Acetaldehyde addition throughout the growth phase alleviates the phenotypic effect of zinc deficiency in Saccharomyces cerevisiae

During experiments to determine the effects of exogenously added acetaldehyde on pure cultures of various yeast strains, we discovered that an early acetaldehyde perfusion during the growth phase allowed several yeasts to partially overcome the phenotypic effects of zinc depletion during alcoholic fermentation. We, therefore, performed genome-wide expression and proteomic analysis on an industrial Saccharomyces cerevisiae yeast strain (VL1) growing in zinc-replete or zinc-depleted conditions in the presence of perfused acetaldehyde to identify molecular markers of this effect. Zinc depletion severely affects ethanol production and therefore nicotinamide adenine dinucleotide (NAD) regeneration, although we observed partial compensation by the upregulation of the poorly efficient Fe-dependent Adh4p in our conditions. A coordinate metabolic response was indeed observed in response to the early acetaldehyde perfusion, and particularly of the lower part of glycolysis, leading to the cellular replenishment of NAD cofactor. These various findings suggest that acetaldehyde exchange between strains may inhibit the growth of some yeast strains while encouraging the growth of others. This phenomenon could be particularly important for understanding the ecology of colonization of complex fermentation media by S. cerevisiae after elimination of non-Saccharomyces yeasts.

Redox interactions between Saccharomyces cerevisiae and Saccharomyces uvarum in mixed culture under enological conditions

Wine yeast starters that contain a mixture of different industrial yeasts with various properties may soon be introduced to the market. The mechanisms underlying the interactions between the different strains in the starter during alcoholic fermentation have never been investigated. We identified and investigated some of these interactions in a mixed culture containing two yeast strains grown under enological conditions. The inoculum contained the same amount (each) of a strain of Saccharomyces cerevisiae and a natural hybrid strain of S. cerevisiae and Saccharomyces uvarum. We identified interactions that affected biomass, by-product formation, and fermentation kinetics, and compared the redox ratios of monocultures of each strain with that of the mixed culture. The redox status of the mixed culture differed from that of the two monocultures, showing that the interactions between the yeast strains involved the diffusion of metabolite(s) within the mixed culture. Since acetaldehyde is a potential effector of fermentation, we investigated the kinetics of acetaldehyde production by the different cultures. The S. cerevisiae-S. uvarum hybrid strain produced large amounts of acetaldehyde for which the S. cerevisiae strain acted as a receiving strain in the mixed culture. Since yeast response to acetaldehyde involves the same mechanisms that participate in the response to other forms of stress, the acetaldehyde exchange between the two strains could play an important role in inhibiting some yeast strains and allowing the growth of others. Such interactions could be of particular importance in understanding the ecology of the colonization of complex fermentation media by S. cerevisiae.