Promoting Butyrate and Caproate Selectivity from CO₂ in Microbial Electrosynthesis: Effect of Na+ Regulation and Microbial Mechanisms

Microbial electrosynthesis (MES) offers a sustainable route for converting CO₂ to value-added chemicals. However, the production of carboxylates such as butyrate and caproate remains constrained by the high demand for reducing power and energy. In this study, we demonstrated that Na⁺ supplementation effectively redirected metabolic flux from acetate toward longer-chain carboxylates, selectively enhancing butyrate and caproate production by 3.2-fold and 3.0-fold, while having no effect on the overall production spectrum (C2–C6 carboxylates). These improvements were driven by a Na⁺-induced restructuring of the microbial community, evidenced by a 19.9% increase in the number of electroactive microorganisms on the cathode and a 2.8-fold enrichment of chain-elongating populations in the suspension. This community shift led to a 1.7-fold increase in current density, suggesting improved electron transfer efficiency. Additionally, Na⁺ upregulated genes associated with ion-gradient-driven energy conservation, rendering chain elongation energetically favorable. This led to increased intracellular ATP generation and NAD(P)H availability, which preferentially activated the fatty acid biosynthesis pathway. This work demonstrates Na⁺ as a simple but effective strategy for steering the production of carboxylate from CO₂ in MES systems.