Bioelectrochemical carbon monoxide methanation in mixed-culture microbial electrolysis cells: Performance, electron transfer pathways and microbial succession.

Bioelectrochemical methanation offers a promising avenue for carbon resource recovery. However, the direct biocathodic conversion of toxic carbon monoxide (CO) into methane, as well as the relevant interfacial extracellular electron transfer pathways and microbial community succession remain largely unexplored. Here, we systematically investigated the CO methanation process within a mixed-culture microbial electrolysis cell. Decreasing the cathodic potential from -0.90 to -1.05 V triggered a significant enhancement in methanogenic activity, boosting the cumulative methane yield from 0.142 to 1.410 mmol, with a maximum production rate of 0.0110 mmol h-1. It was found that the bioelectrochemical CO methanation harnessed a dual electron-uptake network, comprising direct electron transfer coupled with H2-mediated indirect pathway. Electrochemical and Raman spectroscopy analyses revealed that biofilm maturation significantly lowered the interfacial charge transfer resistance from 55.8 to 24.6 Ω, likely mediated by the secretion of endogenous redox-mediating biomolecules such as humic-like substances and carotenoids. Furthermore, 16S rRNA gene sequencing and fluorescence in situ hybridization results showed a robust co-enrichment of hydrogenotrophic Methanobacterium and electroactive Geobacter, indicating a distinct metabolic shift toward highly efficient electromethanogenesis. Overall, this work provides a critical theoretical foundation for engineering sustainable bio-valorization systems to upgrade toxic industrial off-gases into renewable biomethane.