In the search for technology by which economically competitive biofuels can be produced from cellulosic biomass, the combination of sugar-fermenting microbes and ionic liquid solvents looks to be a winner save for one major problem: the ionic liquids used to make cellulosic biomass more digestible for microbes can also be toxic to them. A solution to this conundrum, however, may be in the offing.
Researchers with the U.S. Department of Energy (DOE)’s Joint BioEnergy Institute (JBEI), a multi-institutional partnership led by Berkeley Lab, have identified a tropical rainforest microbe that can endure relatively high concentrations of an ionic liquid used to dissolve cellulosic biomass. The researchers have also determined how the microbe is able to do this, a discovery that holds broad implications beyond the production of advanced biofuels.
"Our findings represent an important first step in understanding the mechanisms of ionic liquid resistance in bacteria and provide a basis for engineering ionic liquid tolerance into strains of fuel-producing microbes for a more efficient biofuel production process," says Blake Simmons, a chemical engineer who heads JBEI’s Deconstruction Division and one of the senior investigators for this research.
Adds Michael Thelen, the principal investigator and a member of JBEI’s Deconstruction Division, "Our study also demonstrates that vigorous efforts to discover and analyze the unique properties of microorganisms can provide an important basis for understanding microbial stress and adaptation responses to anthropogenic chemicals used in industry."
Thelen is the corresponding author and Simmons a co-author of a paper reporting the results of this research in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled "Global transcriptome response to ionic liquid by a tropical rain forest soil bacterium, Enterobacter lignolyticus." Other co-authors are Jane Khudyakov, Patrik D’haeseleer, Sharon Borglin, Kristen DeAngelis, Hannah Wooa, Erika Lindquist and Terry Hazen.
The burning of fossil fuels releases nearly 9 billion metric tons of excess carbon into the atmosphere each year. Meanwhile the global demand for gasoline and other petroleum-based fuels continues to rise. Clean, green and renewable fuels that won’t add excess carbon to the atmosphere are sorely needed. Among the best candidates are advanced biofuels synthesized from the cellulosic biomass in non-food plants. Such fuels could displace petroleum-based fuels on a gallon-for-gallon basis and be incorporated into today’s vehicles and infrastructures with no impact on performance.
To this end, researchers at JBEI have already engineered a strain of E. coli bacteria to digest the cellulosic biomass of switchgrass, a perennial grass that thrives on land not suitable for food crops, and convert its sugars into biofuel replacements for gasoline, diesel and jet fuels. A key to this success was the pretreatment of the switchgrass with an ionic liquid to dissolve it.
"Unlike the starch sugars in grains, the complex polysaccharides in cellulosic biomass are semicrystalline and deeply embedded within a tough woody material called lignin," Simmons says. "Lignin can be removed and cellulose crystallinity can be reduced if the biomass is pretreated with ionic liquids, environmentally benign organic salts often used as green chemistry substitutes for volatile organic solvents."