The effects of ethanol on growth rate and passive proton diffusion in yeasts

Abstract

When cell suspensions of Saccharomyces cerevisiae NRRL-Y132 and Kluyveromyces marxianus IGC-2771 were incubated in the presence of different concentrations of ethanol, the final stable pH values (pH f ) reached in these suspensions increased with increasing ethanol concentration, indicating that ethanol enhanced passive proton diffusion into the cells. The plots of pH f as a function of ethanol concentration were linear but biphasic, displaying different slopes below and above the transition ethanol concentrations. When S. cerevisiae NRRL-Y132 and K. marxianus IGC-2771 were grown in the presence of different concentrations of ethanol, the specific growth rate (μ) similarly depended upon ethanol concentration in a linear, biphasic way. Plots of μ at each ethanol concentration against pH f reached in cell suspensions at that ethanol concentration were linear and monophasic for S. cerevisiae NRRL-Y132 but biphasic for K. marxianus IGC-2771. Ethanol inhibition of growth and enhancement of proton diffusion are therefore correlated in these yeasts. Whereas ethanol inhibition of growth and enhancement of transmembrane proton diffusion were affected to the same degree by ethanol below and above the transition ethanol concentration in S. cerevisiae NRRL-Y132, these two parameters of ethanol inhibition were affected to different degrees below and above the transition in K. marxianus IGC-2771 as indicated by the inflection point in the plot of μ vs pH f . Attempts to extent these findings to other yeasts showed that the correlation between the effects of ethanol on pH f and μ is not a universal phenomenon among yeasts.