Physiological response of Clostridium ljungdahlii DSM 13528 of ethanol production under different fermentation conditions

•A unique alternative acetate re-assimilation pathway triggered ethanol production.•Putative aldehyde oxidoreductase aor1 involved in this alternative pathway.•Accumulation of external acids induced the shift to solventogenesis.•A trace amount of CaCO3 increased ethanol and acetate production.

In this study, cell growth, gene expression and ethanol production were monitored under different fermentation conditions. Like its heterotrophical ABE-producing relatives, a switch from acidogenesis to solventogenesis of Clostridium ljungdahlii during the autotrophic fermentation with CO/CO2 could be observed, which occurred surprisingly in the late-log phase rather than in the transition phase. The gene expression profiles indicated that aor1, one of the putative aldehyde oxidoreductases in its genome played a critical role in the formation of ethanol, and its transcription could be induced by external acids. Moreover, a low amount of CaCO3 was proved to have positive influences on the cell density and substrate utilization, followed by an increase of over 40% ethanol and 30% acetate formation.