Propanol as electron donor for efficient odd-chain carboxylate production by chain elongation with reactor microbiomes.

Microbial consortia that catalyze chain elongation processes have been enriched using different selection strategies, for which the electron donor is an essential one. Propanol is an extraordinarily promising electron donor because it can be generated from renewable resources, including lignocellulosic biomass and protein wastes. Here, propanol was proven in detail to be an efficient electron donor, enhancing the production of odd medium-chain carboxylates during chain elongation. By exploring various electron acceptors, reactor conditions, and electron donor/electron acceptor mol ratios, our study highlights that acetate is the most suitable electron acceptor for the production of both odd- and even-chain carboxylates. The optimal conditions for propanol-based chain elongation were 30 °C and pH 6, achieving 82.8 % selectivity for odd-chain carboxylates. Another critical insight from our work is that a propanol/acetate mol ratio of 1:1 can minimize the inhibitory effect of propanol and maximize the yield of medium-chain carboxylates, with the highest concentration of n-heptanoate reaching 124.5 mmol C/L. This was further illustrated by 16S rRNA amplicon sequencing, which elucidated that the community composition and keystone species in a propanol-based reactor closely resembled that of the ethanol one. The dominant phylum of the propanol-based reactor, Firmicutes showed a significant positive correlation with the concentrations of n-caproate and n-valerate. Additionally, the co-occurrence of Clostridium sensu stricto 12 and Oscillibacter, known as typical chain elongators, was identified within the propanol-based reactor. These findings enhance our understanding of propanol-based chain elongation, offer guiding principles for reactor microbiota assembly, and support efficient odd medium-chain carboxylate production.