CO as electron donor for efficient medium chain carboxylate production by chain elongation: Microbial and thermodynamic insights

CO is a main component of syngas produced by the thermal gasification of biomass. The biological conversion of sole CO into medium-chain carboxylates (MCCs) has attracted considerable attention, although the process has a low efficiency. The present study demonstrated a process to achieve efficient MCCs production from short-chain carboxylates (SCCs), namely, acetate and n-butyrate, with CO as electron donor. Results demonstrated that n-butyrate as an electron acceptor was superior to acetate and the control (without electron acceptor). The highest n-caproate concentration (7.7 mM) was obtained in reactor 160B, while it can reach only 2.7 and 0 mM in reactors 80A and C, respectively. The lag phase (15 days) for n-caproate production with n-butyrate as electron acceptor was significantly lower than that with acetate (90 days) or control (no n-caproate produced). The microbial community composition of chain elongation bioreactors showed that Eubacterium limosum was significantly positively correlated with the production of n-caproate and was found to be responsible for performing chain elongation with acetate or n-butyrate as electron acceptor and CO as electron donor. A thermodynamic model based on the metabolic pathway of E. limosum and demonstrating the feasibility of n-caproate production from n-butyrate and CO was established. Furthermore, the present study verified that n-butyrate is more favorable as electron acceptor with CO as electron donor to achieve chain elongation according to the trade-off between the energy harvest yields and rates.