Biogenic ferrihydrite microtubules strengthen extracellular electron transfer driving anoxic methane oxidation

Wetlands represent a significant global source of methane emissions, yet the oxidation mechanisms of methane within their anoxic interfaces remain poorly understood. This study investigates the metabolic potential of aerobic methanotrophs ( Methylobacterium genus), to utilize iron oxides under anoxic conditions. A systematic comparison of efficacy was conducted for biogenic ferrihydrite (BFH), versus chemically synthesized ferrihydrite (CFH) as electron acceptors under anoxic conditions. Anoxic microcosm experimental systems were established, integrating mineral characterization techniques, extracellular polymeric substances (EPS) compositional analysis, and electrochemical measurements. The results demonstrated that BFH, due to its unique micro-tubular organic-inorganic composite structure, achieved a Fe(III) reduction rate of 33.5 μM/day, representing a 63.41% increase compared to CFH. Further mechanistic analysis revealed that BFH significantly enhanced the secretion of redox components within EPS, like c (c-Cyts) and humic acid-like substances. These components effectively reduced the interfacial charge transfer resistance ( Rct decreased by 34.6%) and facilitated optimized electron transfer pathways at the microbe-mineral interface, involving both direct contact and electron shuttle mechanisms, thereby enabling efficient coupling of methane oxidation with Fe reduction. These findings offer new insights into the interaction between organisms and minerals and provide a theoretical basis for formulating measures to reduce methane emissions in wetland ecosystems.