Iron Oxides Fuel Anaerobic Oxidation of Methane in the Presence of Sulfate in Hypersaline Coastal Wetland Sediment

Wetland methane emissions are the primary natural contributor to the global methane budget, accounting for approximately one-third of total emissions from natural and anthropogenic sources. Anaerobic oxidation of methane (AOM) serves as the major sink of methane in anoxic wetland sediments, where electron acceptors are present, thereby effectively mitigating its emissions. Nevertheless, environmental controls on electron acceptors, in particular, the ubiquitous iron oxides, involved in AOM are poorly understood. Here, we explored methane sinks within a hypersaline pool situated in a coastal wetland. The geochemical profiles reveal a tiering, where microbial sulfate reduction dominates in the organic-rich surface sediment, yielding to iron reduction in the deeper organic-poor yet sulfate-rich subsurface sediment. This shift is attributed to the drilling-induced depression and subsequent diagenetic transformation of the surface sediment. Radiotracer incubations demonstrate a strong association of AOM with sulfate in surface sediment and with iron oxides in subsurface sediment. Despite high concentrations of sulfate in coastal wetlands, Fe-dependent AOM may play a significant, yet often under-considered, role as a sink for methane emissions.