Novel synthetic co‐culture of Acetobacterium woodii and Clostridium drakei using CO(2) and in situ generated H(2) for the production of caproic acid via lactic acid

Acetobacterium woodii is known to produce mainly acetate from CO(2) and H(2), but the production of higher value chemicals is desired for the bioeconomy. Using chain‐elongating bacteria, synthetic co‐cultures have the potential to produce longer‐chained products such as caproic acid. In this study, we present first results for a successful autotrophic co‐cultivation of A. woodii mutants and a Clostridium drakei wild‐type strain in a stirred‐tank bioreactor for the production of caproic acid from CO(2) and H(2) via the intermediate lactic acid. For autotrophic lactate production, a recombinant A. woodii strain with a deleted Lct‐dehydrogenase complex, which is encoded by the lctBCD genes, and an inserted D‐lactate dehydrogenase (LdhD) originating from Leuconostoc mesenteroides, was used. Hydrogen for the process was supplied using an All‐in‐One electrode for in situ water electrolysis. Lactate concentrations as high as 0.5 g L(–1) were achieved with the AiO‐electrode, whereas 8.1 g L(–1) lactate were produced with direct H(2) sparging in a stirred‐tank bioreactor. Hydrogen limitation was identified in the AiO process. However, with cathode surface area enlargement or numbering‐up of the electrode and on‐demand hydrogen generation, this process has great potential for a true carbon‐negative production of value chemicals from CO(2).