Influence of Carbon Substrate and Iron Oxide Minerals on Methane Production and Magnetic Mineral Formation in Salt Marsh Sediments

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 variation is unclear, but one hypothesis is non-competitive carbon substrate usage by salt marsh methanogens where sulfate is available. In low-salinity wetlands crystalline iron minerals have also been found to increase methane production, but this has not been explored in salt marshes. This study documents how different carbon substrates (monomethylamine and ethanol) and Fe(III) minerals (ferrihydrite, magnetite or hematite) influenced methane production by salt marsh microbial communities from the Great Marsh Preserve, DE. Carbon source was the major driver of microbial community composition measured at the end of the microcosm incubations. Monomethylamine microcosms produced more methane than those with ethanol, with the highest values in microcosms with conductive or semi-conductive iron oxides (magnetite and hematite, respectively). Less methane was produced with monomethylamine and non-conductive iron oxides (ferrihydrite) or iron-free incubations. This increased methane production could indicate that interspecies electron transfer was active in some of our treatments. However, instead of the more commonly-described partners, this syntrophic relationship appears to be between methylotrophic methanogens belonging to Methanococcoides and an unidentified iron reducing bacterial group, possibly Ca. Omnitrophus. Ethanol amendments yielded much less measured methane overall and there was no discernible effect of conductive iron oxides. However, the small proportion of anaerobic methane oxidizers that were detected in ethanol-amended incubations suggests cryptic methane cycling may have occurred. Although some iron reduction and Fe2+ production was observed in all treatments, significant transformation of ferrihydrite to magnetite was observed only with ethanol amendments. In parallel, magnetite supported the greatest extent of methane production from monomethylamine. Our monomethylamine and magnetite treatment showed that such a transformation of ferrihydrite to magnetite in the environment has the potential to enhance methane production by salt marsh microbial communities if non-competitive, methylated substrates become available. Our results indicate that interspecies electron transfer to iron minerals is potentially mediated by previously unrecognized syntrophic partners in sulfate-containing salt marshes. Further, this study highlights the importance of organic carbon substrate type and its influence on methane production and iron mineral transformation in salt marsh sediments.