viernes, 19 de julio de 2013

Biofuels: How Research Has Evolved with Government Policy

ORIGINAL: OBR Review
17th July 2013

It has been more than 20 years since the UN framework convention on climate change was ratified by over 150 countries, and in 2008 the UK was the first to establish a legally binding climate change target; to cut greenhouse gases in the UK by 80% by 2050 (1990 baseline). [1] Arguably the most visible response to the daunting task has been the widespread adoption of a ‘Green Mindset,’ leading to more economical energy usage in the home, but there is still a greater need for more complex strategies to replace the non-renewable fossil fuels that power cars and industry. For example, energy consumers in the home contribute to just 15% of total UK carbon emissions, whereas fossil fuel consumption in the energy supply and transport sectors is responsible for an estimated 40% and 24% of the total, respectively (2012 figures). [2] It is in this respect that biofuels may prove a cleaner alternative.

Green Biotech research is now offering new opportunities to replace increments of petrol or diesel with bio-based fuels, the first generation of which has likely been a contributor to an estimated 19% reduction in UK carbon emissions between 1990-2012, as published by government earlier this year. With further advances in Green Biotechnology, it is anticipated that a new generation of biofuels could eventually replace petroleum-based fuels entirely.

First generation bio-based fuels: still a candidate for a cleaner climate? 
Field of Rapeseed
The production of new sustainable fuels has reverted to an age-old strategy, to use biomass, for energy generation. The process of burning wood for heat dates back to prehistory, now combustion of first generation fuels derived from crops such as corn or rapeseed has appeared in the transport energy production pipeline.

Ethanol, which can be chemically extracted from corn, was first established as a viable automotive fuel in the early 20th century. Two centuries earlier, Rudolf Diesel proposed pure vegetable oils could drive agricultural vehicles. The difference between then and now is that the greater need for greener fuels is driving improvements on the methodology, scale, and efficiency by which ethanol can be extracted from crops or vegetable oils and converted into fatty acid alkyl esters, i.e. modern biodiesel. Nonetheless, biodiesel is costly to produce, pushing further rises to energy prices, and to date, biofuels are mostly inferior in energy to petroleum-based fuels, which are chemically optimal for internal combustion. [3]

Despite their disadvantages, there is still a market for first generation biofuels, which is perhaps driven by an absence of other commercially available alternatives, and by UK government policy. For example, the Renewable Transport Fuels Obligation requires transport fuel suppliers to supplement conventional petrol or diesel with at least 5% renewable fuel.

The impetus to extrapolate the production of crop-derived fuel is however somewhat questionable economically; biofuel and food crop production rely on the same agriculturally available land. [4] In 1999, it had been estimated that agriculture already occupied nearly a third of global land surface, [5] and as the global population continues to grow, inevitably so too will demand for food and energy, intensifying competition for land space. It is hardly surprising that in January 2013, a EU proposal was announced to revise the ‘Renewable Energy Directive’, capping food crop-based biofuel supplements at 5% by 2020. [6]

Advanced bio-based fuels: microbes as candidates for fuel production

An exploration of unconventional hosts and previously unknown metabolic biology has yielded new possibilities for advanced bio-based fuels. Microbes that can feed off degraded wood, crop residues, grasses, and other plant waste are considered gold-dust for second generation biofuel research. By metabolising non-food plant feedstock, certain microbes can create ethanol fuel as a fermentation by-product of the natural biochemical pathway. [4]

Yeast has for eons produced ethanol from the break down of simple sugars in the creation of bread, beer and wine. Though alike to other traditional model organisms, Baker’s yeast lacks the specific glycosidase enzymes that metabolise complex cellulosic waste into fermentable, glucose-like sugars. Relationships between specific glycosidases and their cellulosic substrates are also often incompletely characterised, leaving less room to supply microbes with the optimal ‘missing’ enzymes. [7]

Studies of ‘unconventional’ organisms have laid down a framework for engineering a suitable host for microbial biofuel production. Through the action of specific glycosidases, the bacterium Clostridium cellulolyticum was found capable of producing low yields of ethanol from rice straw. [8] More unusually, in an American Chemical Society meeting this year, fungi present in the faeces of horses was highlighted as a potential treasure-trove of cellulase enzymes, which allow the fungi to flourish on lignin-rich grass and release fermentable sugars for horse digestion. [9] A DNA sequencing study on the Limnoriid wood borer has also revealed a transcriptome swamped with putative cellulose-degrading enzyme-encoding genes. [10]

Unconventional hosts are not easy to manipulate in industry, whereas engineering well-established hosts with newly characterised glycosidase-encoding genes may be the key to commercialising microbial fuel production. Processes that exploit yeast and E. coli are already scaled-up to provide populations with many products, such as food and antibiotics. Artificially adapting the glycosidase enzymes by mutagenesis or over-expression may also produce higher specific activities and higher ethanol yields. [4]

The work of University of Exeter-based Professor John Love may well prove a remarkable turning point demonstrating widespread potential in the microbe biofuel field. Ethanol is an incomparable replacement for energy-dense petroleum-based fuels. The existence of a carbon-neutral and exact substitute for fossil fuels is, in natural terms, unlikely. Using modern synthetic biology technologies, Professor Love’s research group, funded by Shell and the BBSRC, was able to manipulate E. coli bacteria to convert glucose to a near-chemical replica of conventional diesel. In a paper published by PNAS earlier this year, the group note further work will entail the synthesis of novel metabolic pathways to enable E. coli to metabolise non-food plant feedstock3.

Advanced bio-based fuels: a future commercial reality?
With biological advances, microbes may well have potential, but the cost of research into de-carbonising the planet must first be met should these few first solid steps into what could be a commercial reality be extended. To chase towards that 2050 UK target, research into a whole range of innovative solutions for large-scale carbon-neutral energy could be the most realistic way forward.

References
  1. GOV.UK. Reducing the UK’s greenhouse gas emissions by 80% by 2050. [online] (updated 13 June 2013) Available at: https://www.gov.uk/government/policies/reducing-the-uk-s-greenhouse-gas-emissions-by-80-by-2050 [Accessed 25 June 2013]
  2. GOV.UK Department of Energy and Climate Change, 2013. Statistical Release, 2012 UK Greenhouse Gas Emissions, Provisional Figures. [online] Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/193414/280313_ghg_national_statistics_release_2012_provisional.pdf [Accessed 25 June 2013]
  3. Howard, T. P., Middelhaufe, S., Moore, K., Edner, C., Kolak, D.M., Taylor, G.N., Parker, D.A., Lee, R., Smirnoff, N., Aves, S. J., Love, J. (2013) ‘Synthesis of customized petroleum-replica fuel molecules by targeted modification of free fatty acid pools in Escherichia coli’, Proceedings of the National Academy of Sciences of the United States of America, 110 (19), pp. 7636-7641.
  4. Ruffing, A. M., 2013. Metabolic engineering of hydrocarbon biosynthesis for biofuel production. In: Z. Fang, ed. 2013. Liquid, gaseous and solid biofuels – conversion techniques. InTech. Ch.8
  5. Hurtt, C. G., Chini L. P., Froling, S., Betts, R. A., Feddema, J., Fischer, G., Fisk, J. P., Hibbard, K., Houghton, R. A., Janetos, A., Jones, C. D., Kindermann, G., Kinoshita, T., Goldewijk, K. K., Riahi, K., Shevliakova, E., Smith, S., Stehfest, E., Thomson, A., Thornton, P., van Vuuren, D. P., Wang, Y. P., 2011. Harmonization of land-use scenarios for the period 1500–2100: 600 years of global gridded annual land-use transitions, wood harvest, and resulting secondary lands. Climatic Change, 109, pp. 117-161.
  6. European Commission. Biofuels – Land Use Change. [online] Available at: http://ec.europa.eu/energy/renewables/biofuels/land_use_change_en.htm [Accessed June 27 2013]
  7. Yang, B., Dai, Z., Ding, S., Wyman, C.E., 2011. Enzymatic hydrolysis of cellulosic biomass. Biofuels, 2 (4), pp. 421-450.
  8. Williams, K., Zheng, Y., McGarvey, J., Fan, Z., Zhang, R., 2013. Ethanol and volatile fatty acid production from lignocellulose by Clostridium cellulolyticum. ISRN Biotechnology, 2013.
  9. American Chemical Society (ACS), 2013. Enzymes from horse feces could hold secrets to streamlining biofuel production. [online] Available at: http://www.acs.org/content/acs/en/pressroom/newsreleases/2013/april/enzymes-from-horse-feces-could-hold-secrets-to-streamlining-biofuel-production.html [Accessed 26 June 2013]
  10. King, A. J., Cragg, S. M., Li, Y., Dymond, J., Guille, M. J., Bowles, D. J., Bruce, N. C., Graham, I. A., McQueen-Mason, S. J., 2010. Molecular insight into lignocellulose digestion by a marine isopod in the absence of gut microbes. Proceedings of the National Academy of Sciences of the United States of America, 107 (12), pp. 5345-5350.


This post was written by: Ami Day View author bio

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