Carbon monoxide partial pressure effects on the metabolic process of syngas fermentation

The fermentation of syngas (CO, CO2, and H2) produced from biomass gasification for the production of ethanol has received increased attention due to the low cost and abundance of cellulosic feedstocks. Since CO plays a critical role in the available reducing equivalents and carbon conversion, this work assessed the effects of constant CO partial pressure (PCO), ranging from 0.35 to 2.0 atm, on cell growth, acetic acid production, and ethanol production using Clostridium carboxidivorans P7T. Several key findings included: (a) the maximum cell concentration increased with increasing PCO, increasing 440% with a PCO increase from 0.35 to 2.0 atm, (b) ethanol production changed from non-growth-associated to growth-associated with increasing PCO, (c) acetic acid production (gram acetic acid per gram cells) decreased for PCO ≥ 1.05 atm relative to PCO ≤ 0.70 atm, and (d) acetic acid appeared to be converted in the latter growth stages for PCO of 1.35 and 2.0 atm. Several explanations point to the potential importance of PCO and the PCO to PCO2PCO2 ratio on electron and ATP production. Since gasification processes that generate syngas could result in differing gas partial pressures, process variations could significantly change growth and product formation as evidenced by metabolic changes observed in this work due to changing PCO and/or the PCO to PCO2PCO2 ratio.