen production by microalgae from biophotolysis to dark fermentation. Bioresour Technol. 2016;227:373–87.

Elliott DC, Biller P, Ross AB, Schmidt AJ, Jones SB. Hydrothermal liquefaction of biomass: developments from batch to continuous process. Bioresour Technol. 2015;178:147–56.

Yu KL, Show PL, Ong HC, Ling TC, Chi-Wei Lan J, Chen WH, et al. Microalgae from wastewater treatment to biochar—feedstock preparation and conversion technologies. Energy Convers Manag. 2017;150:1–13.

Yu KL, Lau BF, Show PL, Ong HC, Ling TC, Chen W-H, et al. Recent developments on algal biochar production and characterization. Bioresour Technol. 2017;246:2–11.

Chaiwong K, Kiatsiriroat T, Vorayos N, Thararax C. Study of bio-oil and bio-char production from algae by slow pyrolysis. Biomass Bioenergy. 2013;56:600–6.

Huang Y, Chen Y, Xie J, Liu H, Yin X, Wu C. Bio-oil production from hydrothermal liquefaction of high-protein high-ash microalgae including wild Cyanobacteria sp. and cultivated Bacillariophyta sp. Fuel. 2016;183:9–19.

Zainan NH, Srivatsa SC, Li F, Bhattacharya S. Quality of bio-oil from catalytic pyrolysis of microalgae Chlorella vulgaris . Fuel. 2018;223:12–9.

Chang Z, Duan P, Xu Y. Catalytic hydropyrolysis of microalgae: influence of operating variables on the formation and composition of bio-oil. Bioresour Technol. 2015;184:349–54.

Shamsul NS, Kamarudin SK, Rahman NA. Conversion of bio-oil to bio gasoline via pyrolysis and hydrothermal: a review. Renew Sust Energ Rev. 2017;80:538–49.

Ferreira A, Soares Dias A, Silva C, Costa M. Bio-oil and bio-char characterization from microalgal biomass. In: V Conferência Nacional de Mecânica dos Fluidos, Termodinâmica e Energia; 11–12 September, 2014; Porto, Portugal. 2014. p. 99–104.

Pandian AK, Ramakrishnan RBB, Devarajan Y. Emission analysis on the effect of nanoparticles on neat biodiesel in unmodified diesel engine. Environ Sci Pollut Res Int. 2017;24:23273–8.

Basha JS, Anand RB. The influence of nano additive blended biodiesel fuels on the working characteristics of a diesel engine. J Braz Soc Mech Sci. 2013;35:257–64.

Krishna VSR, Reddy KVK, Prasad BD. Experimental investigations on LHR diesel engine using biodiesel with nano additive. Int J Lat Eng Manag Res. 2018;3:89–95.

Wang SK, Wang F, Hu YR, Stiles AR, Guo C, Liu CZ. Magnetic flocculant for high efficiency harvesting of microalgal cells. ACS Appl Mater Inter. 2014;6:109–15.

Lee K, Lee SY, Na JG, Jeon SG, Praveenkumar R, Kim DM, et al. Magnetophoretic harvesting of oleaginous Chlorella sp. by using biocompatible chitosan/magnetic nanoparticle composites. Bioresour Technol. 2013;149:575–8.

Download references

The authors would also like to acknowledge University of Technology Sydney for seed fund with (Org Unit 321740) and Account Number (2232397) for supporting this research.

Author information

Authors and affiliations.

Department of Civil and Infrastructure Engineering, School of Engineering, RMIT University, Melbourne, VIC, 3001, Australia

Nazia Hossain

School of Information, Systems and Modeling, Faculty of Engineering and Information Technology, University of Technology Sydney, Sydney, NSW, 2007, Australia

T. M. I. Mahlia

Research Centre for Nano Materials and Energy Technology (RCNMET), School of Science and Technology, Sunway University, No. 5, Jalan University, 47500, Bandar Sunway, Petaling Jaya, Malaysia

Department of Engineering, Lancaster University, Lancaster, LA1 4YW, UK

You can also search for this author in PubMed   Google Scholar

Contributions

The authors solely prepared this review article. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Nazia Hossain .

Ethics declarations

Ethics approval and consent to participate.

The facts and views in the manuscript are ours and we are totally responsible for authenticity, validity, and originality. We also declare that manuscripts are our original work and we have not copied from anywhere else. There is no plagiarism in our manuscripts.

Consent for publication

We undertake and agree that the manuscripts submitted to your journal have not been published elsewhere and have not been simultaneously submitted to other journals.

Competing interests

The authors declare that they have no competing interests. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/ ) applies to the data made available in this article, unless otherwise stated.

Reprints and permissions

About this article

Cite this article.

Hossain, N., Mahlia, T.M.I. & Saidur, R. Latest development in microalgae-biofuel production with nano-additives. Biotechnol Biofuels 12 , 125 (2019). https://doi.org/10.1186/s13068-019-1465-0

Download citation

Received : 30 January 2019

Accepted : 10 May 2019

Published : 20 May 2019

DOI : https://doi.org/10.1186/s13068-019-1465-0

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Microalgal biofuel
• Nano-additives

Biotechnology for Biofuels and Bioproducts

ISSN: 2731-3654

• General enquiries: [email protected]

REVIEW article

Scope of algae as third generation biofuels.

• Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Kapurthala, Punjab, India

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

Introduction

The requirement of energy for the mankind is increasing day by day. The major source of energy is based on fossil fuels only. Thus, the scarcity of fossil fuels, rising price of petroleum based fuels, energy protection, and increased global warming resulted in focusing on renewable energy sources such as solar, wind, hydro, tidal, and biomass worldwide ( Goldemberg and Guardabassi, 2009 ; Dragone et al., 2010 ; Rajkumar et al., 2014 ).

Different biomass from various sources like agricultural, forestry, and aquatic have been taken into consideration as the feedstocks for the production of several biofuels such as biodiesel ( Boyce et al., 2008 ; Yanqun et al., 2008 ), bioethanol ( Behera et al., 2014 ), biohydrogen ( Marques et al., 2011 ), bio-oil ( Shuping et al., 2010 ), and biogas ( Hughes et al., 2012 ; Singh et al., 2014 ). However, the environmental impact raised from burning of fuels has a great impact on carbon cycle (carbon balance), which is related to the combustion of fossil fuels. Besides, exhaustion of different existing biomass without appropriate compensation resulted in huge biomass scarcity, emerging environmental problems such as deforestation and loss of biodiversity ( Goldemberg, 2007 ; Li et al., 2008 ; Saqib et al., 2013 ).

Recently, researchers and entrepreneurs have focused their interest, especially on the algal biomass as the alternative feedstock for the production of biofuels. Moreover, algal biomass has no competition with agricultural food and feed production ( Demirbas, 2007 ). The photosynthetic microorganisms like microalgae require mainly light, carbon dioxide, and some nutrients (nitrogen, phosphorus, and potassium) for its growth, and to produce large amount of lipids and carbohydrates, which can be further processed into different biofuels and other valuable co-products ( Brennan and Owende, 2010 ; Nigam and Singh, 2011 ). Interestingly, the low content of hemicelluloses and about zero content of lignin in algal biomass results in an increased hydrolysis and/or fermentation efficiency ( Saqib et al., 2013 ). Other than biofuels, algae have applications in human nutrition, animal feed, pollution control, biofertilizer, and waste water treatment ( Thomas, 2002 ; Tamer et al., 2006 ; Crutzen et al., 2007 ; Hsueh et al., 2007 ; Choi et al., 2012 ). Therefore, the aim of the current review is to explore the scope of algae for the production of different biofuels and evaluation of its potential as an alternative feedstock.

Algae: Source of Biofuels

Generally, algae are a diverse group of prokaryotic and eukaryotic organisms ranging from unicellular genera such as Chlorella and diatoms to multicellular forms such as the giant kelp, a large brown alga that may grow up to 50 m in length ( Li et al., 2008 ). Algae can either be autotrophic or heterotrophic. The autotrophic algae require only inorganic compounds such as CO 2 , salts, and a light energy source for their growth, while the heterotrophs are non-photosynthetic, which require an external source of organic compounds as well as nutrients as energy sources ( Brennan and Owende, 2010 ). Microalgae are very small in sizes usually measured in micrometers, which normally grow in water bodies or ponds. Microalgae contain more lipids than macroalgae and have the faster growth in nature ( Lee et al., 2014a ). There are about more than 50,000 microalgal species out of which only about 30,000 species have been taken for the research study ( Surendhiran and Vijay, 2012 ; Richmond and Qiang, 2013 ; Rajkumar et al., 2014 ). The short harvesting cycle of algae is the key advantage for its importance, which is better than other conventional crops having harvesting cycle of once or twice in a year ( Chisti, 2007 ; Schenk et al., 2008 ). Therefore, the main focus has been carried out on algal biomass for its application in biofuel area.

There are several advantages of algal biomass for biofuels production: (a) ability to grow throughout the year, therefore, algal oil productivity is higher in comparison to the conventional oil seed crops; (b) higher tolerance to high carbon dioxide content; (c) the consumption rate of water is very less in algae cultivation; (d) no requirement of herbicides or pesticides in algal cultivation; (e) the growth potential of algal species is very high in comparison to others; (f) different sources of wastewater containing nutrients like nitrogen and phosphorus can be utilized for algal cultivation apart from providing any additional nutrient; and (g) the ability to grow under harsh conditions like saline, brackish water, coastal seawater, which does not affect any conventional agriculture ( Spolaore et al., 2006 ; Dismukes et al., 2008 ; Dragone et al., 2010 ). However, there are several disadvantages of algal biomass as feedstock such as the higher cultivation cost as compared to conventional crops. Similarly, harvesting of algae require high energy input, which is approximately about 20–30% of the total cost of production. Several techniques such as centrifugation, flocculation, floatation, sedimentation, and filtration are usually used for harvesting and concentrating the algal biomass ( Demirbas, 2010 ; Ho et al., 2011 ).

The algae can be converted into various types of renewable biofuels including bioethanol, biodiesel, biogas, photobiologically produced biohydrogen, and further processing for bio-oil and syngas production through liquefaction and gasification, respectively ( Kraan, 2013 ). The conversion technologies for utilizing algal biomass to energy sources can be categorized into three different ways, i.e., biochemical, chemical, and thermochemical conversion and make an algal biorefinery, which has been depicted in Figure 1 . The biofuel products derived from algal biomass using these conversion routes have been explored in detail in the subsequent sections.

Figure 1. Algal biomass conversion process for biofuel production .

Biodiesel Production

Biodiesel is a mixture of monoalkyl esters of long chain fatty acids [fatty acid methyl esters (FAME)], which can be obtained from different renewable lipid feedstocks and biomass. It can be directly used in different diesel engines ( Clark and Deswarte, 2008 ; Demirbas, 2009 ). Studies to explore the microalgae as feedstock for the production of liquid fuels had been started for the mid-1980s. In order to solve the energy crisis, the extraction of lipids from diatoms was attempted by some German scientists during the period of World War-II ( Cohen et al., 1995 ). The higher oil yield in algal biomass as compared to oil seed crops makes the possibility to convert into the biodiesel economically using different technologies. A comparative study between algal biomass and terrestrial plants for the production of biodiesel has been depicted in Table 1 . The oil productivity (mass of oil produced per unit volume of the microalgal broth per day) depends on the algal growth rate and the biomass content of the species. The species of microalgae such as Kirchneriella lunaris , Ankistrodesmus fusiformis , Chlamydocapsa bacillus , and Ankistrodesmus falcatus with high levels of polyunsaturated FAME are generally preferred for the production of biodiesel ( Nascimento et al., 2013 ). They commonly multiply their biomass with doubling time of 24 h during exponential growth. Oil content of microalgae is generally found to be very high, which exceed up to 80% by weight of its dry biomass. About 5,000–15,000 gal of biodiesel can be produced from algal biomass per acre per year, which reflects its potentiality ( Spolaore et al., 2006 ; Chisti, 2007 ).

Table 1 . Comparative study between algal biomass and terrestrial plants for biodiesel production .

However, there are some standards such as International Biodiesel Standard for Vehicles (EN14214) and American Society for Testing and Materials (ASTM), which are required to comply with the algal based biodiesel on the physical and chemical properties for its acceptance as substitute to fossil fuels ( Brennan and Owende, 2010 ). The higher degree of polyunsaturated fatty acids of algal oils as compared to vegetable oils make susceptible for oxidation in the storage and further limits its utilization ( Chisti, 2007 ). Some researchers have reported the different advantages of the algal biomass for the biodiesel production due to its high biomass growth and oil productivity in comparison to best oil crops ( Chisti, 2007 ; Hossain et al., 2008 ; Hu et al., 2008 ; Rosenberg et al., 2008 ; Schenk et al., 2008 ; Rodolfi et al., 2009 ; Mutanda et al., 2011 ).

Algal biodiesel production involves biomass harvesting, drying, oil extraction, and further transesterification of oil, which have been described as below.

Harvesting and Drying of Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. Harvesting of algal biomass and further drying is important prior to mechanical and solvent extraction for the recovery of oil. Macroalgae can be harvested using nets, which require less energy while microalgae can be harvested by some conventional processes, which include filtration ( Rossignol et al., 1999 ) flocculation ( Liu et al., 2013 ; Prochazkova et al., 2013 ), centrifugation ( Heasman et al., 2008 ), foam fractionation ( Csordas and Wang, 2004 ), sedimentation, froth floatation, and ultrasonic separation ( Bosma et al., 2003 ). Selection of harvesting method depends on the type of algal species.

Drying is an important method to extend shelf-life of algal biomass before storage, which avoids post-harvest spoilage ( Munir et al., 2013 ). Most of the efficient drying methods like spray-drying, drum-drying, freeze drying or lyophilization, and sun-drying have been applied on microalgal biomass ( Leach et al., 1998 ; Richmond, 2004 ; Williams and Laurens, 2010 ). Sun-drying is not considered as a very effective method due to presence of high water content in the biomass ( Mata et al., 2010 ). However, Prakash et al. (2007) used simple solar drying device and succeed in drying Spirulina and Scenedesmus having 90% of moisture content. Widjaja et al. (2009) showed the effectiveness of drying temperature during lipid extraction of algal biomass, which affects both concentration of triglycerides and lipid yield. Further, all these processes possess safety and health issues ( Singh and Gu, 2010 ).

Extraction of Oil from Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. In the literature, there are different methods of oil extraction from algae, such as mechanical and solvent extraction ( Li et al., 2014 ). However, the extraction of lipids from microalgae is costly and energy intensive process.

Mechanical oil extraction

The oil from nuts and seeds is extracted mechanically using presses or expellers, which can also be used for microalgae. The algal biomass should be dried prior to this process. The cells are just broken down with a press to leach out the oil. About 75% of oil can be recovered through this method and no special skill is required ( Munir et al., 2013 ). Topare et al. (2011) extracted oil through screw expeller by mechanical pressing (by piston) and osmotic shock method and recovered about 75% of oil from the algae. However, more extraction time is required as compared to other methods, which make the process unfavorable and less effective ( Popoola and Yangomodou, 2006 ).

Solvent based oil extraction

Oil extraction using solvent usually recovers almost all the oil leaving only 0.5–0.7% residual oil in the biomass. Therefore, the solvent extraction method has been found to be suitable method rather than the mechanical extraction of oil and fats ( Topare et al., 2011 ). Solvent extraction is another method of lipid extraction from microalgae, which involves two stage solvent extraction systems. The amount of lipid extracted from microalgal biomass and further yield of highest biodiesel depends mainly on the solvent used. Several organic solvents such as chloroform, hexane, cyclo-hexane, acetone, and benzene are used either solely or in mixed form ( Afify et al., 2010 ). The solvent reacts on algal cells releasing oil, which is recovered from the aqueous medium. This occurs due to the nature of higher solubility of oil in organic solvents rather than water. Further, the oil can be separated from the solvent extract. The solvent can be recycled for next extraction. Out of different organic solvents, hexane is found to be most effective due to its low toxicity and cost ( Rajvanshi and Sharma, 2012 ; Ryckebosch et al., 2012 ).

In case of using mixed solvents for oil extraction, a known quantity of the solvent mixture is used, for example, chloroform/methanol in the ratio 2:1 (v/v) for 20 min using a shaker and followed by the addition of mixture, i.e., chloroform/water in the ratio of 1:1 (v/v) for 10 min ( Shalaby, 2011 ). Similarly, Pratoomyot et al. (2005) extracted oil from different algal species using the solvent system chloroform/methanol in the ratio of 2:1 (v/v) and found different fatty acid content. Ryckebosch et al. (2012) optimized an analytical procedure and found chloroform/methanol in the ratio 1:1 as the best solvent mixture for the extraction of total lipids. Similarly, Lee et al. (1998) extracted lipid from the green alga Botryococcus braunii using different solvent system and obtained the maximum lipid content with chloroform/methanol in the ratio of 2:1. Hossain et al., 2008 used hexane/ether in the ratio 1:1 (v/v) for oil extraction and allowed to settle for 24 h. Using a two-step process, Fajardo et al. (2007) reported about 80% of lipid recovery using ethanol and hexane in the two steps for the extraction and purification of lipids. Therefore, a selection of a most suitable solvent system is required for the maximum extraction of oil for an economically viable process.

Lee et al. (2009) compared the performance of various disruption methods, including autoclaving, bead-beating, microwaves, sonication, and using 10% NaCl solution in the extraction of Botryococcus sp., Chlorella vulgaris , and Scenedesmus sp, using a mixture of chloroform and methanol (1:1).

Transesterification

This is a process to convert algal oil to biodiesel, which involves multiple steps of reactions between triglycerides or fatty acids and alcohol. Different alcohols such as ethanol, butanol, methanol, propanol, and amyl alcohol can be used for this reaction. However, ethanol and methanol are used frequently for the commercial development due to its low cost and its physical and chemical advantages ( Bisen et al., 2010 ; Surendhiran and Vijay, 2012 ). The reaction can be performed in the presence of an inorganic catalyst (acids and alkalies) or lipase enzyme. In this method, about 3 mol of alcohol are required for each mole of triglyceride to produce 3 mol of methyl esters (biodiesel) and 1 mol of glycerol (by-product) ( Meher et al., 2006 ; Chisti, 2007 ; Sharma and Singh, 2009 ; Surendhiran and Vijay, 2012 ; Stergiou et al., 2013 ) (Figure 2 ). Glycerol is denser than biodiesel and can be periodically or continuously removed from the reactor in order to drive the equilibrium reaction. The presence of methanol, the co-solvent that keeps glycerol and soap suspended in the oil, is known to cause engine failure ( Munir et al., 2013 ). Thus, the biodiesel is recovered by repeated washing with water to remove glycerol and methanol ( Chisti, 2007 ).

Figure 2. Transesterification of oil to biodiesel . R 1–3 are hydrocarbon groups.

The reaction rate is very slow by using the acid catalysts for the conversion of triglycerides to methyl esters, whereas the alkali-catalyzed transesterification reaction has been reported to be 4000 times faster than the acid-catalyzed reaction ( Mazubert et al., 2013 ). Sodium and potassium hydroxides are the two commercial alkali catalysts used at a concentration of about 1% of oil. However, sodium methoxide has become the better catalyst rather than sodium hydroxide ( Singh et al., 2006 ).

Kim et al. (2014) used Scenedesmus sp. for the biodiesel production through acid and alkali transesterification process. They reported 55.07 ± 2.18%, based on lipid by wt of biodiesel conversion using NaOH as an alkaline catalyst than using H 2 SO 4 as 48.41 ± 0.21% of biodiesel production. In comparison to acid and alkalies, lipases as biocatalyst have different advantages as the catalysts due to its versatility, substrate selectivity, regioselectivity, enantioselectivity, and high catalytic activity at ambient temperature and pressure ( Knezevic et al., 2004 ). It is not possible by some lipases to hydrolyze ester bonds at secondary positions, while some other group of enzymes hydrolyzes both primary and secondary esters. Another group of lipases exhibits fatty acids selectivity, and allow to cleave ester bonds at particular type of fatty acids. Luo et al. (2006) cloned the lipase gene lipB68 and expressed in Escherichia coli BL21 and further used it as a catalyst for biodiesel production. LipB68 could catalyze the transesterification reaction and produce biodiesel with a yield of 92% after 12 h, at a temperature of 20°C. The activity of the lipase enzyme with such a low temperature could provide substantial savings in energy consumption. However, it is rarely used due to its high cost ( Sharma et al., 2001 ).

Extractive transesterification

It involves several steps to produce biodiesel such as drying, cell disruption, oils extraction, transesterification, and biodiesel refining ( Hidalgo et al., 2013 ). The main problems are related with the high water content of the biomass (over 80%), which overall increases the cost of whole process.

In situ transesterification

This method skips the oil extraction step. The alcohol acts as an extraction solvent and an esterification reagent as well, which enhances the porosity of the cell membrane. Yields found are higher than via the conventional route, and waste is also reduced. Industrial biodiesel production involves release of extraction solvent, which contributes to the production of atmospheric smog and to global warming. Thus, simplification of the esterification processes can reduce the disadvantages of this attractive bio-based fuel. The single-step methods can be attractive solutions to reduce chemical and energy consumption in the overall biodiesel production process ( Patil et al., 2012 ). A comparison of direct and extractive transesterification is given in Table 2 .

Table 2 . Comparison of extractive transesterification and in situ methods ( Haas and Wagner, 2011 ) .

Bioethanol Production

Several researchers have been reported bioethanol production from certain species of algae, which produce high levels of carbohydrates as reserve polymers. Owing to the presence of low lignin and hemicelluloses content in algae in comparison to lignocellulosic biomass, the algal biomass have been considered more suitable for the bioethanol production ( Chen et al., 2013 ). Recently, attempts have been made (for the bioethanol production) through the fermentation process using algae as the feedstocks to make it as an alternative to conventional crops such as corn and soyabean ( Singh et al., 2011 ; Nguyen and Vu, 2012 ; Chaudhary et al., 2014 ). A comparative study of algal biomass and terrestrial plants for the production of bioethanol has been given in Table 3 . There are different micro and macroalgae such as Chlorococcum sp., Prymnesium parvum , Gelidium amansii , Gracilaria sp., Laminaria sp., Sargassum sp., and Spirogyra sp., which have been used for the bioethanol production ( Eshaq et al., 2011 ; Rajkumar et al., 2014 ). These algae usually require light, nutrients, and carbon dioxide, to produce high levels of polysaccharides such as starch and cellulose. These polysaccharides can be extracted to fermentable sugars through hydrolysis and further fermentation to bioethanol and separated through distillation as shown in Figure 3 .

Table 3 . Comparative study between algal biomass and terrestrial plants for bioethanol production .

Figure 3. Process for bioethanol production from microalgae .

Pre-Treatment and Saccharification

It has been reported that, the cell wall of some species of green algae like Spirogyra and Chlorococcum contain high level of polysaccharides. Microalgae such as C. vulgaris contains about 37% of starch on dry weight basis, which is the best source for bioethanol with 65% conversion efficiency ( Eshaq et al., 2010 ; Lam and Lee, 2012 ). Such polysaccharide based biomass requires additional processing like pre-treatment and saccharification before fermentation ( Harun et al., 2010 ). Saccharification and fermentation can also be carried out simultaneously using an amylase enzyme producing strain for the production of ethanol in a single step. Bioethanol from microalgae can be produced through the process, which is similar to the first generation technologies involving corn based feedstocks. However, there is limited literature available on the fermentation process of microalgae biomass for the production of bioethanol ( Schenk et al., 2008 ; John et al., 2011 ).

The pre-treatment is an important process, which facilitates accessibility of biomass to enzymes to release the monosaccharides. Acid pre-treatment is widely used for the conversion of polymers present in the cell wall to simple forms. The energy consumption in the pre-treatment is very low and also it is an efficient process ( Harun and Danquah, 2011a , b ). Yazdani et al. (2011) found 7% (w/w) H 2 SO 4 as the promising concentration for the pre-treatment of the brown macroalgae Nizimuddinia zanardini to obtain high yield of sugars without formation of any inhibitors. Candra and Sarinah (2011) studied the bioethanol production using red seaweed Eucheuma cottonii through acid hydrolysis. In this study, 5% H 2 SO 4 concentration was used for 2 h at 100°C, which yielded 15.8 g/L of sugars. However, there are other alternatives to chemical hydrolysis such as enzymatic digestion and gamma radiation to make it more sustainable ( Chen et al., 2012 ; Yoon et al., 2012 ; Schneider et al., 2013 ).

Similar to starch, there are certain polymers such as alginate, mannitol, and fucoidan present in the cell wall of various algae, which requires additional processing like pre-treatment and saccharification before fermentation. Another form of storage carbohydrate found in various brown seaweeds and microalgae is laminarin, which can be hydrolyzed by β-1,3-glucanases or laminarinases ( Kumagai and Ojima, 2010 ). Laminarinases can be categorized into two groups such as exo- and endo-glucanases based on the mode of hydrolysis, which usually produces glucose and smaller oligosaccharides as the end product. Both the enzymes are necessary for the complete digestion of laminarin polymer ( Lee et al., 2014b ).

Markou et al. (2013) saccharified the biomass of Spirulina ( Arthrospira platensis ), fermented the hydrolyzate and obtained the maximum ethanol yield of 16.32 and 16.27% (g ethanol /g biomass ) produced after pre-treatment with 0.5 N HNO 3 and H 2 SO 4 , respectively. Yanagisawa et al. (2011) investigated the content of polysaccharide materials present in three types of seaweeds such as sea lettuce ( Ulva pertusa ), chigaiso ( Alaria crassifolia ), and agar weed ( Gelidium elegans ). These seaweeds contain no lignin, which is a positive signal for the hydrolysis of polysaccharides without any pre-treatment. Singh and Trivedi (2013) used Spirogyra biomass for the production of bioethanol using Saccharomyces cerevisiae and Zymomonas mobilis . In a method, they followed acid pre-treatment of algal biomass and further saccharified using α-amylase producing Aspergillus niger . In another method, they directly saccharified the biomass without any pre-treatment. The direct saccharification process resulted in 2% (w/w) more alcohol in comparison to pretreated and saccharified algal biomass. This study revealed that the pre-treatment with different chemicals are not required in case of Spyrogyra , which reflects its economic importance for the production of ethanol. Also, cellulase enzyme has been used for the saccharification of algal biomass containing cellulose. However, this enzyme system is more expensive than amylases and glucoamylases, and doses required for effective cellulose saccharification are usually very high. Trivedi et al. (2013) applied different cellulases on green alga Ulva for saccharification and found highest conversion efficiency of biomass into reducing sugars by using cellulase 22119 rather than viscozyme L, cellulase 22086 and 22128. In this experiment, they found a maximum yield of sugar 206.82 ± 14.96 mg/g with 2% (v/v) enzyme loading for 36 h at a temperature of 45°C.

Fermentation

There are different groups of microorganisms like yeast, bacteria, and fungi, which can be used for the fermentation of the pretreated and saccharified algal biomass under anaerobic process for the production of bioethanol ( Nguyen and Vu, 2012 ). Nowadays, S. cerevisiae and Z. mobilis have been considered as the bioethanol fermenting microorganisms. However, fermentation of mannitol, a polymer present in certain algae is not possible in anaerobic condition using these well known microorganisms and requires supply of oxygen during fermentation, which is possible only by Zymobacter palmae ( Horn et al., 2000 ).

Certain marine red algae contain agar, a polymer of galactose and galactopyranose, which can be used for the production of bioethanol ( Yoon et al., 2010 ). The biomass of red algae can be depolymerized into different monomeric sugars like glucose and galactose. In addition to mannitol and glucose, brown seaweeds contain about 14% of extra carbohydrates in the form of alginate ( Wargacki et al., 2012 ). Horn et al. (2000) reported the presence of alginate, laminaran, mannitol, fucoidan, and cellulose in some brown seaweeds, which are good source of sugars. They fermented brown seaweed extract having mannitol using bacteria Z. palmae and obtained an ethanol yield of about 0.38 g ethanol/g mannitol.

In the literature, there are many advantages supporting microalgae as the promising substrate for bioethanol production. Hon-Nami (2006) used Chlamydomonas perigranulata algal culture and obtained different by-products such as ethanol and butanediol. Similarly, Yanagisawa et al. (2011) obtained glucose and galactose through the saccharification of agar weed (red seaweed) containing glucan and galactan and obtained 5.5% of ethanol concentration through fermentation using S. cerevisiae IAM 4178. Harun et al. (2010) obtained 60% more ethanol in case of lipid extracted microalgal biomass rather than intact algal biomass of Chlorococcum sp. This shows the importance of algal biomass for the production of both biodiesel and bioethanol.

Biogas Production

Recently, biogas production from algae through anaerobic digestion has received a remarkable attention due to the presence of high polysaccharides (agar, alginate, carrageenan, laminaran, and mannitol) with zero lignin and low cellulose content. Mostly, seaweeds are considered as the excellent feedstock for the production of biogas. Several workers have demonstrated the fermentation of various species of algae like Scenedesmus , Spirulina , Euglena , and Ulva for biogas production ( Samson and Leduy, 1986 ; Yen and Brune, 2007 ; Ras et al., 2011 ; Zhong et al., 2012 ; Saqib et al., 2013 ). The production of biogas using algal biomass in comparison to some terrestrial plants is shown in Table 4 .

Table 4 . Comparative study between algal biomass and terrestrial plants for biogas production .

Biogas is produced through the anaerobic transformation of organic matter present in the biodegradable feedstock such as marine algae, which releases certain gases like methane, carbon dioxide, and traces of hydrogen sulfide. The anaerobic conversion process involves basically four main steps. In the first step, the insoluble organic material and higher molecular mass compounds such as lipids, carbohydrates, and proteins are hydrolyzed into soluble organic material with the help of enzyme released by some obligate anaerobes such as Clostridia and Streptococci . The second step is called as acidogenesis, which releases volatile fatty acids (VFAs) and alcohols through the conversion of soluble organics with the involvement of enzymes secreted by the acidogenic bacteria. Further, these VFAs and alcohols are converted into acetic acid and hydrogen using acetogenic bacteria through the process of acetogenesis, which finally metabolize to methane and carbon dioxide by the methanogens ( Cantrell et al., 2008 ; Vergara-Fernandez et al., 2008 ; Brennan and Owende, 2010 ; Romagnoli et al., 2011 ).

Sangeetha et al. (2011) reported the anaerobic digestion of green alga Chaetomorpha litorea with generation of 80.5 L of biogas/kg of dry biomass under 299 psi pressure. Vergara-Fernandez et al. (2008) evaluated digestion of the marine algae Macrocystis pyrifera and Durvillaea antarctica marine algae in a two-phase anaerobic digestion system and reported similar biogas productions of 180.4 (±1.5) mL/g dry algae/day with a methane concentration around 65%. However, in case of algae blend, same methane content was observed with low biogas yield. Mussgnug et al. (2010) reported biogas production from some selected green algal species like Chlamydomonas reinhardtii and Scenedesmus obliquus and obtained 587 and 287 mL biogas/g of volatile solids, respectively. Further, there are few studies, which have been conducted with microalgae showing the effect of different pre-treatment like thermal, ultrasound, and microwave for the high production of biogas ( Gonzalez-Fernandez et al., 2012a , b ; Passos et al., 2013 ).

However, there are different factors, which limit the biogas production such as requirement of larger land area, infrastructure, and heat for the digesters ( Collet et al., 2011 ; Jones and Mayfield, 2012 ). The proteins present in algal cells increases the ammonium production resulting in low carbon to nitrogen ratio, which affects biogas production through the inhibition of growth of anaerobic microorganisms. Also, anaerobic microorganisms are inhibited by the sodium ions. Therefore, it is recommended to use the salt tolerating microorganisms for the anaerobic digestion of algal biomass ( Yen and Brune, 2007 ; Brennan and Owende, 2010 ; Jones and Mayfield, 2012 ).

Biohydrogen Production

Recently, algal biohydrogen production has been considered to be a common commodity to be used as the gaseous fuels or electricity generation. Biohydrogen can be produced through different processes like biophotolysis and photo fermentation ( Shaishav et al., 2013 ). Biohydrogen production using algal biomass is comparative to that of terrestrial plants (Table 5 ). Park et al. (2011) found Gelidium amansii (red alga) as the potential source of biomass for the production of biohydrogen through anaerobic fermentation. Nevertheless, they found 53.5 mL of H 2 from 1 g of dry algae with a hydrogen production rate of 0.518 L H 2 /g VSS/day. The authors found an inhibitor, namely, 5-hydroxymethylfurfural (HMF) produced through the acid hydrolysis of G. amansii that decreases about 50% of hydrogen production due to the inhibition. Thus, optimization of the pre-treatment method is an important step to maximize biohydrogen production, which will be useful for the future direction ( Park et al., 2011 ; Shi et al., 2011 ). Saleem et al. (2012) reduced the lag time for hydrogen production using microalgae Chlamydomonas reinhardtii by the use of optical fiber as an internal light source. In this study, the maximum rate of hydrogen production in the presence of exogenic glucose and optical fiber was reported to be 6 mL/L culture/h, which is higher than other reported values.

Table 5 . Comparative study between algal biomass and terrestrial plants for biohydrogen production .

Some of microalgae like blue green algae have glycogen instead of starch in their cells. This is an exception, which involves oxidation of ferrodoxin by the hydrogenase enzyme activity for the production of hydrogen in anaerobic condition. However, another function of this enzyme is to be involved in the detachment of electrons. Therefore, different researchers have focused for the identification of these enzyme activities having interactions with ferrodoxin and the other metabolic functions for microalgal photobiohydrogen production. They are also involved with the change of these interactions genetically to enhance the biohydrogen production ( Gavrilescu and Chisti, 2005 ; Hankamer et al., 2007 ; Wecker et al., 2011 ; Yacoby et al., 2011 ; Rajkumar et al., 2014 ).

Bio-Oil and Syngas Production

Bio-oil is formed in the liquid phase from algal biomass in anaerobic condition at high temperature. The composition of bio-oil varies according to different feedstocks and processing conditions, which is called as pyrolysis ( Iliopoulou et al., 2007 ; Yanqun et al., 2008 ). There are several parameters such as water, ash content, biomass composition, pyrolysis temperature, and vapor residence time, which affect the bio-oil productivity ( Fahmi et al., 2008 ). However, due to the presence of water, oxygen content, unsaturated and phenolic moieties, crude bio-oil cannot be used as fuel. Therefore, certain treatments are required to improve its quality ( Bae et al., 2011 ). Bio-oils can be processed for power generation with the help of external combustion through steam and organic rankine cycles, and stirling engines. However, power can also be generated through internal combustion using diesel and gas-turbine engines ( Chiaramonti et al., 2007 ). In literature, there are limited studies on algae pyrolysis compared to lignocellulosic biomass. Although, high yields of bio-oil occur through fluidized-bed fast pyrolysis processes, there are several other pyrolysis modes, which have been introduced to overcome their inherent disadvantages of a high level of carrier gas flow and excessive energy inputs ( Oyedun et al., 2012 ). Demirbas (2006) investigated suitability of the microalgal biomass for bio-oil production and found the superior quality than the wood. Porphy and Farid (2012) produced bio-oil from pyrolysis of algae ( Nannochloropsis sp.) at 300°C after lipid extraction, which composed of 50 wt% acetone, 30 wt% methyl ethyl ketone, and 19 wt% aromatics such as pyrazine and pyrrole. Similarly, Choi et al. (2014) carried out pyrolysis study on a species of brown algae Saccharina japonica at a temperature of 450°C and obtained about 47% of bio-oil yield.

Gasification is usually performed at high temperatures (800–1000°C), which converts biomass into the combustible gas mixture through partial oxidation process, called syngas or producer gas. Syngas is a mixture of different gases like CO, CO 2 , CH 4 , H 2 , and N 2 , which can also be produced through normal gasification of woody biomass. In this process, biomass reacts with oxygen and water (steam) to generate syngas. It is a low calorific gas, which can be utilized in the gas turbines or used directly as fuel. Different variety of biomass feedstocks can be utilized for the production of energy through the gasification process, which is an added advantage ( Carvalho et al., 2006 ; Prins et al., 2006 ; Lv et al., 2007 ).

Conclusion and Future Perspectives

Recently, it is a challenge for finding different alternative resources, which can replace fossil fuels. Due to presence of several advantages in algal biofuels like low land requirement for biomass production and high oil content with high productivity, it has been considered as the best resource, which can replace the liquid petroleum fuel. However, one of its bottlenecks is the low biomass production, which is a barrier for industrial production. Also, another disadvantage includes harvesting of biomass, which possesses high energy inputs. For an economic process development in comparison to others, a cost-effective and energy efficient harvesting methods are required with low energy input. Producing low-cost microalgal biofuels requires better biomass harvesting methods, high biomass production with high oil productivity through genetic modification, which will be the future of algal biology. Therefore, use of the standard algal harvesting technique, biorefinery concept, advances in photobioreactor design and other downstream technologies will further reduce the cost of algal biofuel production, which will be a competitive resource in the near future.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are thankful to Prof. Y. K. Yadav, Director, NIRE, Kapurthala for his consistent support to write this review paper. The authors greatly acknowledge the Ministry of New and Renewable Energy, New Delhi, Govt. of India, for providing funds to carry out research work.

Afify, A. M. M., Shanab, S. M., and Shalaby, E. A. (2010). Enhancement of biodiesel production from different species of algae. Grasas y Aceites 61, 416–422. doi: 10.3989/gya.021610

CrossRef Full Text | Google Scholar

Al-Juhaimi, F. Y. (2014). Biogas production through the anaerobic digestion of date palm tree wastes – process optimization. Bioresources 9, 3323–3333. doi:10.15376/biores.9.2

Awolu, O. O., and Layokun, S. K. (2013). Optimization of two-step transesterification production of biodiesel from neem ( Azadirachta indica ) oil. Int. J. Energy Environ. 4, 39. doi:10.1186/2251-6832-4-39

Bae, Y. J., Ryu, C., Jeon, J. K., Park, J., Suh, D. J., Suh, Y. W., et al. (2011). The characteristics of bio-oil produced from the pyrolysis of three marine macroalgae. Bioresour. Technol. 102, 3512–3520. doi:10.1016/j.biortech.2010.11.023

Pubmed Abstract | Pubmed Full Text | CrossRef Full Text | Google Scholar

Behera, S., Mohanty, R. C., and Ray, R. C. (2011). Ethanol production from mahula ( Madhuca latifolia L.) flowers using free and immobilized (in Luffa cylindrical L. sponge discs) cells of Zymomonas mobilis MTCC 92. Ann. Microbiol. 61, 469–474. doi:10.1007/s13213-010-0160-y

Behera, S., Mohanty, R. C., and Ray, R. C. (2014). Batch ethanol production from cassava ( Manihot esculenta Crantz.) flour using Saccharomyces cerevisiae cells immobilized in calcium alginate. Ann. Microbiol. doi:10.1007/s13213-014-0918-8

Bisen, P. S., Sanodiya, B. S., Thakur, G. S., Baghel, R. K., and Prasad, G. B. K. S. (2010). Biodiesel production with special emphasis on lipase-catalyzed transesterification. Biotechnol. Lett. 32, 1019–1030. doi:10.1007/s10529-010-0275-z

Bosma, R., Van Spronsen, W. A., Tramper, J., and Wijffels, R. H. (2003). Ultrasound a new s separation technique to harvest microalgae. J. Appl. Phycol. 15, 143–153. doi:10.1023/A:1023807011027

Boyce, A. N., Chowd-Hury, P., and Naqiuddin, M. (2008). Biodiesel fuel production from algae as renewable energy. Am. J. Biochem. Biotechnol. 4, 250–254. doi:10.3844/ajbbsp.2008.250.254

Brennan, L., and Owende, P. (2010). Biofuels from microalgae-a review of technologies for production, processing, and extractions of biofuels and co-products. Renew. Sustain. Energ. Rev. 14, 557–577. doi:10.1016/j.rser.2009.10.009

Candra, K. P., and Sarinah, S. (2011). Study on bioethanol production using red seaweed Eucheuma cottonii from Bontang sea water. J. Coastal Dev. 15, 45–50. doi:10.4172/1410-5217.10000328

Cantrell, K. B., Ducey, T., Ro, K. S., and Hunt, P. G. (2008). Livestock waste-to-bioenergy generation opportunities. Bioresour. Technol. 99, 7941–7953. doi:10.1016/j.biortech.2008.02.061

Carvalho, A. P., Meireles, L. A., and Malcata, F. X. (2006). Microalgal reactors: a review of enclosed system designs and performances. Biotechnol. Prog. 22, 1490–1506. doi:10.1002/bp060065r

Chaudhary, L., Pradhan, P., Soni, N., Singh, P., and Tiwari, A. (2014). Algae as a feedstock for bioethanol production: new entrance in biofuel world. Int. J. Chem. Technol. Res. 6, 1381–1389.

Google Scholar

Chen, C. Y., Zhao, X. Q., Yen, H. W., Ho, S. H., Cheng, C. L., Bai, F., et al. (2013). Microalgae-based carbohydrates for biofuel production. Biochem. Eng. J. 78, 1–10. doi:10.1016/j.bej.2013.03.006

Chen, R., Yue, Z., Deitz, L., Liu, Y., and Liao, W. (2012). Use of an algal hydrolysate to improve enzymatic hydrolysis of lignocellulose. Bioresour. Technol. 108, 149–154. doi:10.1016/j.biortech.2011.12.143

Cheng, X. Y., and Liu, C. Z. (2011). Hydrogen production via thermophilic fermentation of cornstalk by Clostridium thermocellum . Energy Fuel 25, 1714–1720. doi:10.1021/ef2000344

Chiaramonti, D., Oasmaa, A., and Solantausta, Y. (2007). Power generation using fast pyrolysis liquids from biomass. Renew. Sustain. Energ. Rev. 11, 1056–1086. doi:10.1016/j.rser.2005.07.008

Chisti, Y. (2007). Biodiesel from microalgae. Biotechnol. Adv. 25, 294–306. doi:10.1016/j.biotechadv.2007.02.001

Choi, J. H., Woo, H. C., and Suh, D. J. (2014). Pyrolysis of seaweeds for bio-oil and bio-char production. Chem. Eng. Trans. 37, 121–126. doi:10.1016/j.biortech.2014.09.068

Choi, W., Han, J., Lee, C., Song, C., Kim, J., Seo, Y., et al. (2012). Bioethanol production from Ulva pertusa kjellman by high-temperature liquefaction. Chem. Biochem. Eng. 26, 15–21.

Clark, J., and Deswarte, F. (2008). Introduction to Chemicals from Biomass . Wiley series in renewable resources. Hoboken: John Wiley & Sons.

Cohen, Z., Norman, H. A., and Heimer, Y. M. (1995). Microalgae as a source of x-3 fatty acids. World Rev. Nutr. Diet. 77, 1–31.

Collet, P., Helias, A., Lardon, L., Ras, M., Goy, R. A., and Steyer, J. P. (2011). Life-cycle assessment of microalgae culture coupled to biogas production. Bioresour. Technol. 102, 207–214. doi:10.1016/j.biortech.2010.06.154

Crutzen, P. J., Mosier, A. R., Smith, K. A., and Winiwarter, W. (2007). N 2 O release from agro-biofuel production negates global warming reduction by replacing fossil fuels. Atmos. Chem. Phys. 7, 11191–11205. doi:10.5194/acpd-7-11191-2007

Csordas, A., and Wang, J. (2004). An integrated photobioreactor and foam fractionation unit for growth and harvest of Chaetoceros sp in open systems. Aquac. Eng. 30, 15–30. doi:10.1016/j.aquaeng.2003.07.001

Demirbas, A. (2006). Oily products from mosses and algae via pyrolysis. Energy Source 28, 933–940. doi:10.1080/009083190910389

Demirbas, A. (2007). Progress and recent trends in biofuels. Prog. Energy Combust. Sci. 33, 1–18. doi:10.1016/j.pecs.2006.06.001

Demirbas, A. (2009). Progress and recent trends in biodiesel fuels. Energy Convers. Manag. 50, 14–34. doi:10.1016/j.enconman.2008.09.001

Demirbas, A. (2010). Use of algae as biofuel sources. Energy Convers. Manag. 51, 2738–2749. doi:10.1016/j.enconman.2010.06.010

Dismukes, G. C., Carrieri, D., Bennette, N., Ananyev, G. M., and Posewitz, M. C. (2008). Aquatic phototrophs: efficient alternatives to land-based crops for biofuels. Curr. Opin. Biotechnol. 19, 235–240. doi:10.1016/j.copbio.2008.05.007

Dragone, G., Fernandes, B., Vicente, A. A., and Teixeira, J. A. (2010). “Third generation biofuels from microalgae,” in Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology , ed. A. Mendez-Vilas (Madrid: Formatex), 1315–1366

Eshaq, F. S., Ali, M. N., and Mohd, M. K. (2010). Spirogyra biomass a renewable source for biofuel (bioethanol) production. Int. J. Eng. Sci. Technol. 2, 7045–7054.

Eshaq, F. S., Ali, M. N., and Mohd, M. K. (2011). Production of bioethanol from next generation feed-stock alga Spirogyra species. Int. J. Eng. Sci. Technol. 3, 1749–1755.

Fahmi, R., Bridgwater, A. V., Donnison, I., and Yates, N. (2008). The effect of lignin and inorganic species in biomass on pyrolysis oil yield, quality and stability. Fuel 87, 1230–1240. doi:10.1016/j.fuel.2007.07.026

Fajardo, A. R., Cerdan, L. E., Medina, A. R., Fernandez, F. G. A., Moreno, P. A. G., and Grima, E. M. (2007). Lipid extraction from the microalga Phaeodactylum tricornutum . Eur. J. Lipid Sci. Technol. 109, 120–126. doi:10.1002/ejlt.200600216

Gavrilescu, M., and Chisti, Y. (2005). Biotechnology – a sustainable alternative for chemical industry. Biotechnol. Adv. 23, 471–499. doi:10.1016/j.biotechadv.2005.03.004

Ghadge, S. V., and Raheman, H. (2005). Biodiesel production from mahua ( Madhuca indica ) oil having high free fatty acids. Biomass Bioenergy 28, 601–605. doi:10.1016/j.biombioe.2004.11.009

Goldemberg, J. (2007). Ethanol for a sustainable energy future. Science 315, 808–810. doi:10.1126/science.1137013

Goldemberg, J., and Guardabassi, P. (2009). Are biofuels a feasible option? Energy Policy 37, 10–14. doi:10.1016/j.enpol.2008.08.031

Gonzalez-Fernandez, C., Sialve, B., Bernet, N., and Steyer, J. P. (2012a). Comparison of ultrasound and thermal pretreatment of Scenedesmus biomass on methane production. Bioresour. Technol. 110, 610–616. doi:10.1016/j.biortech.2012.01.043

Gonzalez-Fernandez, C., Sialve, B., Bernet, N., and Steyer, J. P. (2012b). Thermal pretreatment to improve methane production of Scenedesmus biomass. Biomass Energy 40, 105–111. doi:10.1016/j.biombioe.2012.02.008

Haas, M. I., and Wagner, K. (2011). Simplifying biodiesel production: the direct or in situ transesterification of algal biomass. Eur. J. Lipid Sci. Technol. 113, 1219–1229. doi:10.1002/ejlt.201100106

Hankamer, B., Lehr, F., Rupprecht, J., Mussgnug, J. H., Posten, C., and Kruse, O. (2007). Photosynthetic biomass and H 2 production by green algae: from bioengineering to bioreactor scale up. Physiol. Plant. 131, 10–21. doi:10.1111/j.1399-3054.2007.00924.x

Harun, R., and Danquah, M. K. (2011a). Influence of acid pre-treatment on microalgal biomass for bioethanol production. Process Biochem . 46, 304–309. doi:10.1016/j.procbio.2010.08.027

Harun, R., and Danquah, M. K. (2011b). Enzymatic hydrolysis of microalgae biomass for bioethanol production. Chem. Eng. J. 168, 1079–1084. doi:10.1016/j.cej.2011.01.088

Harun, R., Danquah, M. K., and Forde-Gareth, M. (2010). Microalgal biomass as a fermentation feedstock for bioethanol production. J. Chem. Technol. Biotechnol. 85, 199–203. doi:10.1002/jctb.2287

Harun, R., Jason, W. S. Y., Cherrington, T., and Danquah, M. K. (2011). Exploring alkaline pretreatment of microalgal biomass for bioethanol production. Appl. Energy 88, 3464–3467. doi:10.1016/j.apenergy.2010.10.048

Heasman, M., Diemar, J. O., Connor, W., Shushames, T., Foulkes, L., and Nell, J. (2008). Development of extended shelf-life microalgae concentrate diets harvested by centrifugation for bivalve molluscs – a summary. Aquac. Res. 31, 637–659. doi:10.1046/j.1365-2109.2000.318492.x

Hidalgo, P., Toro, C., Ciudad, G., and Navia, R. (2013). Advances in direct transesterification microalgal biomass for biodiesel production. Rev. Environ. Sci. Biotechnol. 12, 179–199. doi:10.1007/s11157-013-9308-0

Ho, S. H., Chen, C. Y., Lee, D. J., and Chang, J. S. (2011). Perspectives on microalgal Co 2 -emission mitigation systems-a review. Biotechnol. Adv. 29, 189–198. doi:10.1016/j.biotechadv.2010.11.001

Hon-Nami, K. (2006). A unique feature of hydrogen recovery in endogenous starch to alcohol fermentation of the marine microalga Chlamydomonas perigranulata . Appl. Biochem. Biotechnol. 131, 808–828. doi:10.1385/ABAB:131:1:808

Horn, H. J., Aasen, I. M., and Østgaard, M. (2000). Production of ethanol from mannitol by Zymobacter palmae . J. Ind. Microbiol. Biotechnol. 24, 51–57. doi:10.1038/sj.jim.2900771

Hossain, A. B. M. S., Salleh, A., Boyce, A. N., Chowd-Hury, P., and Naqiuddin, M. (2008). Biodiesel fuel production from algae as renewable energy. Am. J. Biochem. Biotechnol. 4, 250–254. doi:10.3844/ajbbsp.2008.250.254

Hsueh, H. T., Chu, H., and Yu, S. T. (2007). A batch study on the bio-fixation of carbon dioxide in the absorbed solution from a chemical wet scrubber by hot spring and marine algae. Chemosphere 66, 878–886. doi:10.1016/j.chemosphere.2006.06.022

Hu, Q., Sommerfeld, M., Jarvis, E., Ghirardi, M., Posewitz, M., Seibert, M., et al. (2008). Microalgal triacylglycerols as feedstocks for biofuels production: perspectives and advances. Plant J. 54, 621–639. doi:10.1111/j.1365-313X.2008.03492.x

Hughes, A. D., Kelly, M. S., Black, K. D., and Stanley, M. S. (2012). Biogas from macroalgae: is it time to revisit the idea? Biotechnol. Biofuels 5, 1–7. doi:10.1186/1754-6834-5-86

Iliopoulou, E. F., Antonakou, E. V., Karakoulia, S. A., Vasalos, I. A., Lappas, A. A., and Triantafyllidis, K. S. (2007). Catalytic conversion of biomass pyrolysis products by mesoporous materials: effect of steam stability and acidity of Al-MCM-41 catalysts. Chem. Eng. J. 134, 51–57. doi:10.1016/j.cej.2007.03.066

Ivanova, G., Rakhely, G., and Kovacs, K. (2009). Thermophilic biohydrogen production from energy plants by Caldicellulosiruptor saccharolyticus and comparison with related studies. Int. J. Hydrogen Energy 34, 3659–3670. doi:10.1016/j.ijhydene.2009.02.082

John, R. P., Anisha, G. S., Nampoothiri, K. M., and Pandey, A. (2011). Micro and macroalgal biomass: a renewable source for bioethanol. Bioresour. Technol. 102, 186–193. doi:10.1016/j.biortech.2010.06.139

Jones, C. S., and Mayfield, S. P. (2012). Algae biofuels: versatility for the future of bioenergy. Curr. Opin. Biotechnol. 23, 346–351. doi:10.1016/j.copbio.2011.10.013

Kim, G. V., Choi, W. Y., Kang, D. H., Lee, S. Y., and Lee, H. Y. (2014). Enhancement of biodiesel production from marine alga, Scenedesmus sp. through in situ transesterification process associated with acidic catalyst. BioMed. Res. Int. 2014:391542 doi:10.1155/2014/391542

Knezevic, Z. D., Siler-Marinkovic, S. S., and Mojovic, L. V. (2004). Immobilized lipases as practical catalysts. Acta Period. Technol. 35, 151–164. doi:10.2298/APT0435151K

Kraan, S. (2013). Mass cultivation of carbohydrate rich microalgae, a possible solution for sustainable biofuel production. Mitig. Adapt. Strateg. Glob. Change . 18, 27–46. doi:10.1007/s11027-010-9275-5

Kumagai, Y., and Ojima, T. (2010). Isolation and characterization of two types of β-1,3-glucanases from the common sea hare Aplysia kurodai . Comp. Biochem. Physiol. Biochem. Mol. Biol. 155, 138–144. doi:10.1016/j.cbpb.2009.10.013

Kumar, S., Dheeran, P., Singh, S. P., Mishra, I. M., and Adhikari, D. K. (2014). Bioprocessing for bagasse hydrolysate for ethanol and xylitol production using thermotolerant yeast. Biprocess Biosyst. Eng. 38, 39–47. doi:10.1007/s00449-014-1241-2

Lam, M. K., and Lee, K. T. (2012). Microalgae biofuels: a critical review of issues, problems and the way forward. Biotechnol. Adv. 30, 630–690. doi:10.1016/j.biotechadv.2011.11.008

Leach, G., Oliveira, G., and Morais, R. (1998). Spray-drying of Dunaliella salina to produce a b-carotene rich powder. J. Ind. Microbiol. Biotechnol. 20, 82–85. doi:10.1038/sj.jim.2900485

Lee, A., Lewis, D., and Ashman, P. (2009). Microbial flocculation, a potentially low-cost harvesting technique for marine microalgae for the production of biodiesel. J. Appl. Phycol . 21, 559–567. doi:10.1007/s10811-008-9391-8

Lee, K., Eisterhold, M. L., Rindi, F., Palanisami, S., and Nam, P. K. (2014a). Isolation and screening of microalgae from natural habitats in the Midwestern United States of America for biomass and biodiesel sources. J. Nat. Sci. Biol. Med. 5, 333–339. doi:10.4103/0976-9668.136178

Lee, K. C., Takamitsu, A., Ibrahim, D., Kosugi, A., Prawitwong, P., Lan, D., et al. (2014b). Purification and characterization of a thermostable laminarinase from Penicillium rolfsii c3-2(1) IBRL. Bioresource 9, 1072–1084. doi:10.15376/biores.9.1

Lee, S. J., Yoon, B. D., and Oh, H. M. (1998). Rapid method for the determination of lipid from the green alga Botryococcus braunii . Biotechnol. Tech. 12, 553–556. doi:10.1023/A:1008811716448

Li, Y., Horsman, M., Wu, N., Lan, C. Q., and Dubois-Calero, N. (2008). Biofuels from microalgae. Biotechnol. Progr. 24, 815–820. doi:10.1021/bp070371k

Li, Y., Naghdi, F. G., Garg, G., Adarme-Vega, T. C., Thurecht, K. J., Ghafor, W. A., et al. (2014). A comparative study: the impact of different lipid extraction methods on current microalgal lipid research. Microb. Cell Fact. 13, 1–9. doi:10.1186/1475-2859-13-14

Liu, J., Zhu, Y., Tao, Y., Zhang, Y., Li, A., Li, T., et al. (2013). Freshwater microalgae harvested via flocculation induced by pH decrease. Biotechnol. Biofuels 6, 98. doi:10.1186/1754-6834-6-98

Luo, Y., Zheng, Y., Jiang, Z., Ma, Y., and Wei, D. (2006). A novel psychrophilic lipase from Pseudomonas fluorescens with unique property in chiral resolution and biodiesel production via transesterification. Appl. Microbiol. Biotechnol. 73, 349–355. doi:10.1007/s00253-006-0478-3

Lv, P., Yuan, Z., Wu, C., Ma, L., Chen, Y., and Tsubaki, N. (2007). Biosyngas production from biomass catalytic gasification. Energy. Convers. Manag. 48, 1132–1139. doi:10.1016/j.enconman.2006.10.014

Mamilla, V. R., Mallikarjun, M. V., and Rao, G. L. N. (2011). Preparation of biodiesel from Karanja oil. Int. J. Energy Eng. 1, 94–100. doi:10.5963/IJEE0102008

Markou, G., Angelidaki, I., Nerantzis, E., and Georgakakis, D. (2013). Bioethanol production by carbohydrate-enriched biomass of Arthrospira ( Spirulina ) platensis . Energies 6, 3937–3950. doi:10.3390/en6083937

Marques, A. E., Barbosa, A. T., Jotta, J., Coelho, M. C., Tamagnini, P., and Gouveia, L. (2011). Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: light, nickel, propane, carbon dioxide and nitrogen. Biomass Bioenergy 35, 4426–4434. doi:10.1016/j.biombioe.2011.08.014

Martin, M., Pires, R. F., Alves, M. J., Hori, C. E., Reis, M. H. M., and Cardoso, V. L. (2013). Transesterification of soybean oil for biodiesel production using hydrotalcite as basic catalyst. Chem. Eng. Trans. 32, 817–822. doi:10.3303/CET1332137

Mata, T. M., Martins, A., and Caetano, N. S. (2010). Microalgae for biodiesel production and other applications: a review. Ren. Sust. Energy. Rev . 14, 217–232. doi:10.1016/j.rser.2009.07.020

Mazubert, A., Poux, M., and Aubin, J. (2013). Intensified processes for FAME production from waste cooking oil: a technological review. Chem. Eng. J. 233, 201–223. doi:10.1016/j.cej.2013.07.063

Meher, L. C., Vidya, S. D., and Naik, S. N. (2006). Technical aspects of biodiesel production by transesterification – a review. Renew. Sustain. Energ. Rev. 10, 248–268. doi:10.1016/j.biotechadv.2014.10.003

Munir, N., Sharif, N., Naz, S., Saleem, F., and Manzoor, F. (2013). Harvesting and processing of microalgae biomass fractions for biodiesel production (a review). Sci. Tech. Dev. 32, 235–243.

Mussgnug, J. H., Klassen, V., Schluter, A., and Kruse, O. (2010). Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotechnol. 150, 51–56. doi:10.1016/j.jbiotec.2010.07.030

Mutanda, T., Ramesh, D., Karthikeyan, S., Kumari, S., Anandraj, A., and Bux, F. (2011). Bioprospecting for hyper-lipid producing microalgal strains for sustainable biofuel production. Bioresour. Technol. 102, 57–70. doi:10.1016/j.biortech.2010.06.077

Muthukumar, A., Elayaraja, S., Ajithkumar, T. T., Kumaresan, S., and Balasubramanian, T. (2012). Biodiesel production from marine microalgae Chlorella marina and Nannochloropsis salina . J. Petrol. Technol Altern. Fuels 3, 58–62. doi:10.5897/JPTAF12.010

Naik, M., Meher, L. C., Waik, S. N., and Das, L. M. (2008). Production of biodiesel from high free fatty acid Karanja ( Pongamia pinnata ) oil. Biomass Bioenergy 32, 354–357. doi:10.1016/j.biombioe.2007.10.006

Nascimento, I. A., Marques, S. S. I., Cabanelas, I. T. D., Pereira, S. A., Druzian, J. I., de Souza, C. O., et al. (2013). Screening microalgae strains for biodiesel production: lipid productivity and estimation of fuel quality based on fatty acids profiles as selective criteria. Bioenerg. Res. 6, 1–13. doi:10.1007/s12155-012-9222-2

Nasirian, N., Almassi, M., Minaei, S., and Widmann, R. (2011). Development of a method for biohydrogen production from wheat straw by dark fermentation. Int. J. Hydrogen Energy 36, 411–420. doi:10.1016/j.ijhydene.2010.09.073

Nautiyal, P., Subramanian, K. A., and Dastidar, M. G. (2014). Kinetic and thermodynamic studies on biodiesel production from Spirulina platensis algae biomass using single stage extraction-transesterification process. Fuel 135, 228–234. doi:10.1016/j.fuel.2014.06.063

Nguyen, T. H. M., and Vu, V. H. (2012). Bioethanol production from marine algae biomass: prospect and troubles. J. Viet. Env. 3, 25–29. doi:10.13141/jve.vol3.no1

Nigam, P. S., and Singh, A. (2011). Production of liquid biofuels from renewable resources. Prog. Energy Combust. Sci. 37, 52–68. doi:10.1016/j.pecs.2010.01.003

Oyedun, A. O., Lam, K. L., Gebreegziabher, T., Lee, H. K. M., and Hui, C. W. (2012). Optimisation of operating parameters in multi-stage pyrolysis. Chem. Engg. Trans. 29, 655–660. doi:10.3303/CET1229110

Park, J. H., Yoon, J. J., Park, H. D., Kim, Y. J., Lim, D. J., and Kim, S. H. (2011). Feasibility of biohydrogen production from Gelidium amansii . Int. J. Hydrogen Energy 36, 13997–14003. doi:10.1016/j.ijhydene.2011.04.003

Passos, F., Sole, M., Garcia, J., and Ferrer, I. (2013). Biogas production from microalgae grown in wastewater: effect of microwave pretreatment. Appl. Energy 108, 168–175. doi:10.1016/j.watres.2013.10.013

Patil, P. D., Gude, V. G., Mannarswamy, A., Cooke, P., Nirmalakhandan, N., Lammers, P., et al. (2012). Comparison of direct transesterification of algal biomass under supercritical methanol and microwave irradiation conditions. Fuel 97, 822–831. doi:10.1016/j.fuel.2012.02.037

Popoola, T. O. S., and Yangomodou, O. D. (2006). Extraction, properties and utilization potentials of cassava seed oil. Biotechnology 5, 38–41. doi:10.3923/biotech.2006.38.41

Porphy, S. J., and Farid, M. M. (2012). Feasibility study for production of biofuel and chemicals from marine microalgae Nannochloropsis sp. based on basic mass and energy analysis. ISRN Renew. Energ. 2012:156824. doi:10.5402/2012/156824

Prakash, J., Pushparaj, B., Carlozzi, P., Torzillo, G., Montaini, E., and Materassi, R. (2007). Microalgal biomass drying by a simple solar device. Int. J. Solar Energy 18, 303–311. doi:10.1080/01425919708914325

Pratoomyot, J., Srivilas, P., and Noiraksar, T. (2005). Fatty acids composition of 10 microalgal species. J. Sci. Technol. 27, 1179–1187.

Prins, M. J., Ptasinski, K. J., and Janssen, F. J. J. G. (2006). More efficient biomass gasification via torrefaction. Energy 31, 3458–3470. doi:10.1016/j.energy.2006.03.008

Prochazkova, G., Safarik, I., and Branyik, T. (2013). Harvesting microalgae with microwave synthesized magnetic microparticles. Bioresour. Technol. 130, 472–477. doi:10.1016/j.biortech.2012.12.060

Rajkumar, R., Yaakob, Z., and Takriff, M. S. (2014). Potential of the micro and macro algae for biofuel production: a brief review. Bioresour. 9, 1606–1633. doi:10.15376/biores.9.1

Rajvanshi, S., and Sharma, M. P. (2012). Microalgae: a potential source of biodiesel. J. Sustain. Bioenergy Syst. 2, 49–59. doi:10.4236/jsbs.2012.23008

Ras, M., Lardon, L., Bruno, S., Bernet, N., and Steyer, J. P. (2011). Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris . Bioresour. Technol. 102, 200–206. doi:10.1016/j.biortech.2010.06.146

Richmond, A. (2004). Handbook of Microalgal Culture: Biotechnology and Applied Phycology . Osney Mead, Oxford: Blackwell Science Ltd.

Richmond, A., and Qiang, H. (2013). Handbook of Microalgal Culture: Applied Phycology and Biotechnology , Second Edn. Hoboken, NJ: Wiley-Blackwell.

Rodolfi, L., Zittelli, G. C., Bassi, N., Padovani, G., Biondi, N., Bonini, G., et al. (2009). Microalgae for oil: strain selection, induction of lipid synthesis and outdoor mass cultivation in a low-cost photobioreactor. Biotechnol. Bioeng. 102, 100–112. doi:10.1002/bit.22033

Romagnoli, F., Blumberga, D., and Gigli, E. (2011). Biogas from marine macroalgae: a new environmental technology-life cycle inventory for a further LCA. Environ. Climate Technol. 4, 97–108. doi:10.2478/v10145-010-0024-5

Rosenberg, J. N., Oyler, G. A., Wilkinson, L., and Betenbaugh, M. J. (2008). A green light for engineered algae: redirecting metabolism to fuel a biotechnology revolution. Curr. Opin. Biotechnol. 19, 430–436. doi:10.1016/j.copbio.2008.07.008

Rossignol, N., Vandanjon, L., Jaoue, P., and Quemeneur, F. (1999). Membrane technology for the continuous separation of microalgae/culture medium: compared performances of cross-flow microfiltration and ultrafiltration. Aquac. Eng. 20, 191–208. doi:10.1016/S0144-8609(99)00018-7

Ryckebosch, E., Muylaert, K., and Foubert, I. (2012). Optimization of an analytical procedure for extraction of lipids from microalgae. J. Am. Oil Chem. Soc. 89, 189–198. doi:10.1007/s11746-011-1903-z

Saleem, M., Chakrabarti, M. H., Raman, A. A. A., Hasan, D. B., Daud, W. M. A. W., and Mustafa, A. (2012). Hydrogen production by Chlamydomonas reinhardtii in a two-stage process with and without illumination at alkaline pH. Int. J. Hydrogen Energy 37, 4930–4934. doi:10.1016/j.ijhydene.2011.12.115

Samson, R., and Leduy, A. (1986). Detailed study of anaerobic digestion of Spirulina maxima algal biomass. Biotechnol. Bioeng. 28, 1014–1023. doi:10.1002/bit.260280712

Sangeetha, P., Babu, S., and Rangasamy, R. (2011). Potential of green alga Chaetomorpha litorea (Harvey) for biogas production. Int. J. Curr. Sci. 1, 24–29.

Saqib, A., Tabbssum, M. R., Rashid, U., Ibrahim, M., Gill, S. S., and Mehmood, M. A. (2013). Marine macroalgae Ulva : a potential feed-stock for bioethanol and biogas production. Asian J. Agri. Biol. 1, 155–163.

Schenk, P., Thomas-Hall, S., Stephens, E., Marx, U., Mussgnug, J., Posten, C., et al. (2008). Second generation biofuels: high efficiency microalgae for biodiesel production. Bioenergy Res . 1, 20–43. doi:10.1007/s12155-008-9008-8

Schneider, R. C. S., Bjerk, T. R., Gressler, P. D., Souza, M. P., Corbelline, V. A., and Lobo, E. A. (2013). “Potential production of biofuel from microalgae biomass produced in wastewater,” in Biodiesel – Feedstocks, Production and Applications , ed. Z. Fang (Rijeka: InTech). doi:10.5772/52439

Shaishav, S., Singh, R. N., and Satyendra, T. (2013). Biohydrogen from algae: fuel of the future. Int. Res. J. Env. Sci. 2, 44–47.

Shalaby, E. A. (2011). “Algal biomass and biodiesel production,” in Biodiesel – Feedstocks and Processing Technologies , ed. S. Margarita (Rijeka: InTech), 111–132. ISBN: 978-953-307-713-0.

Sharma, R., Chisti, Y., and Banerjee, U. C. (2001). Production, purification, characterization, and applications of lipases. Biotechnol. Adv. 19, 627–662. doi:10.1016/S0734-9750(01)00086-6

Sharma, Y. C., and Singh, B. (2009). Development of biodiesel: current scenario. Renew. Sustain. Energ. Rev. 13, 1646–1651. doi:10.1016/j.biortech.2010.06.057

Shi, X., Jung, K. W., Kim, H., Ahn, Y. T., and Shin, H. S. (2011). Direct fermentation of Laminaria japonica for biohydrogen production by anaerobic mixed cultures. Int. J. Hydrogen Energy 36, 5857–5864. doi:10.1016/j.ijhydene.2011.01.125

Shuping, Z., Yulong, W., Mingde, Y., Kaleem, I., Chun, L., and Tong, J. (2010). Production and characterization of bio-oil from hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake. Energy 35, 5406–5411. doi:10.1016/j.energy.2010.07.013

Singh, A., He, B., Thompson, J., and Gerpen, J. V. (2006). Process optimization of biodiesel production using alkaline catalysts. Appl. Eng. Agric. 22, 597–600. doi:10.13031/2013.21213

Singh, A., Nigam, P. S., and Murphy, J. D. (2011). Mechanism and challenges in commercialisation of algal biofuels. Bioresour. Technol. 102, 26–34. doi:10.1016/j.biortech.2010.06.057

Singh, D. P., and Trivedi, R. K. (2013). Production of biofuel from algae: an economic and eco-friendly resource. Int. J. Sci. Res. 2, 352–357.

Singh, J., and Gu, S. (2010). Commercialization potential of microalgae for biofuels production. Renew. Sustain. Energ. Rev. 14, 2596–2610. doi:10.1016/j.rser.2010.06.014

Singh, R., Behera, S., Yadav, Y. K., and Kumar, S. (2014). “Potential of wheat straw for biogas production using thermophiles,” in Recent Advances in Bio-Energy Research , eds S. Kumar, A. K. Sarma, S. K. Tyagi, and Y. K. Yadav (Kapurthala: SSS-National Institute of Renewable Energy), 242–249.

Spolaore, P., Joannis-Cassan, C., Duran, E., and Isambert, A. (2006). Commercial applications of microalgae. J. Biosci. Bioeng. 101, 87–96. doi:10.1263/jbb.101.87

Stergiou, P. Y., Foukis, A., Filippou, M., Koukouritaki, M., Parapouli, M., Theodorou, L. G., et al. (2013). Advances in lipase-catalyzed esterification reactions. Biotechnol. Adv. 31, 1846–1859. doi:10.1016/j.biotechadv.2013.08.006

Sukumaran, R. K., Singhania, R., Mathew, G. M., and Pandey, A. (2008). Cellulase production using biomass feed stock and its application in lignocellulose saccharification for bio-ethanol production. Renew. Energ. 34, 421–424. doi:10.1016/j.renene.2008.05.008

Surendhiran, D., and Vijay, M. (2012). Microalgal biodiesel-acomprehensive review on the potential and alternative biofuel. Res. J. Chem. Sci. 2, 71–82.

Susilaningsih, D., Djohan, A. C., Widyaningrum, D. N., and Anam, K. (2009). Biodiesel from indigenous Indonesian marine microalgae Nanochloropsis sp. J. Biotechnol. Res. Trop. Reg. 2, 1–4.

Tamer, E., Amin, M. A., Ossama, E. T., Bo, M., and Benoit, G. (2006). Biological treatment of industrial wastes in a photobioreactor. Water Sci. Technol. 53, 117–125. doi:10.2166/wst.2006.344

Thomas, D. N. (2002). Seaweeds . Washington, DC: Smithsonian Books, Natural History Museum.

Topare, N., Rout, S. J., Renge, V. C., Khedkar, S. V., Chavan, Y. P., and Bhagat, S. L. (2011). Extraction of oil from algae by solvent extraction and oil expeller method. Int. J. Chem. 9, 1746–1750.

Trivedi, N., Gupta, V., Reddy, C. R., and Jha, B. (2013). Enzymatic hydrolysis and production of bioethanol from common macrophytic green alga Ulva fasciata Delile. Bioresour. Technol. 150, 106–112. doi:10.1016/j.biortech.2013.09.103

Vanegas, C. H., and Bartlett, J. (2013). Green energy from marine algae: biogas production and composition from the anaerobic digestion of Irish seaweed species. Environ. Technol. 34, 2277–2283. doi:10.1080/09593330.2013.765922

Vergara-Fernandez, A., Vargas, G., Alarcon, N., and Velasco, A. (2008). Evaluation of marine algae as a source of biogas in a two-stage anaerobic reactor system. Biomass Bioenergy 32, 338–344. doi:10.1016/j.biombioe.2007.10.005

Wargacki, A. J., Leonard, E., Win, M. N., Regitsky, D. D., Santos, C. N., Kim, P. B., et al. (2012). An engineered microbial platform for direct biofuel production from brown macroalgae. Science 335, 308–313. doi:10.1126/science.1214547

Wecker, M. S. A., Meuser, J. E., Posewitz, M. C., and Ghirardi, M. L. (2011). Design of a new biosensor for algal H 2 production based on the H 2 -sensing system of Rhodobacter capsulatus . Int. J. Hydrogen Energy 36, 11229–11237. doi:10.1016/j.ijhydene.2011.05.121

Widjaja, A., Chien, C. C., and Ju, Y. H. (2009). Study of increasing lipid production from fresh water microalgae Chlorella vulgaris . J. Taiwan Inst. Chem. Eng. 40, 13–20. doi:10.1016/j.jtice.2008.07.007

Williams, P. J. L. B., and Laurens, L. M. L. (2010). Microalgae as biodiesel and biomass feedstocks: review and analysis of the biochemistry, energetics and economics. Energy Environ. Sci 3, 554–590. doi:10.1039/b924978h

Wu, X., Li, Q., Dieudonne, M., Cong, Y., Zhou, J., and Long, M. (2010). Enhanced H 2 gas production from bagasse using adhE inactivated Klebsiella oxytoca HP1 by sequential dark-photo fermentations. Bioresour. Technol. 101, 9605–9611. doi:10.1016/j.biortech.2010.07.095

Yacoby, I., Pochekailov, S., Topotrik, H., Ghirardi, M. L., King, P. W., and Zhang, S. (2011). Photosynthetic electron partitioning between (FeFe)-hydrogenase and ferrodoxin: NADP+-oxidoreductase (FNR) enzymes in vitro. Proc. Natl. Acad. Sci. U.S.A. 108, 9396–9401. doi:10.1073/pnas.1103659108

Yanagisawa, M., Nakamura, K., Ariga, O., and Nakasaki, K. (2011). Production of high concentrations of bioethanol from seaweeds that contain easily hydrolysable polysaccharides. Process Biochem. 46, 2111–2116. doi:10.4161/bioe.23396

Yanqun, L., Mark, H., Nan, W., Christopher, Q. L., and Nathalie, D. C. (2008). Biofuels from microalgae. Biotechnol. Progress. 24, 815–820.

Yazdani, P., Karimi, K., and Taherzadeh, M. J. (2011). “Improvement of enzymatic hydrolysis of a marine macro-alga by dilute acid hydrolysis pretreatment,” in World Renewable Energy Congress (Bioenergy Technology), 8-13 May , ed. B. Moshfegh (Linkoping, Sweden), 186–191.

Yen, H. W., and Brune, D. E. (2007). Anaerobic co-digestion of algal sludge and waste paper to produce methane. Bioresour. Technol. 98, 130–134. doi:10.1016/j.biortech.2005.11.010

Yoon, J. J., Kim, Y. J., Kim, S. H., Ryu, H. J., Choi, J. Y., Kim, G. S., et al. (2010). Production of polysaccharides and corresponding sugars from red seaweed. Adv. Mater. Res. 9, 463–466. doi:10.1016/j.biortech.2008.11.053

Yoon, M., Choi, J. I., Lee, J. W., and Park, D. H. (2012). Improvement of saccharification process for bioethanol production from Undaria sp. by gamma irradiation. Radiat. Phys. Chem. 81, 999–1002. doi:10.1016/j.radphyschem.2011.11.035

Yuan, H. (2014). Effect of ammoniation pretreatment at low moisture content on anaerobic digestion performance of rice straw. Bioresources 9, 6707–6718. doi:10.15376/biores.9.4

Yuan, X., Shi, X., Zhang, D., Qiu, Y., Guo, R., and Wang, L. (2011). Biogas production and microcystin biodegradation in anaerobic digestion of blue algae. Energy Environ. Sci. 4, 1511–1515. doi:10.1039/c0ee00452a

Zhang, C. (2013). Alkaline pretreatment for enhancement of biogas production. Bioresour. Technol. 149, 353–358. doi:10.1016/j.biortech.2013.09.070

Zheng, Y. (2009). Anaerobic digestion of saline creeping wild ryegrass for biogas production and pretreatment of particleboard material. Bioresour. Technol. 100, 1582–1588. doi:10.1016/j.biortech.2008.08.048

Zhong, W., Zhang, Z., Luo, Y., Qiao, W., Xiao, M., and Zhang, M. (2012). Biogas productivity by co-digesting Taihu blue algae with corn straw as an external carbon source. Bioresour. Technol. 114, 181–186. doi:10.1016/j.biortech.2012.02.111

Keywords: algae, microalgae, biofuels, bioethanol, biogas, biodiesel, biohydrogen

Citation: Behera S, Singh R, Arora R, Sharma NK, Shukla M and Kumar S (2015) Scope of algae as third generation biofuels. Front. Bioeng. Biotechnol. 2 :90. doi: 10.3389/fbioe.2014.00090

Received: 31 July 2014; Accepted: 29 December 2014; Published online: 11 February 2015.

Reviewed by:

Copyright: © 2015 Behera, Singh, Arora, Sharma, Shukla and Kumar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Sachin Kumar, Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Jalandhar-Kapurthala Road, Wadala Kalan, Kapurthala 144601, Punjab, India e-mail: sachin.biotech@gmail.com

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

Information

• Author Services

Initiatives

You are accessing a machine-readable page. In order to be human-readable, please install an RSS reader.

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited. For more information, please refer to https://www.mdpi.com/openaccess .

Feature papers represent the most advanced research with significant potential for high impact in the field. A Feature Paper should be a substantial original Article that involves several techniques or approaches, provides an outlook for future research directions and describes possible research applications.

Feature papers are submitted upon individual invitation or recommendation by the scientific editors and must receive positive feedback from the reviewers.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

Original Submission Date Received: .

• Active Journals
• Find a Journal
• Proceedings Series
• For Authors
• For Reviewers
• For Editors
• For Librarians
• For Publishers
• For Societies
• For Conference Organizers
• Open Access Policy
• Institutional Open Access Program
• Special Issues Guidelines
• Editorial Process
• Research and Publication Ethics
• Article Processing Charges
• Testimonials
• Preprints.org
• SciProfiles
• Encyclopedia

Article Menu

• Subscribe SciFeed
• Google Scholar
• on Google Scholar
• Table of Contents

Find support for a specific problem in the support section of our website.

Please let us know what you think of our products and services.

Visit our dedicated information section to learn more about MDPI.

JSmol Viewer

Yeast for the production of biochemicals and biofuels.

Conflicts of Interest

List of contributions.

• Liu, J.; Sun, S.; Sun, Y.; Liu, D.; Kang, J.; Ye, Z.; Song, G.; Ge, J. Effect of Short-Chain Fatty Acids on the Yield of 2,3-Butanediol by Saccharomyces cerevisiae W141: The Synergistic Effect of Acetic Acid and Dissolved Oxygen. Fermentation   2023 , 9 , 236. https://doi.org/10.3390/fermentation9030236 .
• Ao, G.; Sun, S.; Liu, L.; Guo, Y.; Tu, X.; Ge, J.; Ping, W. Production of 2,3-Butanediol by S. cerevisiae L7 in Fed-Batch Fermentation with Optimized Culture Conditions. Fermentation   2023 , 9 , 694. https://doi.org/10.3390/fermentation9070694 .
• Liu, J.; Yao, G.; Wan, X.; Wang, F.; Han, P.; Bao, S.; Wang, K.; Song, T.; Jiang, H. Highly Efficient Biosynthesis of γ -Bisabolene with a New Sesquiterpene Synthase AcTPS5 by Dual Cytoplasmic-Peroxisomal Engineering in Saccharomyces cerevisiae . Fermentation   2023 , 9 , 779. https://doi.org/10.3390/fermentation9090779 .
• Chen, J.; Gui, L.; Chen, B.; Sun, Y.; Zhao, Y.; Lu, F.; Li, M. Improving Expression of Pepsinogen A from Homo sapiens in Aspergillus niger by Using a Multi-Copy Gene Knock-in Strategy. Fermentation   2023 , 9 , 538. https://doi.org/10.3390/fermentation9060538 .
• Zeng, Y.; Wang, R.; Liang, J.; Zhang, H.; Yi, J.; Liu, Z. Strategies for Recovery, Purification and Quantification of Torularhodin Produced by Rhodotorula mucilaginosa Using Different Carbon Sources. Fermentation   2023 , 9 , 846. https://doi.org/10.3390/fermentation9090846 .
• Ahuja, V.; Arora, A.; Chauhan, S.; Thakur, S.; Jeyaseelan, C.; Paul, D. Yeast-Mediated Biomass Valorization for Biofuel Production: A Literature Review. Fermentation   2023 , 9 , 784. https://doi.org/10.3390/fermentation9090784 .
• Zhang, H.; Zhang, P.; Wu, T.; Ruan, H. Bioethanol Production Based on Saccharomyces cerevisiae : Opportunities and Challenges. Fermentation   2023 , 9 , 709. https://doi.org/10.3390/fermentation9080709 .
• Paul, D.; Bohacz, J.; Bhatia, S.K. Editorial: Biowaste valorization utilizing microbial systems. Front. Microbiol. 2023 , 14 , 1213598. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Yaashikaa, P.R.; Kumar, P.; Varjani, S. Valorization of agro-industrial wastes for biorefinery process and circular bioeconomy: A critical review. Bioresour. Technol. 2022 , 343 , 126126. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Aidonojie, P.A.; Ukhurebor, K.E.; Oaihimire, I.E.; Ngonso, B.F.; Egielewa, P.E.; Akinsehinde, B.O.; Kusuma, H.S.; Darmokoesoemo, H. Bioenergy revamping and complimenting the global environmental legal framework on the reduction of waste materials: A facile review. Heliyon 2023 , 6 , e12860. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Parasar, D.P.; Ramakrishnan, E.; Kabilan, S.; Kotoky, J.; Sarma, H.K. Characterization of β-cryptoxanthin and other carotenoid derivatives from Rhodotorula taiwanensis , a novel yeast isolated from traditional starter culture of Assam. Chem. Biodivers. 2020 , 17 , e2000198. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Sokolova, N.; Peng, B.; Haslinger, K. Design and engineering of artificial biosynthetic pathways-where do we stand and where do we go? FEBS Lett. 2023 , 597 , 2897–2907. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Lu, H.; Chen, H.; Tang, X.; Yang, Q.; Zhang, H.; Chen, Y.Q.; Chen, W. Metabolomics analysis reveals the role of oxygen control in the nitrogen limitation induced lipid accumulation in Mortierella alpina . J. Biotechnol. 2021 , 325 , 325–333. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Magdouli, S.; Guedri, T.; Tarek, R.; Brar, S.K.; Blais, J.F. Valorization of raw glycerol and crustacean waste into value added products by Yarrowia lipolytica . Bioresour. Technol. 2017 , 243 , 57–68. [ Google Scholar ] [ CrossRef ] [ PubMed ]
• Deeba, F.; Kiran Kumar, K.; Ali Wani, S.; Singh, A.; Sharma, J.; Gaur, N.A. Enhanced biodiesel and β-carotene production in Rhodotorula pacifica INDKK using sugarcane bagasse and molasses by an integrated biorefinery framework. Bioresour. Technol. 2022 , 351 , 127067. [ Google Scholar ] [ CrossRef ] [ PubMed ]

Click here to enlarge figure

Share and Cite

Paul, D. Yeast for the Production of Biochemicals and Biofuels. Fermentation 2024 , 10 , 451. https://doi.org/10.3390/fermentation10090451

Paul D. Yeast for the Production of Biochemicals and Biofuels. Fermentation . 2024; 10(9):451. https://doi.org/10.3390/fermentation10090451

Paul, Debarati. 2024. "Yeast for the Production of Biochemicals and Biofuels" Fermentation 10, no. 9: 451. https://doi.org/10.3390/fermentation10090451

Article Metrics

Article access statistics, further information, mdpi initiatives, follow mdpi.

Subscribe to receive issue release notifications and newsletters from MDPI journals

Maintenance work is planned from 22:00 BST on Monday 16th September 2024 to 22:00 BST on Tuesday 17th September 2024.

During this time the performance of our website may be affected - searches may run slowly, some pages may be temporarily unavailable, and you may be unable to access content. If this happens, please try refreshing your web browser or try waiting two to three minutes before trying again.

We apologise for any inconvenience this might cause and thank you for your patience.

Journal of Materials Chemistry A

Photo-nanozyme coupling catalyzes glucose oxidation for high-performance enzymatic biofuel cells.

Glucose biofuel cells (GBFCs) are special energy conversion devices using naturally abundant glucose as fuel. However, achieving high power output and stability remains a challenge in existing GBFC. In this study, we created a photoelectric coupling nanozyme catalyst of Au/BiVO 4 with triple synergistic promotion effects: the surface plasmon resonance of Au significantly broadened the photo-absorption region, enhanced the light absorption intensity, and increased the carrier density of the BiVO 4 ; furthermore, the outstanding electron transfer capacity of Au accelerated the photoelectrons separation from the vacancies in BiVO 4 , endowing the BiVO 4 with excellent photo-corrosion resistance; additionally, the three-dimensional structure of BiVO 4 provides abundant sites for Au, remarkably improving the loading and catalytic stability of Au. Consequently, the Au/BiVO 4 catalytic for GBFC can simultaneously convert solar and chemical energy stored in glucose into electrical energy, providing an extraordinarily high power density and open-circuit voltage (575 μW cm -2 , 0.86 V) and working steadily for 20 hours. Altogether, high power output and high stability are achieved in the Au/BiVO 4 catalytic GBFC. Thus, this study will significantly propel the development of GBFC through the innovative application of the photoelectric coupling nanozyme catalytic strategy.

• This article is part of the themed collection: Journal of Materials Chemistry A HOT Papers

Supplementary files

• Supplementary information PDF (1928K)

Transparent peer review

To support increased transparency, we offer authors the option to publish the peer review history alongside their article.

View this article’s peer review history

Article information

Download citation, permissions.

D. Hu, Q. Su, Y. Gao, J. Zhang, L. Wang and J. Zhu, J. Mater. Chem. A , 2024, Accepted Manuscript , DOI: 10.1039/D4TA04675G

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page .

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page .

Read more about how to correctly acknowledge RSC content .

Social activity

Search articles by author.

This article has not yet been cited.

Advertisements

APA Style for beginners

Then check out some frequently asked questions:

What is APA Style?

Why use apa style in high school, how do i get started with apa style, what apa style products are available, your help wanted.

APA Style is the most common writing style used in college and career. Its purpose is to promote excellence in communication by helping writers create clear, precise, and inclusive sentences with a straightforward scholarly tone. It addresses areas of writing such as how to

• format a paper so it looks professional;
• credit other people’s words and ideas via citations and references to avoid plagiarism; and
• describe other people with dignity and respect using inclusive, bias-free language.

APA Style is primarily used in the behavioral sciences, which are subjects related to people, such as psychology, education, and nursing. It is also used by students in business, engineering, communications, and other classes. Students use it to write academic essays and research papers in high school and college, and professionals use it to conduct, report, and publish scientific research .

High school students need to learn how to write concisely, precisely, and inclusively so that they are best prepared for college and career. Here are some of the reasons educators have chosen APA Style:

• APA Style is the style of choice for the AP Capstone program, the fastest growing AP course, which requires students to conduct and report independent research.
• APA Style helps students craft written responses on standardized tests such as the SAT and ACT because it teaches students to use a direct and professional tone while avoiding redundancy and flowery language.
• Most college students choose majors that require APA Style or allow APA Style as an option. It can be overwhelming to learn APA Style all at once during the first years of college; starting APA Style instruction in high school sets students up for success.

High school students may also be interested in the TOPSS Competition for High School Psychology Students , an annual competition from the APA Teachers of Psychology in Secondary Schools for high school students to create a short video demonstrating how a psychological topic has the potential to benefit their school and/or local community and improve people’s lives.

Most people are first introduced to APA Style by reading works written in APA Style. The following guides will help with that:

Many people also write research papers or academic essays in APA Style. The following resources will help with that:

Instructors will also benefit from using the following APA Style resources:

In addition to all the free resources on this website, APA publishes several products that provide comprehensive information about APA Style:

The APA Style team is interested in developing additional resources appropriate for a beginner audience. If you have resources you would like to share, or feedback on this topic, please contact the APA Style team . 

Free newsletter

Apa style monthly.

Subscribe to the APA Style Monthly newsletter to get tips, updates, and resources delivered directly to your inbox.

Welcome! Thank you for subscribing.

• Follow us on Facebook
• Follow us on Twitter
• Follow us on LinkedIn
• Watch us on Youtube
• Audio and video Explore the sights and sounds of the scientific world
• Podcasts Our regular conversations with inspiring figures from the scientific community
• Video Watch our specially filmed videos to get a different slant on the latest science
• Webinars Tune into online presentations that allow expert speakers to explain novel tools and applications
• Latest Explore all the latest news and information on Physics World
• Research updates Keep track of the most exciting research breakthroughs and technology innovations
• News Stay informed about the latest developments that affect scientists in all parts of the world
• Features Take a deeper look at the emerging trends and key issues within the global scientific community
• Opinion and reviews Find out whether you agree with our expert commentators
• Interviews Discover the views of leading figures in the scientific community
• Analysis Discover the stories behind the headlines
• Blog Enjoy a more personal take on the key events in and around science
• Physics World Live
• Impact Explore the value of scientific research for industry, the economy and society
• Events Plan the meetings and conferences you want to attend with our comprehensive events calendar
• Innovation showcases A round-up of the latest innovation from our corporate partners
• Collections Explore special collections that bring together our best content on trending topics
• Artificial intelligence Explore the ways in which today’s world relies on AI, and ponder how this technology might shape the world of tomorrow
• #BlackInPhysics Celebrating Black physicists and revealing a more complete picture of what a physicist looks like
• Nanotechnology in action The challenges and opportunities of turning advances in nanotechnology into commercial products
• The Nobel Prize for Physics Explore the work of recent Nobel laureates, find out what happens behind the scenes, and discover some who were overlooked for the prize
• Revolutions in computing Find out how scientists are exploiting digital technologies to understand online behaviour and drive research progress
• The science and business of space Explore the latest trends and opportunities associated with designing, building, launching and exploiting space-based technologies
• Supercool physics Experiments that probe the exotic behaviour of matter at ultralow temperatures depend on the latest cryogenics technology
• Women in physics Celebrating women in physics and their contributions to the field
• IOP Publishing
• Enter e-mail address
• Show Enter password
• Remember me Forgot your password?
• Access more than 20 years of online content
• Manage which e-mail newsletters you want to receive
• Read about the big breakthroughs and innovations across 13 scientific topics
• Explore the key issues and trends within the global scientific community
• Choose which e-mail newsletters you want to receive

Reset your password

Please enter the e-mail address you used to register to reset your password

Note: The verification e-mail to change your password should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact [email protected]

Registration complete

Thank you for registering with Physics World If you'd like to change your details at any time, please visit My account

• Everyday science

Researchers cut to the chase on the physics of paper cuts

If you have ever been on the receiving end of a paper cut, you will know how painful they can be.

Kaare Jensen from the Technical University of Denmark (DTU), however, has found intrigue in this bloody occurrence. “I’m always surprised that thin blades, like lens or filter paper, don’t cut well, which is unexpected because we usually consider thin blades to be efficient,” Jensen told Physics World .

To find out why paper is so successful at cutting skin, Jensen and fellow DTU colleagues carried out over 50 experiments with a range of paper thicknesses to make incisions into a piece of gelatine at various angles.

Through these experiments and modelling, they discovered that paper cuts are a competition between slicing and “buckling”. Thin paper with a thickness of about 30 microns, or 0.03 mm, doesn’t cut so well because it buckles – a mechanical instability that happens when a slender object like paper is compressed. Once this occurs, the paper can no longer transfer force to the tissue, so is unable to cut.

Thick paper, with a thickness greater than around 200 microns, is also ineffective at making an incision. This is because it distributes the load over a greater area, resulting in only small indentations.

The team found, however, a paper cut “sweet spot” at around 65 microns and when the incision was made at an angle of about 20 degrees from the surface. This paper thickness just happens to be close to that of the paper used in print magazines, which goes some way to explain why it annoyingly happens so often.

Using the results from the work, the researchers created a 3D-printed scalpel that uses scrap paper for the cutting edge. Using this so-called “papermachete” they were able to slice through apple, banana peel, cucumber and even chicken.

Jensen notes that the findings are interesting for two reasons. “First, it’s a new case of soft-on-soft interactions where the deformation of two objects intertwines in a non-trivial way,” he says. “Traditional metal knives are much stiffer than biological tissues, while paper is still stiffer than skin but around 100 times weaker than steel.”

The second is that it is a “great way” to teach students about forces given that the experiments are straightforward to do in the classroom. “Studying the physics of paper cuts has revealed a surprising potential use for paper in the digital age: not as a means of information dissemination and storage, but rather as a tool of destruction,” the researchers write.

Want to read more?

Note: The verification e-mail to complete your account registration should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder.

If you haven't received the e-mail in 24 hours, please contact [email protected] .

• E-mail Address

Physics World Careers

Providing valuable careers advice and a comprehensive employer directory

• Dark matter and energy

LUX-ZEPLIN puts new limit on dark-matter mass

Discover more from physics world.

Pumping on a half-pipe: physicists model a skateboarding skill

• Culture, history and society

Physics World reveals its top 10 Breakthroughs of the Year for 2023

• Nanophotonics
• Research update

Metasurface makes thermal sources emit laser-like light

Related jobs, international marketing manager - uk, publishing services assistant - 12 month ftc, ebooks coordinator, related events.

• Medical physics | Workshop ISMRM Workshop on Motion Correction in MR 3—6 September 2024 | Québec City, Canada
• Particle and nuclear | Symposium World Nuclear Symposium 2024 4—6 September 2024 | London, UK
• Everyday science | Conference Pittcon Conference and Exposition 1—5 March 2025 | Boston, US

• SUGGESTED TOPICS
• The Magazine
• Newsletters
• Managing Yourself
• Managing Teams
• Work-life Balance
• The Big Idea
• Data & Visuals
• Reading Lists
• Case Selections
• HBR Learning
• Topic Feeds
• Account Settings
• Email Preferences

Research: How to Build Consensus Around a New Idea

• Devon Proudfoot
• Wayne Johnson

Strategies for overcoming the disagreements that can stymie innovation.

Previous research has found that new ideas are seen as risky and are often rejected. New research suggests that this rejection can be due to people’s lack of shared criteria or reference points when evaluating a potential innovation’s value. In a new paper, the authors find that the more novel the idea, the more people differ on their perception of its value. They also found that disagreement itself can make people view ideas as risky and make them less likely to support them, regardless of how novel the idea is. To help teams get on the same page when it comes to new ideas, they suggest gathering information about evaluator’s reference points and developing criteria that can lead to more focused discussions.

Picture yourself in a meeting where a new idea has just been pitched, representing a major departure from your company’s standard practices. The presenter is confident about moving forward, but their voice is quickly overtaken by a cacophony of opinions from firm opposition to enthusiastic support. How can you make sense of the noise? What weight do you give each of these opinions? And what does this disagreement say about the idea?

• DP Devon Proudfoot is an Associate Professor of Human Resource Studies at Cornell’s ILR School. She studies topics related to diversity and creativity at work.
• Wayne Johnson is a researcher at the Utah Eccles School of Business. He focuses on evaluations and decisions about new information, including persuasion regarding creative ideas and belief change.

Partner Center

Content Search

Yemen + 4 more

Crimes of Enforced Disappearance: A Research Paper Approaching the Issues and Envisioning Solutions [EN/AR]

Attachments.

Crimes of Enforced Disappearance

A Research Paper Addressing Challenges and Proposing Solutions

Mwatana for Human Rights, in partnership with the Smith Human Rights Clinic at Columbia Law School, has released a research paper titled "Crimes of Enforced Disappearance: Addressing Challenges and Proposing Solutions," coinciding with the International Day of the Victims of Enforced Disappearances, observed annually on August 30. The paper sheds light on the crimes of enforced disappearance committed by successive authorities over decades in Yemen, including those committed since the 1960s and during the ongoing armed conflict that began in September 2014.

The paper focuses on exploring potential mechanisms to address the burdened file of enforced disappearances in Yemen within the framework of transitional justice programs in the post-conflict phase. It discusses the challenges and issues related to the complete absence of accountability mechanisms, truth-seeking, uncovering the fate of the disappeared, and mechanisms for reparations for violations committed over the past decades up to the present.

The paper reviews the experiences of dealing with enforced disappearance and truth-seeking efforts in Syria, Sri Lanka, Argentina, and Colombia—contexts that have suffered from enforced disappearances for decades by different authorities and have developed effective mechanisms to address these issues. These experiences serve as valuable references when designing programs related to enforced disappearances in Yemen in the post-conflict phase.

Radhya Al-Mutawakel, Chairperson of Mwatana for Human Rights, stated, "It is time to explore possible ways to address the issue of enforced disappearance in Yemen and to work towards a future free of this brutal violation and its consequences." She added, "Studying other experiences that share similarities with Yemen's situation is crucial for understanding how other nations have dealt with complex issues like enforced disappearance in Yemen, the challenges they faced, and the ways they overcame them. It is also important to learn from the mistakes that accompanied those experiences to avoid them."

Mwatana emphasized that this paper is part of the organization’s efforts to address urgent elements and topics of transitional justice, which can provide well-considered inputs to guide transitional justice programs in Yemen and ensure their effectiveness. This will contribute to achieving sustainable peace, under which the dignity and freedoms of individuals are preserved, and the recurrence of human rights violations, particularly enforced disappearances, are prevented in the future.

Mwatana noted that thousands of Yemenis remain victims of enforced disappearance as a result of cycles of political conflict—crimes that continue to be passed down by the authorities and parties that have successively held power and led security agencies with their heavy legacy of victims of enforced disappearance, including forcibly disappeared individuals, their families, loved ones, and communities. This also includes hundreds of victims of enforced disappearance related to the current cycle of conflict, involving all local and regional parties.

Mwatana called on the international community, the United Nations, and international entities and institutions working in Yemen to ensure that their efforts and interventions in Yemen include comprehensive solutions for the crimes of enforced disappearance, uncovering the fate of the disappeared, reparations, accountability, compensation, and commemoration within the framework of comprehensive transitional justice. They also urged the formation of a criminal mechanism to investigate human rights violations, particularly enforced disappearance, and to work on strengthening the functioning of independent civil society organizations in monitoring the security and justice sectors, raising awareness, and building capacities.

Related Content

24 women receive economic empowerment grants, at least 214 arbitrary detentions recorded in august 2024, including 13 children and seven women [en/ar], rapid displacement tracking - yemen idp dashboard reporting period: 25 to 31 august 2024, driven by compassion: acts of kindness that change lives.

• Skip to main content
• Keyboard shortcuts for audio player

Research News

The physics behind the most annoying thing that could ever happen to you: a paper cut, the physics behind a very annoying thing that could ever happen to you: a paper cut.

Scientists have figured out what type of paper is the most prone to cut skin. Kaare Jensen, associate professor of physics at the Technical University of Denmark, explains.

Copyright © 2024 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Navigation Menu

Search code, repositories, users, issues, pull requests..., provide feedback.

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly.

To see all available qualifiers, see our documentation .

• Notifications You must be signed in to change notification settings

[SIGGRAPH Asia 2024, Journal Track] ToonCrafter: Generative Cartoon Interpolation

Doubiiu/ToonCrafter

Folders and files.

Repository files navigation

Tooncrafter: generative cartoon interpolation, 🔆 introduction.

⚠️ Please check our disclaimer first.

🤗 ToonCrafter can interpolate two cartoon images by leveraging the pre-trained image-to-video diffusion priors. Please check our project page and paper for more information.

1.1 Showcases (512x320)

1.2 Sparse sketch guidance

2. Applications

2.1 cartoon sketch interpolation (see project page for more details).

2.2 Reference-based Sketch Colorization

📝 Changelog

• Add sketch control and colorization function.
• [2024.05.29] : 🔥🔥 Release code and model weights.
• [2024.05.28] : Launch the project page and update the arXiv preprint.

Currently, our ToonCrafter can support generating videos of up to 16 frames with a resolution of 512x320. The inference time can be reduced by using fewer DDIM steps.

Install Environment via Anaconda (Recommended)

💫 inference, 1. command line.

Download pretrained ToonCrafter_512 and put the model.ckpt in checkpoints/tooncrafter_512_interp_v1/model.ckpt .

2. Local Gradio demo

Download the pretrained model and put it in the corresponding directory according to the previous guidelines.

📢 Disclaimer

Calm down. Our framework opens up the era of generative cartoon interpolation, but due to the variaity of generative video prior, the success rate is not guaranteed.

⚠️ This is an open-source research exploration, instead of commercial products. It can't meet all your expectations.

This project strives to impact the domain of AI-driven video generation positively. Users are granted the freedom to create videos using this tool, but they are expected to comply with local laws and utilize it responsibly. The developers do not assume any responsibility for potential misuse by users.

• Python 99.3%

The Federalist Papers

7 h total length

Discover the Genius of the Constitution

Thomas Jefferson described The Federalist Papers as “the best commentary on the principles of government, which ever was written.”

In this free ten-lecture course you will gain a deeper understanding of the purpose and structure of the American Founding by studying the arguments of America’s most influential Founders. Written between October 1787 and August 1788, The Federalist Papers  is a collection of newspaper essays written by James Madison, Alexander Hamilton, and John Jay in defense of the Constitution.

Taught by Hillsdale College’s politics faculty, this course explores the major themes of this classic work of American politics, such as the problem of majority faction, the importance of separation of powers, the nature of the three branches of government, and the argument concerning the Bill of Rights.

By enrolling in this course you will receive free access to the course lectures, readings, and quizzes to aid you in this examination of the greatest Constitution ever written.

We invite you to join us today in this urgent study of the Constitution and what has been lost by the modern assault on it.  

Expand Course Details

Lessons in this course.

Introduction: Articles of Confederation and the Constitutional Convention

Written following the Constitutional Convention of 1787,  The Federalist Papers  is the foremost American contribution to political thought. Originally published as newspaper essays in New York, they were written by Alexander Hamilton, James Madison, and John Jay under the pen name Publius. The essays defended the merits of the Constitution as a necessary and good replacement for the Articles of Confederation, which had proven defective as a means of governance.

The Improved Science of Politics

Publius argued that the “science of politics . . . has received great improvement” in his own day. These improvements include separation of powers, legislative checks and balances, judges who serve a life term during good behavior, and what he called “the ENLARGEMENT of the ORBIT” of government. Contrary to the practice of previous republics, Publius argued that a republic had a much greater chance of achieving success if it is spread out over a large or extended territory, rather than a small or contracted one.

The Problem of Majority Faction

In  Federalist  10, Publius confronts one of the most important Anti-Federalist arguments against ratification: Republican government is impossible on a territory as large as the United States. In fact, such an undertaking had never been successful. In response, Publius proposes that the principal remedy for the political disease of faction—a disease “most incident to republican government”—is the large or extended sphere or territory.

Federalism and Republicanism

In the summer of 1787, the Framers labored to set up a political regime that would not only secure liberty and republicanism, but also bring energy and stability to the national government. Because of the failures of government under the Articles of Confederation, American political institutions were at risk. In order to secure these institutions, the Framers constructed a republican form of government that—among many other important features—instituted a new form of federalism.

Separation of Powers

In constituting a new government, the Framers knew that written rules—what Publius calls “parchment barriers”—would not be enough by themselves to protect liberty and prevent tyranny. Instead, Publius looks to the “interior structure” as the best means for keeping the branches properly and effectively separated. Separation of powers, the most important of the Constitution’s “auxiliary precautions,” works to prevent governmental tyranny, and by keeping each branch within its proper sphere of authority allows each branch to do its job well.

The Legislative: House and Senate

The Founders understood that the legislative branch is by nature the most powerful in a republican government. Experience of government under the Articles of Confederation, when state legislatures routinely encroached on executive and judicial powers, confirmed this. Thus, the Framers divided the legislative branch into two parts—the House and the Senate. In addition, they differentiated them as much as possible, consistent with the principles of republican government, with the goal of preventing tyranny and encouraging good government.

The Executive

Following their experience under the Articles of Confederation, and armed with the improved science of politics, the Framers instituted a unitary executive in the office of the president. Unlike the executive office in any previous republic, it was designed so as to ensure energy and responsibility in the executive, which are absolutely essential for good execution of the laws, and therefore for good government.

The Judiciary

In the Declaration of Independence, one charge leveled against King George III was that he had “made Judges dependent on his Will alone.” In framing a republican government, the Founders believed that an independent judiciary was indispensable. Publius argues that the term of life tenure during good behavior and a protected salary ensure this independence.

“The Constitution Is Itself . . . a Bill of Rights”

In  Federalist  84, Publius writes, “The truth is, after all the declamation we have heard, that the constitution is itself, in every rational sense, and to every useful purpose, A BILL OF RIGHTS.” In other words, the structure of the Constitution protects the rights of the people. In addition, the American people retain all powers not granted to the federal and state governments.

Conclusion: Constitutionalism Today

American government today is much different from the constitutional republic outlined in  The Federalist Papers —which relies on structure, representation, and limitations on the functions of the federal government. Administrative regulations and entitlements are two distinguishing features of modern government. These new features require a kind of government that is unlimited, disregards separation of powers, and violates the supreme law under which it claims to operate.

View All Lessons

Enroll in this free online course on the federalist papers today.

What Current Students Are Saying

The instructors are very good. The tests make you think. There are other helpful discussion screens and the whole process makes it feel much like a classroom setting. I feel the Federalist Papers are somewhat difficult to read on your own, so this course makes it much easier to understand.

Create your FREE account today!

All you need to access our courses and start learning today is your email address.