Intracellular metabolic changes in Saccharomyces cerevisiae and promotion of ethanol tolerance during the bioethanol fermentation process

Abstract

During the batch bioethanol fermentation process, although Saccharomyces cerevisiae cells are challenged by accumulated ethanol, our previous work showed that the ethanol tolerance of S. cerevisiae increased as fermentation time increased. However, the exact molecular mechanisms underlying the increased ethanol tolerance of S. cerevisiae are still poorly understood. In this study, a gas chromatography-mass spectrometry-based metabolomics strategy was used to determine the fermentation process-associated intracellular metabolic changes in S. cerevisiae cells. With the aid of partial least squares-discriminant analysis between two of the three fermentation stages (i.e., the lag, exponential, and stationary phases), 40 differential metabolites with variable importance and a projection value greater than 1 were identified. During the bioethanol fermentation process, S. cerevisiae cells could continuously remodel their membrane composition and structure to obtain higher ethanol tolerance. During the lag–exponential phase transition, in spite of a down-regulated TCA cycle, the increased ergosterol content combined with decreased saturated fatty acid content might be the most significant factor in making yeast cells more robust and ethanol-tolerant. During the exponential–stationary phase transition, a re-activated TCA cycle could provide plenty of energy, and the increased energy production together with the increased energy requirements might be partly responsible for the increased ethanol tolerance in the stationary phase. Moreover, the increased content of glycerol, trehalose, ergosterol and some amino acids also might jointly confer the yeast cells with higher ethanol tolerance. These results highlighted our knowledge about the relationship between the bioethanol fermentation process and ethanol tolerance, and could contribute to the construction of feasible ethanologenic strains with higher ethanol tolerance.