Electron shuttling mediated by humic substances fuels anaerobic methane oxidation and carbon burial in wetland sediments

Key pathways for the anaerobic oxidation of methane (AOM) have remained elusive, particularly in organic rich ecosystems. In this work, the occurrence of AOM driven by humus-catalyzed dissimilatory iron reduction was investigated in sediments from a coastal mangrove swamp. Anoxic sediment incubations supplied with both goethite (α-FeOOH) and leonardite (humic substances (HS)) displayed an average AOM rate of 10.7 ± 0.8 μmol CH4 cm−3 day−1, which was 7 and 3 times faster than that measured in incubations containing only goethite or HS, respectively. Additional incubations performed with 13C-methane displayed Pahokee Peat HS-mediated carbonate precipitation linked to 13CH4 oxidation and ferrihydrite reduction (~1.3 μmol carbonate cm−3 day−1). These results highlight the role of HS on mitigating greenhouse gases released from wetlands, not only by mediating the AOM process, but also by enhancing carbon sequestration as inert minerals (calcite, aragonite and siderite).