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News and Trends
(full access to article may require subscription of payment) Fuels from cellulosic biomass are one of the leading alternatives in meeting sustainability and energy security challenges associated with fossil fuels. Biological conversion processes involving microbial fermentation (after biomass pretreatment) are among the leading options for producing these cellulosic biofuels. However, in the use of microorganisms for biofuel production, the microorganism's low ethanol tolerance (i.e. the maximum ethanol concentration in the fermentation medium in which the microorganism could still produce ethanol) limits the maximum yield of the bioethanol product during fermentation. A team of researchers from the Oak Ridge National Laboratory (ORNL), the Thayer School of Engineering, Dartmouth College and the University of Tennessee (United States), recently discovered a single key gene responsible for the high ethanol tolerance in Clostridium thermocellum. In their study, the researchers first resequenced the genome of an ethanol tolerant mutant and compared it with the normal strain of Clostridium thermocellum. Then, mutations in the genome were identified and mutations associated with ethanol metabolism/tolerance were pinpointed. The researchers verified the genome's ethanol-tolerant trait by putting a particular copy of the pinpointed alcohol tolerance gene into a wild type strain. They then determined the transformed organism's capability to grow in elevated levels of ethanol. From the results of their study, the researchers found that the increased ethanol tolerance of the C. thermocellum is due to a mutated bifunctional acetaldehyde-CoA/alcohol dehydrogenase gene (adhE) and the possible mechanism for the increased ethanol tolerance is the alteration of the NADH cofactor binding specificity in the protein. The simplicity of the genetic basis for the ethanol-tolerant phenotype observed, according to the researchers, is a major breakthrough that could improve cellulosic ethanol production by the utilization of more ethanol-tolerant mutant microbial strains. The full paper is published in the journal, Proceedings of the National Academy of Sciences of the United States of America (PNAS) (URLabove).
Scientists from the University of Miami Rosenstiel School of Marine & Atmospheric Science (United States) report the use of stable isotopic ratio measurements of carbon-13 to carbon-12 to detect and distinguish ethanol from automotive exhausts and biogenic ethanol emissions from tropical plants. Unburned biofuel ethanol in vehicles can be emitted into the atmosphere and can have potential impacts on air quality. At the same time, "natural ethanol emissions" are said to exist in living plants. The researchers discovered that unique "ethanol signatures" from these emissions can be distinguished by looking at the ratio of carbon-13 to carbon-12 isotopes in the samples. They report that "ethanol emitted in exhaust is distinctly different from that emitted by tropical plants and can serve as a unique stable isotopic tracer for transportation-related inputs to the atmosphere". When they used the technique to analyze air samples in downtown Miami and the Everglades National Park, they found "that 75% of ethanol in Miami's urban air came from synthetic biofuels, while the majority of ethanol in the Everglades air was emitted from plants, even though a small quantity of city pollution from a nearby road floats into the park". One possible application of the technique, as suggested by the research team would be during aircraft sampling campaigns to identify and track plumes as they drift away from urban areas." The complete paper is published in the journal, Environmental Science and Technology (URL above). Energy Crops and Feedstocks for Biofuels Production
(complete access to journal article may require payment or subscription) Macroalgae are multicellular organisms that are capable of generating higher dry biomass at a faster rate, compared to terrestrial plants. These features have made macroalgae a potential biofuel feedstock. Among the macroalgae, the largest growing species are reportedly those belonging to the Class Phaeophyceae and are termed as "kelp". However, previous studies on kelp cultivation harvested the macroalgae at only one period of the year, and the effects of seasonal variations, which can drastically change the macroalgae's chemical composition, were not considered. In order to fill these knowledge gaps, researchers from the Institute of Biological, Environmental and Rural Sciences (IBERS), Aberystwyth University and Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering, University of Leeds (United Kingdom), recently investigated the effect of changing seasons in the chemical composition of the macroalgae, Laminaria digitata. In their study, the researchers harvested the kelps at different months during a one-year period. After which, they measured the chemical composition of macroalgae. Finally, they determined the ideal month for harvesting the kelps. From the results of their study, the researchers found that for the production of biofuel, July would be the best month for Laminaria harvesting, since the metal concentrations were low, carbohydrate concentrations were high, and the high heating values (as determined from the proximate and ultimate analysis) were also the highest. The macroalgal harvest seems suited for biofuel production by biological conversion. Thermochemical conversion of the macroalgae to biofuels (for example, by combustion or pyrolysis), may not be recommended, due to the high water and metal contents. The full paper is published in the journal, Bioresource Technology (URL above). Biofuels ProcessingXylose is considered as the second most abundant 5-carbon sugar (a pentose) from pretreated/hydrolyzed lignocellulosic biomass, which can potentially be converted to ethanol by fermentation. The most abundant sugar in pretreated/hydrolyzed lignocellulosic biomass is usually glucose (a 6-carbon sugar, or hexose). However, in many ethanol fermentations from pretreated/saccharified lignocellulosic substrates, xylose is not as efficiently utilized to ethanol (compared to glucose), due to the absence of pentose-metabolizing capabilities of the fermenting yeast (Saccharomyces cerevisiae). While molecular biology techniques have attempted to engineer Saccharomyces cerevisiae strains which harbor genes for pentose metabolism, the fermentative capacity of these strains "pales in comparison to glucose". Thus, industrial fermentation of pentoses (such as xylose) by Saccharomyces cerevisiae still has limited economic feasibility. Recently, scientists from the University of Wisconsin, the Great Lakes Bioenergy Research Center, the US Department of Energy Joint Genome Institute, and Michigan State University report a new approach for understanding pentose metabolism in yeasts. While previous studies used approaches involving "metabolic modeling, single-species genome/expression analysis and directed evolution", they used "comparative genomics". They sequenced the genomes of xylose-fermenting yeast species, and applied a cross-species comparative genomics approach. In so doing, they identified several genes, that when expressed in Saccharomyces cerevisiae, can "significantly improve xylose-dependent growth and xylose assimilation". The approach can be a strategy for improving the economic feasibility of industrial ethanol fermentation from pentoses. The full paper is published in the Proceedings of the National Academy of Sciences (PNAS) (URL above).
(complete access to journal article may require payment or subscription) A team of researchers from the Department of Chemical and Biomolecular Engineering and Department of Bioengineering, Rice University (United States), recently developed an "efficient technique in converting simple glucose into biofuels and petrochemical substitutes" through the use of reverse beta oxidation cycle. The beta oxidation cycle is said to be one of the most efficient and most fundamental metabolic pathways used by organisms to break down fatty acids into energy. In their study, the researchers reversed the beta oxidation cycle by selectively manipulating about a dozen genes in the bacteria Escherichia coli. By harnessing the efficiency of the beta oxidation pathway, the researchers successfully converted glucose to biobutanol, an "advanced" biofuel that can be substituted for gasoline in most engines, at a "breakneck pace". According to the study, some advantages of this novel method are (1) the selective manipulations of certain genes could produce fatty acids of particular lengths, including long-chain molecules like stearic acid and palmitic acid, which have chains of more than a dozen carbon atoms; (2) the rate of biobutanol production is ten times faster than any previous method on a cell-per-cell basis; and (3) any other industrial organisms, such as yeast or algae, could be used in this method since the beta-oxidation pathway is present in almost every organism. The full paper is published in the journal, Nature (URL above). Biofuels Policy and Economics
(complete access to journal article may require payment or subscription) Life Cycle Assessment (LCA) is a common approach for the assessment of biofuel sustainability. It is currently used by regulatory bodies, such as California's Low Carbon Fuel Standard and the US Renewable Fuel Standard, for evaluating environmental impacts of different biofuel types. However, drawbacks of this approach (such as "critical but difficult-to-verify assumptions, limits of available data and disputes about system boundaries) have prompted the development of other approaches in the assessment of sustainability. Dr. John DeCicco from the School of Natural Resources and Environment, University of Michigan (United States) recently proposed an alternative approach in the evaluation of biofuel sustainability. In his paper, he recommended the use of annual basis carbon (ABC) accounting to track the stocks and flows of carbon and other relevant greenhouse gases (GHGs) throughout fuel supply chains. In ABC accounting, "the fuel and feedstock production facilities are the focus of accounting while treating the carbon dioxide emissions from fuel end-use at face value regardless of the origin of the fuel carbon". The ABC approach is said to differ from the LCA approach in the following ways: (1) the ABC approach does not automatically credit biogenic carbon (biogenic carbon can be interpreted as the carbon released from biological activities or from the processing of biological material), (2) "the ABC policy does not fundamentally rely on a need for baselines because it is not attempting to treat the issue as an offsets problem", (3) the ABC approach has a "source focus", rather than a "product focus". There are three elements to the ABC approach, as summarized by the Greencar Congress website: (1) full accounting of carbon dioxide emissions from fuel end-use regardless of the fuel's origin; (2) an attributional accounting protocol that relies on facilities-level GHG balances to report net carbon dioxide uptake and track otherwise unregulated GHG emissions throughout fuel and feedstock supply chains; and (3) a mechanism for mitigating consequential impacts, particularly, the leakage due to indirect land use change. The full paper is published in the journal, Biofuels and Carbon Management (URL above).
http://papers.ssrn.com/pape.tar?abstract_id=1805008 With the potential of better energy security and better environmental quality, biofuels development has become a national policy agenda in many countries. Various forms of regulatory schemes, such as federal income tax provisions, biofuel-mandated blending and agricultural program subsidies, are placed in order to provide incentive for the use of biofuels. However, law professors from the University of Illinois Energy Biosciences Institute recently argue that the current regulatory framework also hinders the commercialization of the innovations in the biofuel industry. In their paper, they first provided a normative analysis of the regulatory schemes incentivizing and governing the commercialization of biofuel-related technological innovations. Then, they applied the insights via a detailed case study that focuses on biobutanol, an emerging biofuel with the potential to act as a socially optimal alternative to petroleum-based transportation fuels. Finally, they provided suggestions that will help mitigate unjustified regulatory hurdles to the commercialization of biofuel-based technological innovations. According to the researchers, the two main reasons in current regulatory schemes that hinder commercialization of biofuel innovation are: (1) legally accepted biofuel must be substantially similar to the current commercial biofuel; and (2) the time-consuming and costly process of getting regulatory approval for new biofuels. The full paper is available at the Social Science Research Network website (URL above). |
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