Magnetite Nanoparticles Enhancing H₂‐Driven Biomethanation in a Mixed Microbial Community

Biological methanation is increasingly considered for biogas upgrading. Here, the supplementation of conductive magnetite (Fe 3 O 4 ) nanoparticles is investigated as a strategy to enhance H 2 ‐driven biomethanation in a mixed hydrogenotrophic methanogenic community. An enrichment culture, maintained for over 180 days in a fill‐and‐draw anaerobic bioreactor under H 2 /CO 2 feeding, is used to inoculate batch microcosms containing 0, 1.25, and 2.5 gFe L −1 of magnetite. Magnetite addition resulted in a dose‐dependent increase in maximum methane production rates—up to 13‐fold compared to controls—and sustained high hydrogen‐to‐methane conversion yields (78–107%). 16S rRNA gene sequencing reveals that archaeal community composition remained dominated by hydrogenotrophic Methanobrevibacter and Methanobacterium spp., whereas bacterial populations shifted from acetogenic Sporomusa and Acetobacterium spp. toward H 2 ‐oxidizing Paracoccus and Thauera spp. at higher magnetite concentrations. Electron microscopy and energy‐dispersive X‑ray spectroscopy show that magnetite nanoparticles formed conductive networks bridging microbial cells, and fluorescence in situ hybridization confirmed co‐localization of methanogens and Paracoccus within these aggregates. The findings support a direct interspecies electron transfer (DIET) mechanism facilitated by magnetite, whereby Paracoccus spp. oxidize H 2 and shuttle electrons to methanogens, accelerating biomethanation. These results highlight the potential of magnetite‐mediated DIET to improve power‐to‐methane processes and advance biogas upgrading technologies.