Exploring the tolerance of marine yeast to inhibitory compounds for improving bioethanol production

Inhibitor tolerance is one of the key challenges in lignocellulosic bioethanol production. The presence of inhibitors in a lignocellulosic hydrolysate induces stress in microorganisms and thus reduces bioethanol synthesis efficiency. In this study, 166 marine yeasts isolated from different marine environments were compared with 78 terrestrial yeasts for their tolerance to inhibitory compounds such as acetic acid, formic acid, furfural, vanillin and salt, which are commonly found in hydrolysates derived from lignocellulosic materials. Marine yeasts showed higher tolerance to all the inhibitors tested than terrestrial yeasts. The most tolerant marine yeast was Wickerhamomyces anomalus M15 which had IC50 values of 10.7% (w/w) and 83.9 mM for salt and acetic acid, respectively, while those for an industrial terrestrial yeast Saccharomyces cerevisiae NCYC2592 were 6.0% (w/w) and 75.9 mM, respectively. Statistical analysis revealed that marine yeast clustered separately from terrestrial yeast. In fermentation, using simulated wheat straw hydrolysates containing inhibitors and prepared using seawater, three selected marine yeasts Saccharomyces cerevisiae AZ65, W. anomalus M15, and Candida membranifaciens M2 produced 23-24 g L-1 bioethanol from 60 g L-1 glucose, while terrestrial yeast Saccharomyces cerevisiae produced only 12.5 g L-1 bioethanol. Marine yeasts were significantly more tolerant to the presence of inhibitory compounds and are phenotypically distinct from terrestrial yeasts. Significantly, a higher quantity of bioethanol was obtained in fermentation using selected marine yeasts with media containing inhibitory compounds. The study has highlighted the potential of finding microorganisms with industrially favourable characteristics in marine ecosystems and other adverse and challenging environments.