Impact of cyclic pH shifts on carbon monoxide fermentation to ethanol by Clostridium autoethanogenum

•Cyclic pH shifts allow enhanced ethanol production during CO fermentation.•Cyclic pH shifts result in significant consumption of produced acids at low pH.•Fast high conversion of acetic acid to ethanol can be obtained at lower pH.•Mixotrophic fermentation leads to biomass increase and some lactic acid production.

Fermentation medium and fermentation pH are among the main parameters that control acidogenesis and solventogenesis in clostridia during CO bioconversion. The approach of cyclic pH shifts from high (pH 5.75) to low (pH 4.75) levels, in continuous gas-fed bioreactors, allowed Clostridium autoethanogenum to consume and convert CO and the produced acetic acid from CO to alcohols using one single bioreactor. Increasing amounts of ethanol accumulate after each pH shift cycle, while acids are used up and converted during each cycle. In this study, a total of three cyclic pH shifts along with partial medium replenishment, resulted in a high ethanol production, reaching a concentration of 7143 mg/L. The mixotrophic fermentation using xylose as additional carbon source besides carbon monoxide allowed to conclude that xylose is a preferred carbon source compared to CO and that it allows to increase the cell mass concentration (421 mg/L).