Aerobic methanotrophic activity stimulates iron reduction in lake sediments

Recent observations from shallow anoxic lacustrine sediments around the world show an unexpected presence of bacterial methanotrophs, usually typified to be aerobic, together with anaerobic microorganisms, such as methanogens and iron reducers, which may result from undetectable traces of oxygen or a yet to be understood biochemical process producing oxygen. Both mcr gene-bearing archaea and pmoA gene-bearing bacterial methanotrophs were suggested to mediate methane oxidation in Lake Kinneret sediments. In these sediments, iron reduction was shown to be coupled to methane oxidation; however, with an unclear mechanism linked to methanotrophy. Here we show a new set of geochemical and microbial data from slurry experiments that tested the effect of exposure of oxygen on this aerobic and anaerobic activity. Surprisingly, exposure of oxygen levels up to 1% promoted aerobic methanotrophy and increased net iron reduction in anoxic lake sediments. The iron reduction was microbially mediated and performed by either Desulfuromonas or Geobacter or Methylomonas. The experiments provide insight into the complex life and biogeochemical cycles in anoxic lake sediments.