Laccase applications in biofuels production: current status and future prospects

The desire to reduce dependence on the ever diminishing fossil fuel reserves coupled with the impetus towards green energy has seen increased research in biofuels as alternative sources of energy. Lignocellulose materials are one of the most promising feedstocks for advanced biofuels production. However, their utilisation is dependent on the efficient hydrolysis of polysaccharides, which in part is dependent on cost-effective and benign pretreatment of biomass to remove or modify lignin and release or expose sugars to hydrolytic enzymes. Laccase is one of the enzymes that are being investigated not only for potential use as pretreatment agents in biofuel production, mainly as a delignifying enzyme, but also as a biotechnological tool for removal of inhibitors (mainly phenolic) of subsequent enzymatic processes. The current review discusses the major advances in the application of laccase as a potential pretreatment strategy, the underlying principles as well as directions for future research in the search for better enzyme-based technologies for biofuel production. Future perspectives could include synergy between enzymes that may be required for optimal results and the adoption of the biorefinery concept in line with the move towards the global implementation of the bioeconomy strategy.     

Hydrothermal liquefaction of Malaysia's algal biomass for high‐quality bio‐oil production

Currently, fossil materials form the majority of our energy and chemical source. Many global concerns force us to rethink about our current dependence on the fossil energy. Limiting the use of these energy sources is a key priority for most countries that pledge to reduce greenhouse gas emissions. The application of biomass, as substitute fossil resources for producing biofuels, plastics and chemicals, is a widely accepted strategy for sustainable development. Aquatic plants including algae possess competitive advantages as biomass resources compared to the terrestrial plants in this current global situation. Bio‐oil production from algal biomass is technically and economically viable, cost competitive, requires no capacious lands and minimal water use and reduces atmospheric carbon dioxide. The aim of this paper is to review the potential of converting algal biomass, as an aquatic plant, into high‐quality crude bio‐oil through applicable processes in Malaysia. In particular, bio‐based materials and fuels from algal biomass are considered as one of the reliable alternatives for clean energy. Currently, pyrolysis and hydrothermal liquefaction (HTL) are two foremost processes for bio‐oil production from biomass. HTL can directly convert high‐moisture algal biomass into bio‐oil, whereas pyrolysis requires feedstock drying to reduce the energy consumption during the process. Microwave‐assisted HTL, which can be conducted in aqueous environment, is suitable for aquatic plants and wet biomass such as algae.     

Biofuels from microbes

Today, biomass covers about 10% of the world’s primary energy demand. Against a backdrop of rising crude oil prices, depletion of resources, political instability in producing countries and environmental challenges, besides efficiency and intelligent use, only biomass has the potential to replace the supply of an energy hungry civilisation. Plant biomass is an abundant and renewable source of energy-rich carbohydrates which can be efficiently converted by microbes into biofuels, of which, only bioethanol is produced on an industrial scale today. Biomethane is produced on a large scale, but is not yet utilised for transportation. Biobutanol is on the agenda of several companies and may be used in the near future as a supplement for gasoline, diesel and kerosene, as well as contributing to the partially biological production of butyl-t-butylether, BTBE as does bioethanol today with ETBE. Biohydrogen, biomethanol and microbially made biodiesel still require further development. This paper reviews microbially made biofuels which have potential to replace our present day fuels, either alone, by blending, or by chemical conversion. It also summarises the history of biofuels and provides insight into the actual production in various countries, reviewing their policies and adaptivity to the energy challenges of foreseeable future.     

Association with an ammonium-excreting bacterium allows diazotrophic culture of oil-rich eukaryotic microalgae

Concerns regarding the depletion of the world's reserves of oil and global climate change have promoted an intensification of research and development toward the production of biofuels and other alternative sources of energy during the last years. There is currently much interest in developing the technology for third-generation biofuels from microalgal biomass mainly because of its potential for high yields and reduced land use changes in comparison with biofuels derived from plant feedstocks. Regardless of the nature of the feedstock, the use of fertilizers, especially nitrogen, entails a potential economic and environmental drawback for the sustainability of biofuel production. In this work, we have studied the possibility of nitrogen biofertilization by diazotrophic bacteria applied to cultured microalgae as a promising feedstock for next-generation biofuels. We have obtained an Azotobacter vinelandii mutant strain that accumulates several times more ammonium in culture medium than wild-type cells. The ammonium excreted by the mutant cells is bioavailable to promote the growth of nondiazotrophic microalgae. Moreover, this synthetic symbiosis was able to produce an oil-rich microalgal biomass using both carbon and nitrogen from the air. This work provides a proof of concept that artificial symbiosis may be considered an alternative strategy for the low-N-intensive cultivation of microalgae for the sustainable production of next-generation biofuels and other bioproducts.     

Sustainable liquid biofuels from biomass: the writing's on the walls

Domination of the global biosphere by human beings is unprecedented in the history of the planet, and our impact is such that substantive changes in ecosystems, and the global environment as a whole, are now becoming apparent. Our activity drives the steady increase in global temperature observed in recent decades. The realization of the adverse effects of greenhouse gas emissions on the environment, together with declining petroleum reserves, has ensured that the quest for sustainable and environmentally benign sources of energy for our industrial economies and consumer societies has become urgent in recent years. Consequently, there is renewed interest in the production and use of fuels from plants. The 'first-generation' biofuels made from starch and sugar appear unsustainable because of the potential stress that their production places on food commodities. Second-generation biofuels, produced from cheap and abundant plant biomass, are seen as the most attractive solution to this problem, but a number of technical hurdles must be overcome before their potential is realized. This review will focus on the underpinning research necessary to enable the cost-effective production of liquid fuels from plant biomass, with a particular focus on aspects related to plant cell walls and their bioconversion.     

Developments and perspectives of photobioreactors for biofuel production

The production of biofuels from microalgae requires efficient photobioreactors in order to meet the tight constraints of energy efficiency and economic profitability. Current cultivation systems are designed for high-value products rather than for mass production of cheap energy carriers. Future bioreactors will imply innovative solutions in terms of energy efficiency, light and gas transfer or attainable biomass concentration to lower the energy demand and cut down production costs. A new generation of highly developed reactor designs demonstrates the enormous potential of photobioreactors. However, a net energy production with microalgae remains challenging. Therefore, it is essential to review all aspects and production steps for optimization potential. This includes a custom process design according to production organism, desired product and production site. Moreover, the potential of microalgae to synthesize valuable products additionally to the energetic use can be integrated into a production concept as well as waste streams for carbon supply or temperature control.     

Recycling and reuse of spent microalgal biomass for sustainable biofuels

The use of microalgal biomass (MAB) for biofuel production has been recognized since long. Despite distinct advantages of algal biofuels, however, their sustainability and economic viability is still doubtful. Overall process cost and low energy recovery need to be significantly improved. The use of MAB, after extracting primary fuels in the form of hydrogen, methane, biodiesel and bioethanol, can be one promising route. This algal biomass, collectively termed as spent microalgal biomass (SMAB), contains even up to 70% of its initial energy level and also retains nutrients including proteins, carbohydrates, and lipids. Potential application routes include diet for animals and fish, the removal of heavy metals and dyes from wastewater, and the production of bioenergy (e.g., biofuels and electricity). Unlike whole algae biomass whose applications are relatively well documented, SMAB has been studied only to limited degree. Therefore, this work gives a brief overview of various ways of SMAB applications. An insight into current status, barriers and future prospects on SMAB research is provided. The feasibility of each application is evaluated on the basis of its energy recovery, economic viability, and future perspectives are provided.     

Renewable fuels from algae: An answer to debatable land based fuels

This article reviews the utilization of first and second-generation biofuels as the suitable alternatives to depleting fossil fuels. Then the concern has been presented over a debate on most serious problem arising from the production of these biofuels; which is the increase of food market prices because of the increased use of arable land for the cultivation of biomass used for the production of first and second-generation biofuels. The solution to this debate has been suggested with the use of non-arable land for the cultivation of algal biomass for the generation of third generation biofuels. The recent research and developments in the cultivation of algal biomass and their use for biofuel production have been discussed.     

Microalgae potential as a biogas source: current status,restraints and future trends

In recent years, the world energy demands have had a recurrent increase. For this reason the alternative to the fossil fuel resources are trend topics in investigation. Microalgae have been extensively studied as a source of biofuels and as one of the most promising alternatives in this new framework. One of the possibilities of obtaining renewable energy from microalgae is biogas production using anaerobic digestion process. This process is considered a significant component for biofuels and waste management, since represent an opportunity for energy generation using different wastewater products; also, the economic viability of microalgae liquid biofuel production could be improved. However, the anaerobic digestion of microalgae biomass is still not optimized because of the numerous technical limitations such as the microalgae characteristics, low carbon:nitrogen ratio, ammonia toxicity and even salinity. The present review summarizes and compares information concerning to anaerobic digestion of microalgal biomass and future directions for research. Besides, specific operational factors and potential inhibitory parameters of the process are analyzed and compared. Additionally, the paper covers the state or art concerning in methane production enhancement from algal biomass.     

Sulla (<Emphasis Type="Italic">Hedysarum coronarium</Emphasis> L.) as Potential Feedstock for Biofuel and Protein

Although sulla (Hedysarum coronarium L.) has many interesting features that could support the production of biofuels (e.g., a high yield and soluble sugar content, N-fixation capacity, low input requirements for its cultivation), no study has assessed the possibility of its use for that purpose. Our objective was to evaluate the potential value for energy production of sulla cut at various stages of growth. Furthermore, the potential of sulla as a dual purpose crop (energy and feed) was investigated. The crop was grown in rainfed conditions in a typical Mediterranean environment (over two complete 2-year crop cycles) and was cut at four different phenological stages. The biomass was divided into two fractions (stems and leaves), weighed, and analyzed to estimate the theoretical production of bioethanol and biomethane and the feed value of the whole biomass and of the two fractions. The total dry matter yield in the 2-year crop cycle was about 18 Mg ha^?1; this level of production is similar to or higher than that of most other crops grown in the same environment in rainfed conditions. The stems had a high content of total soluble sugars (even higher than 200 g kg^?1) and cell wall polysaccharides, markedly higher than the leaves. The leaves contained most of the protein of the plant, representing an actual protein concentrate. Thus, the crop seems particularly suitable for dual purpose use if stems are allocated to the production of biofuels and leaves to the production of livestock feed. Moreover, the results showed that the early seed set stage is the most appropriate cutting time for maximizing yield both for energy and for livestock use.     

Biotreibstoffe aus Entwicklungsländern

Biofuels from developing countries The pressure for reducing greenhouse gas emissions, rising oil prices, but also the lobbying by the agricultural sector and the automotive industry have induced the recent boom on biofuels. Due to limited land availability, competition with food production and high overall environmental impacts, the sustainability market potential for biofuels is assumed to be significantly smaller than 10% of global fuel consumption. Nevertheless, niches for the sustainable production and use of biofuels exist especially in developing countries. It is often more sustainable to use biomass feedstock for local supply of electricity and heat than producing biofuels for export.     

Algal biofuels

The world is facing energy crisis and environmental issues due to the depletion of fossil fuels and increasing CO₂ concentration in the atmosphere. Growing microalgae can contribute to practical solutions for these global problems because they can harvest solar energy and capture CO₂ by converting it into biofuel using photosynthesis. Microalgae are robust organisms capable of rapid growth under a variety of conditions including in open ponds or closed photobioreactors. Their reduced biomass compounds can be used as the feedstock for mass production of a variety of biofuels. As another advantage, their ability to accumulate or secrete biofuels can be controlled by changing their growth conditions or metabolic engineering. This review is aimed to highlight different forms of biofuels produced by microalgae and the approaches taken to improve their biofuel productivity. The costs for industrial-scale production of algal biofuels in open ponds or closed photobioreactors are analyzed. Different strategies for photoproduction of hydrogen by the hydrogenase enzyme of green algae are discussed. Algae are also good sources of biodiesel since some species can make large quantities of lipids as their biomass. The lipid contents for some of the best oil-producing strains of algae in optimized growth conditions are reviewed. The potential of microalgae for producing petroleum related chemicals or ready-make fuels such as bioethanol, triterpenic hydrocarbons, isobutyraldehyde, isobutanol, and isoprene from their biomass are also presented.     

Development and status of dedicated energy crops in the United States

The biofuel industry is rapidly growing because of increasing energy demand and diminishing petroleum reserves on a global scale. A multitude of biomass resources have been investigated, with high-yielding, perennial feedstocks showing the greatest potential for utilization as advanced biofuels. Government policy and economic drivers have promoted the development and commercialization of biofuel feedstocks, conversion technologies, and supply chain logistics. Research and regulations have focused on the environmental consequences of biofuels, greatly promoting systems that reduce greenhouse gas emissions and life-cycle impacts. Numerous biofuel refineries using lignocellulosic feedstocks and biomass-based triglycerides are either in production or pre-commercial development phases. Leading candidate energy crops have been identified, yet require additional efforts to realize their full potential. Advanced biofuels, complementing conventional biofuels and other renewable energy sources such as wind and solar, provide the means to substantially displace humanity’s reliance on petroleum-based energy.     

Meeting the challenge of food and energy security

Growing crops for bioenergy or biofuels is increasingly viewed as conflicting with food production. However, energy use continues to rise and food production requires fuel inputs, which have increased with intensification. Focussing on the question of food or fuel is thus not helpful. The bigger, more pertinent, challenge is how the increasing demands for food and energy can be met in the future, particularly when water and land availability will be limited. Energy crop production systems differ greatly in environmental impact. The use of high-input food crops for liquid transport fuels (first-generation biofuels) needs to be phased out and replaced by the use of crop residues and low-input perennial crops (second/advanced-generation biofuels) with multiple environmental benefits. More research effort is needed to improve yields of biomass crops grown on lower grade land, and maximum value should be extracted through the exploitation of co-products and integrated biorefinery systems. Policy must continually emphasize the changes needed and tie incentives to improved greenhous gas reduction and environmental performance of biofuels.     

Integration of microalgae cultivation with industrial waste remediation for biofuel and bioenergy production: opportunities and limitations

There is currently a renewed interest in developing microalgae as a source of renewable energy and fuel. Microalgae hold great potential as a source of biomass for the production of energy and fungible liquid transportation fuels. However, the technologies required for large-scale cultivation, processing, and conversion of microalgal biomass to energy products are underdeveloped. Microalgae offer several advantages over traditional 'first-generation' biofuels crops like corn: these include superior biomass productivity, the ability to grow on poor-quality land unsuitable for agriculture, and the potential for sustainable growth by extracting macro- and micronutrients from wastewater and industrial flue-stack emissions. Integrating microalgal cultivation with municipal wastewater treatment and industrial CO(2) emissions from coal-fired power plants is a potential strategy to produce large quantities of biomass, and represents an opportunity to develop, test, and optimize the necessary technologies to make microalgal biofuels more cost-effective and efficient. However, many constraints on the eventual deployment of this technology must be taken into consideration and mitigating strategies developed before large scale microalgal cultivation can become a reality. As a strategy for CO(2) biomitigation from industrial point source emitters, microalgal cultivation can be limited by the availability of land, light, and other nutrients like N and P. Effective removal of N and P from municipal wastewater is limited by the processing capacity of available microalgal cultivation systems. Strategies to mitigate against the constraints are discussed.     

Certification to ensure sustainable production of biofuels

Sustainability risks increase with rising biofuel production. Environmental, social and economic sustainability issues in agricultural production and conversion processes have become a major concern. External effects of biofuel production are not covered by a market mechanism. Therefore, an instrument to address the most pressing sustainability issues of biofuel production is required. The article provides the outline of an implementable certification concept that was elaborated in a multi-stakeholder approach with companies and organizations from Europe, the Americas, and Asia. The approach chosen for certification is beneficial for stakeholders, is market based and does not hamper international trade. The proposed certification system is focused on the most pressing sustainability issues, such as conversion of high carbon density and high nature value land. “Major must” and “minor must” criteria have been developed to assess the sustainability. The certification itself should occur through a compact and cost-effective system. A meta system approach will allow the use of already existing standards. Two separate certificates, one for sustainability of biomass production and one for greenhouse gas emissions, are proposed. The certificates are decoupled in a book and claim system from the biomass or biofuels traded and will be traded on a market place. The general concept of this certification system for biomass and biofuels has been discussed intensively with industry and trade companies and with public sector organizations and nongovernmental organizations. The practical feasibility should be tested in a pilot phase with co-operation of the stakeholders who are already involved.     

Photosynthetic terpene hydrocarbon production for fuels and chemicals

Photosynthetic hydrocarbon production bypasses the traditional biomass hydrolysis process and represents the most direct conversion of sunlight energy into the next‐generation biofuels. As a major class of biologically derived hydrocarbons with diverse structures, terpenes are also valuable in producing a variety of fungible bioproducts in addition to the advanced ‘drop‐in’ biofuels. However, it is highly challenging to achieve the efficient redirection of photosynthetic carbon and reductant into terpene biosynthesis. In this review, we discuss four major scientific and technical barriers for photosynthetic terpene production and recent advances to address these constraints. Collectively, photosynthetic terpene production needs to be optimized in a systematic fashion, in which the photosynthesis improvement, the optimization of terpene biosynthesis pathway, the improvement of key enzymes and the enhancement of sink effect through terpene storage or secretion are all important. New advances in synthetic biology also offer a suite of potential tools to design and engineer photosynthetic terpene platforms. The systemic integration of these solutions may lead to ‘disruptive’ technologies to enable biofuels and bioproducts with high efficiency, yield and infrastructure compatibility.     

Biofuel production in Escherichia coli: the role of metabolic engineering and synthetic biology

The microbial production of biofuels is a promising avenue for the development of viable processes for the generation of fuels from sustainable resources. In order to become cost and energy effective, these processes must utilize organisms that can be optimized to efficiently produce candidate fuels from a variety of feedstocks. Escherichia coli has become a promising host organism for the microbial production of biofuels in part due to the ease at which this organism can be manipulated. Advancements in metabolic engineering and synthetic biology have led to the ability to efficiently engineer E. coli as a biocatalyst for the production of a wide variety of potential biofuels from several biomass constituents. This review focuses on recent efforts devoted to engineering E. coli for the production of biofuels, with emphasis on the key aspects of both the utilization of a variety of substrates as well as the synthesis of several promising biofuels. Strategies for the efficient utilization of carbohydrates, carbohydrate mixtures, and noncarbohydrate carbon sources will be discussed along with engineering efforts for the exploitation of both fermentative and nonfermentative pathways for the production of candidate biofuels such as alcohols and higher carbon biofuels derived from fatty acid and isoprenoid pathways. Continued advancements in metabolic engineering and synthetic biology will help improve not only the titers, yields, and productivities of biofuels discussed herein, but also increase the potential range of compounds that can be produced.     

Miscanthus: A fast‐growing crop for environmental remediation and biofuel production

As an herbaceous perennial, Miscanthus has attracted extensive attention in bioenergy refinery and ecological remediation due to its high yield and superior environmental adaptability. This review summarizes current research advances of Miscanthus in several aspects including biological properties, biofuels production, and phytoremediation of contaminated soil. Miscanthus has relatively high biomass yield, calorific value, and cellulose content compared with other lignocellulosic bioenergy crops, which make it one of the most promising feedstocks for the production of second‐generation biofuels. Moreover, Miscanthus can endure soil pollutions caused by various heavy metals and survive in a variety of adverse environmental conditions. Therefore, it also has potential applications in ecological remediation of contaminated soil, and reclamation of polluted soil and water resources. Nevertheless, more endeavors are still needed in the genetic improvement and elite cultivar breeding, large‐scale cultivation on marginal land, and efficient conversion to biofuels, when utilizing Miscanthus as a bioenergy crop. Furthermore, more efforts should also be undertaken to translate Miscanthus into a bioenergy crop with the phytoremediation potential.     

From photons to biomass and biofuels: evaluation of different strategies for the improvement of algal biotechnology based on comparative energy balances

Microalgal based biofuels are discussed as future sustainable energy source because of their higher photosynthetic and water use efficiency to produce biomass. In the context of climate CO₂ mitigation strategies, algal mass production is discussed as a potential CO₂ sequestration technology which uses CO₂ emissions to produce biomass with high-oil content independent on arable land. In this short review, it is presented how complete energy balances from photon to harvestable biomass can help to identify the limiting processes on the cellular level. The results show that high productivity is always correlated with high metabolic costs. The overall efficiency of biomass formation can be improved by a photobioreactor design which is kinetically adapted to the rate-limiting steps in cell physiology. However, taking into account the real photon demand per assimilated carbon and the energy input for biorefinement, it becomes obvious that alternative strategies must be developed to reach the goal of a real CO₂ sequestration.