Linking microbial functional partitioning with methane productivity and stability in microbial electrolysis cell–anaerobic digestion hybrid systems

Bioenergy recovery is a global priority for environmental sustainability and energy transition. The microbial electrolysis cell–anaerobic digestion hybrid system (MEC-AD) offers a promising approach for efficient methane recovery; however, the functional roles of its components remain unclear. In this study, bioanodes, biocathodes, and suspensions were isolated from a stably operated MEC-AD bioreactor to investigate their microbial community characteristics and dominant metabolic functions under controlled conditions. The bioanode was identified as the primary contributor to methane production, accounting for 69%–82% of the methane yield via mixotrophic methanogenesis, dominated by Methanothrix and Methanosarcina, in synergy with Thermoanaerobacter and Geobacter. The biocathode reshaped fermentation patterns, reducing propionate accumulation by enriching Clostridium, Syntrophobacterium, and Methanobacterium. The suspension exhibited fermentation but limited methanogenesis owing to the low abundance of methanogens. These results clarify the functional partitioning of MEC-AD systems and provide a basis for targeted regulation to reduce propionate accumulation and enhance methane productivity. This study demonstrates the distinct roles of components in MEC-AD systems in promoting efficient and stable methane production, with important implications for process optimization and improved system performance.