Physiological responses of Saccharomyces cerevisiae to industrially relevant conditions: slow growth, low pH and high CO2 levels

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 CO2. 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 CO2 (50 %) on slow-growing chemostat and retentostat cultures of the reference strain S. cerevisiae CEN.PK113-7D. Combined exposure to low pH and high CO2 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 CO2 on physiology, high CO2 strongly affected genome-wide transcriptional responses to low pH. Interference of high CO2 with low-pH signaling is consistent with low-pH and high-CO2 signals being relayed via common (MAPK-)signaling pathways, notably the cell wall integrity (CWI), high osmolarity glycerol (HOG) 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. Compare expression of glucose growth S. cerevisiae CEN.PK113-7D in chemostat cultures at d = 0.025 h-1 for low pH (pH3), high CO2 (50%) conditions as well as in retentostats when going from 0.025 h-1 to near-zero growth rates. Both datasets are compared to 'standard laboratory' conditions.