Long-term incubations provide insight into the mechanisms of anaerobic oxidation of methane in methanogenic lake sediments

Abstract. Anaerobic oxidation of methane (AOM) is among the main processes limiting release greenhouse gas from natural environments. Geochemical profiles and experiments with fresh sediments Lake Kinneret (Israel) indicate that iron-coupled AOM (Fe-AOM) sequesters 10 %–15 % produced in methanogenic zone (>20 cm sediment depth). The this environment was shown to be mediated by a combination mcr-gene-bearing archaea pmoA-gene-bearing aerobic bacterial methanotrophs. Here, we used slurry incubations under controlled conditions elucidate electron acceptors microorganisms are involved process over long term (? 18 months). We monitored addition 13C-labeled two stages incubations: (i) enrichment microbial population (ii) dilution manipulations, including several (metal oxides, nitrate, nitrite humic substances) inhibitors (2-bromoethanesulfonate, acetylene sodium molybdate) methanogenesis, methanotrophy sulfate reduction sulfur disproportionation. Carbon isotope measurements dissolved inorganic carbon pool suggest persistence AOM, consuming 3 %–8 at rate 2.0 ± 0.4 nmol per gram dry day. Lipid isotopes metagenomic analyses point towards methanogens as sole microbes performing reverse methanogenesis. Humic substances iron although not sulfate, manganese, nitrate or nitrite, likely for AOM. Our observations support contrast between mechanisms naturally anoxic lake sediments, potentially co-existing aerobes anaerobes, long-term incubations, wherein anaerobes prevail.