Physiological characterization and genetic improvement of xylose-fermenting yeasts

Conversion of all sugars present on lignocellulosic biomass is essential for economics of biofuels and chemicals processes. Xylose is the pentose sugar most abundant in nature and can account up to 30% of total sugar in some lignocellulosic biomasses, such as sugarcane bagasse. In this context, several microbial strains have been identified, characterized and genetic engineered for fuels and chemicals production from xylose. In this work, we isolated and selected new and publicly known yeast strains to characterize and improve genetically, so that they can produce ethanol from xylose. Initially, new yeast strains isolated from Brazilian biodiversity able to ferment xylose were identified and physiologically characterized in terms of xylose utilization. In parallel, sensitivity of known xylose-fermenting yeast strains to lignocellulosic hydrolysate inhibitors was systematically evaluated. Afterwards, three yeast species were genetically improved for fermentation of furfural containing media by evolutionary engineering. Isolates with increased tolerance towards furfural were obtained and systematically evaluated on synthetic media. In addition, genetic traits responsible for increased tolerance of selected isolates were identified. Finally, fermentative performance of selected strains was evaluated under different conditions using sugarcane bagasse hydrolysates as substrate. Analysis of substrate consumption and product formation demonstrated that the newly isolated and the genetically improved strains can efficiently convert xylose to ethanol. Our results demonstrated the potential of bioprospection and evolutionary engineering to obtain new yeast strains for xylose conversion.