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

Microbial electrosynthesis (MES) involves electroactive communities with diverse metabolic capacities, whose competition for electrons remains poorly understood. In particular, the dynamic succession of metabolisms and the impact of sulfur-cycling microorganisms as potential competitors to methanogens and homoacetogens are often overlooked. Here, we investigated a heat-pretreated anaerobic community incubated under acetogenesis favourable conditions and 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 were marked by 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), CEacetate exceeded 65%, and Clostridium and Acetobacterium became dominant. Our findings highlight the underestimated role of sulfur metabolism in MES, and demonstrate how targeted strategies can redirect functional outcomes in complex biocathodic communities.