Impact of sulfur-cycling metabolism during microbial succession in CO₂-reducing biocathodes

Microbial electrosynthesis (MES) involves electroactive communities with diverse metabolic capacities, yet electron competition remains poorly understood. In particular, metabolic succession and the role of sulfur-cycling microorganisms competing with methanogens and homoacetogens are often overlooked. We investigated a heat-pretreated anaerobic community incubated under acetogenesis-favourable conditions at a selected cathode potential in three bioelectrochemical reactors. Despite selective pre-enrichment, the reactors followed a reproducible three-phase pattern: initial sulfidogenesis, transient methanogenesis, and a shift to homoacetogenesis favoured by in situ heat treatment. Early stages showed low current densities, relatively poor coulombic efficiencies (CE < 50 %), and detection of H₂S and CH₄. Taxonomic and transcriptomic data revealed Desulfovibrio, Sulfurimonas, and Methanosarcina dominance before the shift to homoacetogenesis. Post-secondary in situ heat treatment, acetate accumulated (up to 717 mg.L^-1), CE_acetate exceeded 65 %, and Clostridium and Acetobacterium became dominant. These results underscore the overlooked role of sulfur metabolism and show how targeted strategies can redirect MES community function.