Ethanol Produced from Switchgrass Biomass Without Pretreatment
One strategy for reducing costs associated with biomass deconstruction and fermentation of sugars to biomass into advanced biofuels is consolidated bioprocessing (CBP). In CBP, non-pretreated biomass is converted to a biofuel in a single process by a cellulolytic microbe that breaks down the biomass and ferments the sugars. U.S. Department of Energy BioEnergy Research Center (BESC) scientists have been working toward CBP by looking at a variety of thermophilic cellulolytic bacteria. A candidate CBP microbe is Caldicellulosiruptor bescii, a natural thermophilic cellulolytic bacterium for which BESC researchers have developed genetic tools for gene insertion and deletion. In this study, BESC researchers demonstrate the successful CBP of switchgrass cellulosic biomass using an engineered strain of C. bescii.
C. bescii had been shown to ferment untreated switchgrass biomass, but it lacked the genes to make ethanol. As C. bescii is a thermophile and CBP is carried out at elevated temperatures, a gene for a heat-stable enzyme for ethanol synthesis was needed. A candidate gene was identified in Clostridium thermocellum and cloned into C. bescii. The engineered C. bescii strain now produced ethanol from cellobiose, Avicel, and switchgrass. To optimize the fermentation of ethanol, two genes were deleted that would otherwise divert fermentation products. In this new C. bescii strain, roughly 30% of biomass was fermented and 1.7 moles of ethanol was produced for each mole of glucose, close to the theoretical 2.0 moles of ethanol per mole of glucose. While there are opportunities to further improve efficiencies, this is an important step in actualizing the CBP’s potential and provides a platform for engineering the production of advanced biofuels and other bioproducts directly from cellulosic biomass without harsh and expensive pretreatment.
Chung, D., M. Cha, A. M. Guss, and J. Westpheling. 2014. “Direct Conversion of Plant Biomass to Ethanol by Engineered Caldicellulosiruptor bescii,” Proceedings of the National Academy of Sciences (USA) 111, 8931–36. DOI:10.1073/pnas.1402210111.