Physiology of the fuel ethanol strain Saccharomyces cerevisiae PE-2 at low pH indicates a context-dependent performance relevant for industrial applications

Abstract

Selected Saccharomyces cerevisiae strains are used in Brazil to produce the hitherto most energetically efficient first-generation fuel ethanol. Although genome and some transcriptome data are available for some of these strains, quantitative physiological data are lacking. This study investigates the physiology of S. cerevisiae strain PE-2, widely used in the Brazilian fuel ethanol industry, in comparison with CEN.PK113-7D, a reference laboratory strain, focusing on tolerance to low pH and acetic acid stress. Both strains were grown in anaerobic bioreactors, operated as batch, chemostat or dynamic continuous cultures. Despite their different backgrounds, biomass and product formation by the two strains were similar under a range of conditions (pH 5 or pH < 3, with or without 105 mM acetic acid added). PE-2 displayed a remarkably higher fitness than CEN.PK113-7D during batch cultivation on complex Yeast extract - Peptone - Dextrose medium at low pH (2.7). Kinetics of viability loss of non-growing cells, incubated at pH 1.5, indicated a superior survival of glucose-depleted PE-2 cells, when compared with either CEN.PK113-7D or a commercial bakers' strain. These results indicate that the sulfuric acid washing step, used in the fuel ethanol industry to decrease bacterial contamination due to non-aseptic operation, might have exerted an important selective pressure on the microbial populations present in such environments.

Keywords: Saccharomyces cerevisiae; acid stress; fuel ethanol; industrial fermentation; low pH tolerance.