Oxygen-dependent regulation of ion fluxes and intracellular pH in Saccharomyces cerevisiae exposed to acetic acid.

The study of acetic acid (AA) stress in yeast is crucial for understanding the mechanisms of acid stress tolerance and enhancing production efficiency. Although AA toxicity has been extensively studied, the mechanism by which oxygen availability modulates ion flux balance and intracellular proton handling under AA stress remains poorly understood. Here, we investigated the combined effects of AA concentration, acidic pH, and oxygen availability on plasma membrane Na+, K+, and H+ fluxes, intracellular pH, and growth in two Saccharomyces cerevisiae strains with distinct AA tolerance. We show that AA toxicity is strongly concentration-, strain-, and oxygen-dependent. AA exposure increased DCCD-sensitive proton (JH+) and sodium (JNa+) fluxes while progressively impairing potassium uptake (JK+), resulting in nonlinear, strain-specific shifts in Na+/H+ and K+/H+ exchange ratios and a substantial increase in ion exchange imbalance. These effects were amplified under oxygen-limited conditions and correlated with reduced growth and metabolic activity. Moderate AA stress was accompanied by increased intracellular ATP, particularly in ATCC 9804 under oxygen limitation, whereas severe AA stress caused ATP depletion, most strongly in ATCC 13007. Notably, the more tolerant strain maintained lower intracellular proton accumulation and more efficient ion flux coordination, particularly during aerobic growth. Together, our findings demonstrate that oxygen availability fundamentally reshapes ion flux balance and intracellular pH homeostasis during AA stress. These insights have direct implications for strain selection and process optimization in industrial fermentations exposed to weak organic acid inhibition.