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, but these emissions are highly variable and not well-predicted by sulfate concentrations. The cause of this variability is unclear, but one hypothesis is that rather than being outcompeted where sulfate is present, salt marsh methanogens use carbon substrates that sulfate reducers do not. In low-salinity wetlands, crystalline iron minerals have also been found to facilitate increased methane production, 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 or hematite) influence methane production by microbial communities from a polyhaline tidal marsh creek in the Great Marsh Preserve, DE, USA. Carbon source was the major influence on methane production and microbial community composition at the end of microcosm incubations. Microcosms amended with monomethylamine produced more methane than those with ethanol. The highest methane production rates were in microcosms with both monomethylamine and magnetite or hematite amendments. Less methane was produced with monomethylamine and ferrihydrite, a non-conductive iron oxide, and without iron supplementation. 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. Instead of the more commonly described syntrophic partners, however, this interaction appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Much less measured methane overall was measured with ethanol amendment and there was no discernible effect of iron treatment. A small proportion of anaerobic methane oxidizers was detected in ethanol-amended incubations, which suggests cryptic methane cycling may have occurred, however. 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 such 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 such as those released to sediments by Spartina grasses to salt marsh methane production as well as the potential importance of iron mineral composition for predicting methane production and iron reduction rates.