Screening of natural yeast isolates under the effects of stresses associated with second-generation biofuel production.

Robust microorganisms are required for sustainable second-generation biofuel production. We evaluated the growth and fermentation performance of six natural isolates that were derived from grape wine and medicinal herbs using a wide range of carbon sources, rice and wheat straw hydrolysates as well as stress conditions associated with second-generation ethanol production. Sequence analysis of the 5.8S internal transcribed spacer (ITS) and species-specific PCR amplification of the HO gene region assigned the natural isolates to Saccharomyces cerevisiae. Restriction fragment length polymorphism (RFLP) analysis of the mitochondrial DNA revealed that natural yeast isolates are genetically closer to the laboratory strain BY4741 than to the CEN.PK strains. Dextrose fermentation by a natural isolate, MTCC4780, under semi-anaerobic conditions produced maximum ethanol yields of 0.44 g/g and 0.39 g/g, respectively, with and without the stresses encountered during lignocellulosic ethanol fermentation. However, MTCC4780 produced ethanol yields of 0.48 g/g, 0.42 g/g and 0.45 g/g, respectively, with glucose, rice and wheat straw enzymatic hydrolysate fermentation in a bioreactor. The isolates MTCC4781 and MTCC4796 showed higher growth and fermentation performance than did MTCC4780 in the presence of elevated temperature and pre-treatment inhibitors. Taken together, the MTCC4780, MTCC4781 and MTCC4796 strains have the potential to serve as a platform for lignocellulosic ethanol production under stresses associated with second-generation biofuel production.