Tuesday, May 3, 2011
Lignocellulosic biomass is an attractive substrate for bioethanol production. However, there are a number of problems associated with this bioconversion. Firstly, during pretreatment of lignocellulosic substrates, a range of inhibitors are produced which adversely affect yeast growth, viability and fermentation. Secondly, xylose constitutes a significant fraction of the dry weight in woody angiosperms (hardwoods) and herbaceous angiosperms and its utilization is essential for efficient biomass-to-ethanol conversion. Unfortunately, the fermentative yeast Saccharomyces cerevisiae, commonly used for hexose fermentation, is unable to utilize xylose and ferment it to ethanol. Pentose-fermenting yeasts, such as Scheffersomyces (Pichia) stipitis, Pachysolen tannophilus and Candida shehatae can ferment both glucose and xylose to ethanol. However, these yeasts exhibit poor tolerance to the inhibitors generated during lignocellulose pretreatment and are also susceptible to catabolite repression. In this study, a genome shuffled strain of S. stipitis, GS301, with improved tolerance to hardwood spent sulphite liquor (HW SSL), was subjected to UV-mutagenesis to further improve its tolerance to acetic acid and HW SSL. GS301 was also subjected to screening on 2-deoxyglucose (2-DG) to select glucose-depressed cells. Several mutants with higher tolerance to HW SSL, acetic acid and 2-DG were isolated. The selected mutants retained the ability to ferment xylose and glucose efficiently to ethanol. Their growth, viability and fermentation performance is being assessed in chemically defined liquid media and selected lignocellulosic hydrolysates. Ultimately, we aim to use genome shuffling to recombine all the beneficial traits in different mutant pools generated in this strain development program to yield a superior strain.