Notable mixed substrate fermentation ability of native Kodamaea strains isolated from Lagenaria siceraria flowersfor bioethanol production from biomass hydrolysates

Bioethanol, produced by fermenting sugar in cellulosic enzymatic hydrolysates of lignocellulosic biomass, has attracted significant attention. However, unfavorable economics is the foremost impediment in its successful commercial deployment. An efficient pretreatment, lower inhibitor generation, efficient enzymatic hydrolysis, maximum sugar recovery, and complete utilization/fermentation of all the sugars in hydrolysates will make the process cost effective. In addition to cellulose, biomass has hemicelluloses with major pentose xylose. S. cerevisiae, widely used organisms for commercial ethanol production exclusively ferment glucose, whilst xylose is left unfermented. Economic feasibility of bioethanol depends on complete utilization of pentoses and hexoses, seeks a co-fermenting yeast. Several engineering approaches developing a recombinant S. cerevisiae, capable of mixed sugar fermentation have met with limited success. Bioprospecting native pentose assimilating/fermenting yeasts is the contemporary approach. This study reports mixed substrate utilization and fermentation by two native Kodamaea strains isolated from Lagenaria siceraria flowers. Kodamaea spp. utilized wide range of pentoses /hexoses and showed considerable mixed sugar fermentation. Initial screening in minimal medium, glucose and xylose fermentation efficiency was 25%& 5%. Upon medium supplementation (0.1% yeast extract and peptone), glucose fermentation enhanced to 61%, co-fermentation  (glucose5%+ xylose 5%) was 50% with maximum ethanol yield 0.25 g/g. Kodamaea spp were  tolerant to inhibitors HMF, furfural and acetic acid. They grew well on paddy straw hydrolysates, completely utilizing sugars and producing ethanol.  This is the first report of native Kodamaea, displaying relevant traits mixed sugar utilization, co-fermentation, inhibitor tolerance making them strong candidates for further evolution for bioethanol production from biomass hydrolysates.