Abstract
Engineered strains of Saccharomyces cerevisiae are used for industrial production of succinic acid. Optimal process conditions for dicarboxylic-acid yield and recovery include slow growth, low pH, and high CO(2) . To quantify and understand how these process parameters affect yeast physiology, this study investigates individual and combined impacts of low pH (3.0) and high CO(2) (50%) on slow-growing chemostat and retentostat cultures of the reference strain S. cerevisiae CEN.PK113-7D. Combined exposure to low pH and high CO(2) led to increased maintenance-energy requirements and death rates in aerobic, glucose-limited cultures. Further experiments showed that these effects were predominantly caused by low pH. Growth under ammonium-limited, energy-excess conditions did not aggravate or ameliorate these adverse impacts. Despite the absence of a synergistic effect of low pH and high CO(2) on physiology, high CO(2) strongly affected genome-wide transcriptional responses to low pH. Interference of high CO(2) with low-pH signaling is consistent with low-pH and high-CO(2) signals being relayed via common (MAPK) signaling pathways, notably the cell wall integrity, high-osmolarity glycerol, and calcineurin pathways. This study highlights the need to further increase robustness of cell factories to low pH for carboxylic-acid production, even in organisms that are already applied at industrial scale.