Archaea catalyze iron-dependent anaerobic oxidation of methane

TLDR: This study identifies archaea of the order Methanosarcinales, related to “Candidatus Methanoperedens nitroreducens,” which couple the reduction of environmentally relevant particulate forms of iron and manganese to methane oxidation, filling one of the remaining lacunas in anaerobic methane oxidation.

Abstract: Anaerobic oxidation of methane (AOM) is crucial for controlling the emission of this potent greenhouse gas to the atmosphere. Nitrite-, nitrate-, and sulfate-dependent methane oxidation is well-documented, but AOM coupled to the reduction of oxidized metals has so far been demonstrated only in environmental samples. Here, using a freshwater enrichment culture, we show that archaea of the order Methanosarcinales, related to “Candidatus Methanoperedens nitroreducens,” couple the reduction of environmentally relevant forms of Fe^(3+) and Mn^(4+) to the oxidation of methane. We obtained an enrichment culture of these archaea under anaerobic, nitrate-reducing conditions with a continuous supply of methane. Via batch incubations using [^(13)C]methane, we demonstrated that soluble ferric iron (Fe^(3+), as Fe-citrate) and nanoparticulate forms of Fe^(3+) and Mn^(4+) supported methane-oxidizing activity. CO_2 and ferrous iron (Fe^(2+)) were produced in stoichiometric amounts. Our study connects the previous finding of iron-dependent AOM to microorganisms detected in numerous habitats worldwide. Consequently, it enables a better understanding of the interaction between the biogeochemical cycles of iron and methane.