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Engineered microbe can convert grass to fuel

25 Aug, 2014

An engineered microbe that can convert plant matter to ethanol may hold the key to future energy independence in a world with an increasing thirst for fuel, dwindling supplies and negative impacts of greenhouse gases, according to a team of researchers from the US.

An engineered microbe that can convert plant matter to ethanol may hold the key to future energy independence in a world with an increasing thirst for fuel, dwindling supplies and negative impacts of greenhouse gases, according to a team of researchers from the US.

The researchers, all from the US Government-funded BioEnergy Science Center and made up of members from the University of Georgia and Tennessee’s Oak Ridge National Laboratory, took a bacterium, Caldicellulosiruptor bescii, that can convert plant matter into sugars, and added genes from another anaerobic bacterium, Clostridium thermocellum, that can convert the sugars into ethanol. They also removed genes for lactate dehydrogenase (an enzyme), which produces lactate from glucose instead of the desired ethanol product, and other enzymes, to cut down on the amount of acetate produced in the end product.

They were then able to successfully demonstrate the direct conversion of untreated switchgrass, a non-food renewable feedstock, to ethanol. The end fermented fuel was 70% ethanol with 38% less acetate compared to what would have been produced with the wild-type C. bescii.

The thermophilic anaerobic C. bescii was first isolated in 1990 from a geothermally heated freshwater pool in Russia and has since been found in similar environments in other parts of the world. The researchers say the bacterium is ideal for the task as it not only frees glucose from plant matter, it also thrives at the high temperatures used to breakdown plant cell walls.

Although ethanol is the most widely used renewable transport biofuel, the energy and expense associated with creating the fuel from lignocellulosic (having woody plant cell walls) biomass are barriers to ethanol’s wider industrialised production and use. Using lignocellulosic biomass as a feedstock usually involves chemical pretreatments and the addition of enzymes to break down cell walls before yeast fermentation into ethanol. However, the researchers write in their published paper that the direct conversion of biomass to ethanol

represents a new paradigm for consolidated bioprocessing, offering the potential for carbon neutral, cost-effective, sustainable fuel production

The researchers used switchgrass as feedstock in the demonstration as it is an abundant perennial that can grow in poor-quality soil with little energy to cultivate, making it both an environmentally desirable and economically sustainable lignocellulosic plant biomass. It also isn’t a food source for people – current production of ethanol often involves the use of ‘softer’ glucose-rich food crops, such as corn, putting the fuel in direct competition with food-supply demands.

The new process involved mechanically shredding and grinding the switchgrass and adding a low-cost salts medium for the bacteria.

The researchers write that ethanol from corn yields about the same amount of energy as it costs to produce, whereas ethanol from switchgrass, using the engineered microbes, could potentially produce 500% more energy than is required to cultivate and process it.

Challenges remain, however, such as growing commercial quantities of switchgrass, which requires investment and might take several years before harvest. Also, there may be issues with cultivating enough of the engineered microbes, compared to yeast, which is currently used in the fermentation process. In addition, the researchers would like to improve the conversion rate of glucose to ethanol (rather than other byproducts) and are looking to delete more genes that encode various enzymes to produce things such as acetate and hydrogen.

References

Chung, D., Cha, M., Guss, A.M. & Westpheling, J. (2014). Direct conversion of plant biomass to ethanol by engineered Caldicellulosiruptor bescii. PNAS. Published online ahead of print 2 June 2014. doi: 10.1073/pnas.1402210111. Available from www.pnas.org/content/early/2014/05/29/1402210111.

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