Formation of Biogenic Fougèrite (Green Rust) and Magnetite by Anaerobic Methane Oxidation Coupled to Ferric Hydroxide Reduction

Understanding the microbial oxidation of methane in terrestrial subsurface environments is important for determining the biogeochemical controls on this important greenhouse gas. The potential for biogenic methanotrophy coupled to oxygen, nitrate, sulfate, or ferric iron reduction was tested in Australian coal mine groundwater containing a relatively high proportion of an archaea closely related to ‘Candidatus Methanoperedens nitroreducens’ (97% 16S rRNA gene identity). Microbial incubations with a 13CH4 tracer and oxygen or ferrihydrite test positive for microbial methane oxidation. Methane was not oxidized in microbial incubations containing nitrate or sulfate as electron acceptors. In a second experiment, the enrichment culture of ‘Ca. Methanoperedenaceae’ was incubated with methane and various ferric iron minerals, including ferrihydrite, hematite, goethite, and magnetite. After 6 months of incubation with methane and ferrihydrite, biogenic iron reduction coupled to methane oxidation resulted in the formation of magnetite and fougèrite (green rust). This is the first study to demonstrate that anaerobic methane oxidation coupled to iron reduction is a potential microbial metabolic process occurring in Australian coal mine groundwater.