Yeasts under ethanol stress regulate water structure at their membranes

Yeasts are able to support different environmental conditions, including stress situations. Given the broad applications of yeast in the food industry, adaptation of this micro-organism to stress conditions is an active research area. It has been reported that lipid composition of the membrane is affected temporarily or permanently by environmental stresses, and thus, the regulation of the membrane biophysical properties under such conditions may be a key point for yeast adaptation. Therefore, we here study the effect of ethanol stress on the hydration and organization of water molecules at the lipid bilayer of yeast membranes, using the generalized polarization (GP) of the fluorescent probe Laurdan. We studied a laboratory strain of Saccharomyces cerevisiae (BY4741), a mutant with increased permeability due to the lack of ergosterol (erg6), and a commercial baker´s yeast. We found that in the presence of low ethanol levels, the GP values remained roughly constant for all strains. At ethanol proportions higher than 20% (v/v), the GP values decreased abruptly. At these high ethanol levels, cells are no longer viable. We further studied BY4741 yeasts acclimated to high ethanol levels, and found similar behavior than for the other yeasts: GP remained roughly constant at ethanol levels at which cells were viable. However, the GP values for the acclimated yeasts was 1.5 times higher than for BY4741 grown in the absence of ethanol. Furthermore, total lipid extracts of BY4741 yeasts grown in YPD and in ethanol were studied, finding differences in the compositions and the biophysical properties. We conclude that water structure and membrane hydration are important parameters for yeast viability, even more important that the specific lipid composition.