Strain improvement of Scheffersomyces stipitis for the bioconversion of lignocellulosic biomass into ethanol.

Pretreatment of recalcitrant lignocellulosic biomass to release sugars for bioconversion into ethanol produces fermentation inhibitors. Increasing yeast inhibitor tolerance should reduce production time and cost. UV mutagenesis followed by genome shuffling using cross mating was performed on Scheffersomyces stipitis strain GS301, a genome shuffled strain with increased tolerance to spent sulphite liquor (SSL). The main fermentation inhibitors in SSL are acetic acid, hydroxymethylfurfural (HMF), and various phenolics. UV mutagenesis resulted in acetic acid tolerant mutants, but they were phenotypically unstable. However, two rounds of UV mutagenesis followed by five rounds of genome shuffling resulted in strains EVB105, EVB205 and EVB505 with increased SSL tolerance and improved acetic acid and HMF tolerance. When fermenting undiluted SSL at pH 5.5, the three strains utilized sugars faster producing higher maximum ethanol than GS301. This study demonstrates that UV mutagenesis with genome shuffling can significantly improve inhibitor tolerance and fermentation performance of yeast.