Simultaneous syngas fermentation and chain elongation by an adapted dark fermentation microbiome

The production of carboxylates from simultaneous syngas fermentation and chain elongation as platform for biotechnological CO₂/CO fixation through open microbiomes is an emerging approach. In this study, dark fermentation microbiome exhibited homoacetogenic and carboxydotrophic activity, converting syngas into acetate, butyrate, and caproate. Four pressure conditions (1.0, 1.2, 1.4, and 1.6 atm) were tested using syngas with the highest CO (0.23 mmol/h) and H 2 (0.08 mmol/h) consumption rates observed at 1.6 atm. Under this condition, selectivity shifted toward medium chain carboxylates (MCC, 72.7%) with H 2 , CO, and CO 2 removal of 66%, 84% and 86%, respectively. Moreover, the adapted microbiome fermented CO as the only carbon and energy source, producing 27.4 mM caproate, 9.4 mM valerate, and 5.1 mM acetate, with 81% MCC selectivity. Microbial community analysis revealed that syngas fermentation was dominated by Haloimpatiens (57.7 – 78%) and Clostridium sensu stricto 12 (10.5 – 20.2%) both associated with syngas fermentation and chain elongation. In contrast, under CO fermentation, Haloimpatiens dropped to 28.8%, while Citrobacter , a carboxydotrophic bacteria known for its ability to oxidize CO, became dominant (39.7%), and Clostridium sensu stricto 1 and Clostridium sensu stricto 12 were positively correlated with CO consumption. These findings highlight the feasibility of the open adapted microbiome to perform simultaneous syngas fermentation and chain elongation with high selectivity to MCC without an organic carbon source.