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

• Sulfate acted as an electron sink, diverting reducing power from acetogenesis. • In situ heat treatment inhibited methanogenesis, promoting homoacetogenesis. • Post-treatment, homoacetogenesis rose and Acetobacterium emerged • Sulfur cycling shaped electron flow distribution and the metabolic succession. 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 H 2 S and CH 4 . 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.