Impact of Culture Nutrition on the Tolerance of Furan Inhibitors and the Conversion of High Xylose Concentrations to Ethanol by Pichia Stipitis Nrrl Y-7124
Patricia J. Slininger, Z. Lewis Liu, and Steven W. Gorsich. Crop BioProtection Research Unit, NCAUR USDA-ARS, 1815 N. University, Peoria, IL 61604
Efficient fermentation processes to produce ethanol from both the hexose and pentose sugars available in low-cost lignocellulosic biomass are sought to support the expansion of the biofuels industry. Such an expansion is expected to strengthen our nation by lessening dependence on foreign sources of fuel, preserving our environment and national resources, and boosting our rural economy. Stress tolerant microorganisms are needed that are able to consume both hexose and pentose sugars and also withstand, survive, and function in the presence of stress factors common to fermentations of lignocellulose hydrolysates, including various chemical fermentation inhibitors such as furfural, hydroxymethylfurfural (HMF), and ethanol. Furfural and HMF are key byproducts of the dilute acid pretreatment hydrolysis of lignocellulosic biomass, the most economical method of releasing hemicellulosic sugars for fermentation to ethanol biofuel. The availability of tolerant microbial catalysts would allow efficient fermentation of low-cost acid hydrolysates despite the presence of inhibitory byproducts. Our research has shown that natural strains of the yeasts Saccharomyces cerevisiae and Pichia stipitis can survive and adapt to the presence of furfural and HMF and that this survival is linked in part to a fully functioning pentose phosphate pathway, likely key in maintaining the cofactor balance needed for the in situ detoxification of furfural and HMF to their less toxic alcohols (furfuryl alcohol and 2,5-bis-hydroxymethylfuran, respectively). Data will be presented showing the impact of mineral and nitrogen source composition on the ability of P. stiptis to survive and detoxify furan inhibitors and to convert high xylose concentrations efficiently to ethanol. Implications of these findings in context of the current literature on biomass to ethanol conversion and stress tolerance will be discussed. Process-based strategies to produce a tolerant initial population and then to foster and sustain tolerance during growth and ethanol fermentation will be considered.