Abstract
High-temperature ethanol fermentation has several benefits including a reduction in cooling cost, minimizing risk of bacterial contamination, and enabling simultaneous saccharification and fermentation. To achieve the efficient ethanol fermentation at high temperature, yeast strain that tolerates to not only high temperature but also the other stresses present during fermentation, e.g., ethanol, osmotic, and oxidative stresses, is indispensable. The C3253, C3751, and C4377 Saccharomyces cerevisiae strains, which have been previously isolated as thermotolerant yeasts, were found to be multiple stress-tolerant. In these strains, continuous expression of heat shock protein genes and intracellular trehalose accumulation were induced in response to stresses causing protein denaturation. Compared to the control strains, these multiple stress-tolerant strains displayed low intracellular reactive oxygen species levels and effective cell wall remodeling upon exposures to almost all stresses tested. In response to simultaneous multi-stress mimicking fermentation stress, cell wall remodeling and redox homeostasis seem to be the primary mechanisms required for protection against cell damage. Moreover, these strains showed better performances of ethanol production than the control strains at both optimal and high temperatures, suggesting their potential use in high-temperature ethanol fermentation.
Highlights
The consumption of ethanol as an alternative fuel has been steadily rising
Yeast cells are exposed to several environmental stresses including ethanol, osmotic, cell wall, and oxidative stresses, the ability of these strains to tolerate these stresses was investigated by examining the growth on YPDA agar plates containing 17% (v/v) ethanol, 18% (w/v) glucose, 18% (w/v) sorbitol, 100 mg L−1 cell wall stress-inducing agent calcofluor white (CFW), or 8 mM H2O2
The C4377 and C3751 strains were tolerant to all stresses examined, whereas the C3253 was tolerant to three stresses, i.e., ethanol, osmotic, and oxidative stresses (Fig. 1)
Summary
The consumption of ethanol as an alternative fuel has been steadily rising. The budding yeast Saccharomyces cerevisiae is commonly used in industrial-scale ethanol production due to several advantages it offers, including a highly efficient ethanol fermentation ability and a relatively high tolerance to fermentation stress (Boulton and Quain 2006). The continuous high-level expression of heat stress-responsive genes, including those encoding heat shock proteins (HSPs) and trehalose metabolic enzymes, was shown to be involved in an acquisition of thermotolerance in S. cerevisiae (Auesukaree et al 2012). The TPS2 gene has been shown to be required for tolerance to several stresses present during fermentation, including ethanol, heat, osmotic, and oxidative stresses (Auesukaree et al 2009) Based on these previous findings, it is possible that cellular mechanisms responsible for thermotolerance may play an important role in protecting yeast cells against other environmental stresses. Consistent with this idea, the thermotolerant S. cerevisiae KNU5377 strain has been reported to be tolerant to ethanol, and oxidative stresses (Kim et al 2013)
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