05/14/2012
Ionic Liquids: Degrading Biomass but Not Biofuel-Producing Microbes
The Science
A major hurdle to the development of economically competitive biofuels remains the difficulty of separating long sugar chains from plant biomass (cellulose and hemicellulose) from the tough network of lignin that gives strength and resilience. Pretreatment of plant material by ionic liquids (ILs), a class of salts that are molten at room temperature, is highly effective in disrupting biomass structure and liberating cellulose chains for subsequent conversion to biofuel compounds by fermentative microbes. However, residual IL molecules are highly toxic to biofuel-producing microbes and must be fully removed from the cellulose fraction prior to conversion, an expensive and time-consuming process. To understand this IL toxicity and enable development of resistant strains of microbes, researchers at the Joint Bioenergy Institute (JBEI) examined shifts in gene expression of a novel biomass-degrading bacterium when exposed to an IL. Enterobacter lignolyticus was surprisingly resistant to IL exposure, altering its cell membrane composition, activating a series of pumps to remove IL from the cell interior, and balancing osmotic pressure across the cell membrane. Many of the response mechanisms were specific to IL exposure and were not triggered by exposure to standard salts. These findings provide new insights into the mechanisms used by microbes to tolerate exposure to ionic liquids and may lead to the improvement of IL tolerance in biofuel-producing microbes through targeted genetic engineering.
Summary
To process plant-based renewable biofuels, pretreatment of plant feedstock with ionic liquids has significant advantages over current methods for deconstruction of lignocellulosic feedstocks. However, ionic liquids are often toxic to the microorganisms used subsequently for biomass saccharification and fermentation. Researchers previously isolated Enterobacter lignolyticus strain SCF1, a lignocellulolytic bacterium from tropical rain forest soil, and report here that it can grow in the presence of 0.5 M 1-ethyl-3-methylimidazolium chloride, a commonly used ionic liquid. We investigated molecular mechanisms of SCF1 ionic liquid tolerance using a combination of phenotypic growth assays, phospholipid fatty acid analysis, and RNA sequencing technologies. Potential modes of resistance to 1-ethyl-3-methylimidazolium chloride include an increase in cyclopropane fatty acids in the cell membrane, scavenging of compatible solutes, up-regulation of osmoprotectant transporters and drug efflux pumps, and down-regulation of membrane porins. These findings represent an important first step in understanding mechanisms of ionic liquid resistance in bacteria and provide a basis for engineering microbial tolerance.
Principal Investigator
Michael P. Thelen
Joint BioEnergy Institute
BER Program Manager
Dawn Adin
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Biological Systems Science Division
[email protected]
References
Khudyakov, J. I., P. D’haeseleer, S. E. Borglin, K. M. DeAngelis, H. Woo, E. A. Lindquist, T. C. Hazen, B. A. Simmons, and M. P. Thelen. 2012. “Global Transcriptome Response to Ionic Liquid by a Tropical Rain Forest Soil Bacterium, Enterobacter lignolyticus,” Proceedings of the National Academy of Sciences of the USA 109(32), E2173–82. DOI: 10.1073/pnas.1112750109. (Reference link)