Monday, May 2, 2011
Grand Ballroom C-D, 2nd fl (Sheraton Seattle)
After pretreatment and hydrolysis, lignocellulosic substrates yield a mixture of monomeric hexoses (glucose, mannose, galactose) and pentoses (xylose, arabinose). While Saccharomyces cerevisiae can ferment glucose and mannose efficiently, it is unable to utilize pentoses or ferment them to ethanol. In contrast, pentose-fermenting yeasts can ferment the main hexose and pentose sugars in lignocellulosic hydrolysates to ethanol. However, they suffer from hexose repression, low ethanol tolerance, and perform poorly in the presence of hydrolysate inhibitors. Among the pentose-fermenting yeasts, the mating systems of Pachysolen tannophilus and Scheffersomyces (Pichia) stipitis have been described. This makes it possible to use cross-mating based genome shuffling approach to improve their properties for lignocellulosic biomass conversion. We initiated a strain development program to improve the properties of P. tannophilus by non-recombinant means. Several consecutive rounds of UV mutagenesis and screening were used to select for mutants of P. tannophilus NRRL Y2460 with improved tolerance to ethanol, acetic acid and hardwood spent sulfite liquor (HW SSL). Glucose-derepressed mutants were also isolated through selection for resistance to 2-deoxyglucose. Inhibitor-tolerant mutants were tested for their growth and fermentation performance in chemically defined media and selected lignocellulosic hydrolysates. Results confirmed the mutants retained good xylose and glucose fermenting ability. Research is planned to subject the selected mutant populations to genome shuffling to obtain recombinants with further improved tolerance. Inhibitor-tolerant strains of P. tannophilus with improved viability, growth and fermentation performance are of interest for application in bioconversion of lignocellulosic hydrolysates to ethanol.