Influence of carbon source and iron oxide minerals on methane production and magnetic mineral formation in salt marsh sediments

Abstract. Salt marshes can emit significant methane to the atmosphere. These emissions are highly variable, but the cause of this variability is not well understood. Although methanogenesis should be inhibited by sulfate reduction where sulfate is present due to its thermodynamic unfavorableness, methane emissions are not well predicted by sulfate concentrations; thus, other controls on methane production must be active. One hypothesis is that where sulfate is present, salt marsh methanogens are fueled by methylated carbon substrates that sulfate reducers do not compete for. It has also been shown that crystalline iron minerals can facilitate increased methane production in low-salinity wetlands, but this has not been explored in salt marshes. This study documents how different organic carbon sources (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite, and hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source had a strong influence on microbial community composition by the end of the incubations. More methane was produced with monomethylamine amendment than with ethanol, and the highest methane production rates were in incubations supplied with both monomethylamine and magnetite or hematite. This increased methane production in the presence of (semi)conductive iron minerals could indicate that interspecies electron transfer was active in some of our treatments. However, instead of the more commonly described syntrophic partners, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron-reducing bacterial group, possibly Candidatus Omnitrophus. Much less methane was measured with ethanol and ferrihydrite amendments. However, in ethanol-amended incubations, a small proportion of anaerobic methane oxidizers was detected, which suggests that both methane production and re-oxidation may have occurred, leading to low measured methane production. Although some iron reduction and Fe2+ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendment. If microbially mediated magnetite formation occurs in salt marsh sediment, our observations indicate that the resulting magnetite could enhance methane production by methylotrophic methanogens. This study highlights the importance of methylated compounds to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.