Co-cultivation of Clostridium acetobutylicum and Clostridium carboxidivorans in batch reactors resulted in ethanol oxidation and acetate accumulation

Ethanol fermentation has gained interest in biofuel production. Among strategies used for effective industrial production, synthetic co-cultures have been explored to increase productivity and product spectrum. Nevertheless, the potential negative effects of co-culturing have not been considered. We observed undesired ethanol oxidation to acetate in Clostridium acetobutylicum and Clostridium carboxidivorans co-cultures at a pH set-point of 5.8 and a glucose concentration of 5 g/L. Maximum ethanol oxidation accounted for a 25 % reduction, significantly decreasing production efficiency. Gene expression analysis revealed higher expression levels of bifunctional aldehyde/alcohol dehydrogenase adhE2 and butanol dehydrogenase bdh genes of C. carboxidivorans at the stationary phase, suggesting that the misregulation of these genes was responsible for ethanol oxidation. Co-culturing of the two species led to acetate accumulation but also to an undesired ethanol oxidation for NADH regeneration needs in C. carboxidivorans, highlighting the need for further research into unexpected syntrophic relationships among microbial consortia.