The microbial community behind efficient mixotrophic chain elongation

Microbial communities fed with both organic and inorganic substrates can improve sustainability and feasibility of chain elongation processes. Sustainably-sourced H2, CO2, and CO can be co-fed to microorganisms as a source for acetyl-CoA, while a small amount of an ATP-rich organic substrate helps overcome the kinetic hindrances associated with autotrophic carboxylate production. Here, we operated two bioreactor systems with continuous recirculation of H2, CO2, and CO while co-feeding an organic feedstock model (lactate and acetate) to understand how a mixotrophic community is shaped during carboxylate production. Contrary to the assumption that H2, CO2, and CO support chain elongation via ethanol production in open cultures, significant correlations (p=0.01) indicated that relatives of Clostridium luticellarii and Eubacterium aggregans produced carboxylates (acetate to n-caproate) while consuming H2, CO2, CO, and lactate themselves. After 100 days, the enriched community was dominated by these two bacterial groups coexisting in cyclic dynamics shaped by the CO partial pressure. Pure autotrophs occurred only when the acetate concentration was low (3.2 g L-1), while heterotrophs had lesser roles: Pseudoramibacter, Oscillibacter, and Colidextribacter contributed to n-caproate production and Clostridium tyrobutyricum and Acidipropionibacterium spp. grew opportunistically producing n-butyrate and propionate, respectively. For a fixed amount of lactate consumed, the mixotrophic community showed a remarkable electron efficiency in comparison to a heterotrophic one. The extra H2 and CO consumed routed 82% more electrons to carboxylates and 50% more electrons to carboxylates longer than acetate.