Manganese and Iron Support Anaerobic Oxidation of Methane at Cold Seeps

The fate of seafloor methane, hosted in cold seep environments, remains a key area of study with implications for the climatically important global methane cycle. As the organic-rich sediments of cold seeps host diverse chemosynthetic microbial communities, capable of catalyzing a wide spectrum of redox transformations, a complex coupling among elemental cycling often arises – including reactions involving methane oxidation. Indeed, the most important global sink of seafloor-hosted methane is the anaerobic oxidation of methane (AOM), during which archaea oxidize methane to carbon dioxide. Sulfate has canonically been considered the primary electron acceptor of AOM, however, in recent years oxidized forms of iron (Fe) and manganese (Mn) have also been shown to be viable alternate electron acceptors. In order to investigate the coupling of methane oxidation with metal reduction, rates of Fe- and Mn-dependent AOM were measured during incubations using sediments collected at cold seeps on the Cascadia Margin, off the coast of Oregon. Sediment incubations were amended with one of the following electron acceptors: sulfate, oxidized Mn (as either Mn(III)-DFOB, manganate, buserite, or δ-MnO₂), oxidized Fe (as either Fe(III)-DFOB, ferrihydrite, nontronite, or goethite), and a killed control. Headspaces were filled with ¹³C-labeled methane and transfer of ¹³C into the DIC pool was tracked over four subsequent time-points to quantify rates of AOM. At all sites, sulfate-dependent AOM exhibited the highest rates, with comparably elevated rates observed in a subset of Fe and Mn treatments. The majority of Mn and Fe additions showed elevated rates of AOM compared to the unamended control. Differences in rates of AOM were also related to the relative disorder versus crystallinity of the Fe or Mn mineral added. Our results shed light on possible roles that Fe and Mn redox cycling play in supporting these reactions and constrain conditions under which they might be significant (e.g., metal-rich cold seep sediments).