| Summary: | Abstract Background This study aimed at producing C6–C8 medium-chain carboxylates (MCCAs) directly from gaseous CO using mixed culture. The yield and C2–C8 product composition were investigated when CO was continuously fed with gradually increasing partial pressure. Results The maximal concentrations of n-caproate, n-heptylate, and n-caprylate were 1.892, 1.635, and 1.033 mmol L−1, which were achieved at the maximal production rates of 0.276, 0.442, and 0.112 mmol L−1 day−1, respectively. Microbial analysis revealed that long-term acclimation and high CO partial pressure were important to establish a CO-tolerant and CO-utilizing chain-elongating microbiome, rich in Acinetobacter, Alcaligenes, and Rhodobacteraceae and capable of forming MCCAs solely from CO. Conclusions These results demonstrated that carboxylate and syngas platform could be integrated in a shared growth vessel, and could be a promising one-step technique to convert gaseous syngas to preferable liquid biochemicals, thereby avoiding the necessity to coordinate syngas fermentation to short-chain carboxylates and short-to-medium-chain elongation. Thus, this method could provide an alternative solution for the utilization of waste-derived syngas and expand the resource of promising biofuels.
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