S195: Potential of a thermophilic isolate and its enzymes in ethanol fermentation of switchgrass

The discovery of new thermophiles and exploitation of their novel and unique thermostable enzymes could further reduce the enzyme costs of cellulosic ethanol which currently constitute more than 10% of the total costs. Advantages of employing thermostable enzymes for cellulose hydrolysis include: increased conversion rates due to improved mass transfer rates allowing the use of lower enzyme dosages and resulting in higher yields of fermentable sugars; improved substrate solubility and reduced viscosity allowing the use of higher solids loadings; decreased risk of contamination resulting in increased robustness of process. The focus of this presentation is on the enzymatic hydrolysis and ethanol fermentation of switchgrass utilizing a cellulolytic thermophile, Geobacillus sp. R7. This strain was recently isolated from the deep subsurface of the NSF Deep Underground Science and Engineering Laboratory and was shown to possess remarkable thermostability of its hydrolytic enzymes. At 70^oC, biomass was enzymatically liquefied for only 36 h and then fermented to ethanol using an industrial Saccharomyces strain for 72 h with yields of 0.45-0.47 g ethanol/g glucose. The use of higher solids loadings (20%) and temperatures (70^oC) during enzymatic hydrolysis proved beneficial for the significant reduction of hydrolysis times (2.67-times) and enzyme loadings (4-times), with important implications for reduced capital and operating costs of ethanol production. At 20% solids and 70^oC, enzymatic hydrolysis improved glucose yields by 28% over those obtained at 50^oC. Furthermore, under microaerophilic conditions, Geobacillus sp. R7 produced ethanol from lignocellulosic waste in a single step (consolidated bioprocessing) with outstanding potential for cost reductions.