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

[摘要] Anaerobic oxidation of methane (AOM) is among the main processes limitingthe release of the greenhouse gas methane from natural environments.Geochemical profiles and experiments with fresh sediments from Lake Kinneret(Israel) indicate that iron-coupled AOM (Fe-AOM) sequesters 10 %–15 % ofthe methane produced in the methanogenic zone ( > 20  cm sedimentdepth). The oxidation of methane in this environment was shown to bemediated by a combination of mcr -gene-bearing archaea and pmoA -gene-bearing aerobicbacterial methanotrophs. Here, we used sediment slurry incubations undercontrolled conditions to elucidate the electron acceptors and microorganismsthat are involved in the AOM process over the long term ( ∼  18 months). We monitored the process with the addition of 13 C-labeledmethane and two stages of incubations: (i) enrichment of the microbialpopulation involved in AOM and (ii) slurry dilution and manipulations,including the addition of several electron acceptors (metal oxides, nitrate,nitrite and humic substances) and inhibitors (2-bromoethanesulfonate,acetylene and sodium molybdate) of methanogenesis, methanotrophy and sulfatereduction and sulfur disproportionation. Carbon isotope measurements in thedissolved inorganic carbon pool suggest the persistence of AOM, consuming3 %–8 % of the methane produced at a rate of 2.0  ±  0.4 nmol per gram of dry sediment per day. Lipid carbon isotopes and metagenomic analysespoint towards methanogens as the sole microbes performing the AOM process byreverse methanogenesis. Humic substances and iron oxides, although not sulfate,manganese, nitrate or nitrite, are the likely electron acceptors used forthis AOM. Our observations support the contrast between methane oxidationmechanisms in naturally anoxic lake sediments, with potentially co-existingaerobes and anaerobes, and long-term incubations, wherein anaerobes prevail.

[学科分类] 大气科学