Microbial methanoges-methanotrophs and iron reducers in incubation experiments of sediments from the sea of Galilee
In anaerobic Lake Kinneret sediments, mcr gene-bearing archaea and pmoA gene-bearing methanotrophic bacteria were suggested to mediate methane oxidation coupled with iron reduction. However, the requirement for oxygen in methane oxidation and the link between the two processes have remained unclear. Here we present new results from slurry experiments amended with 13C labeled methane, hematite, and different levels of oxygen in nitrogen or methane headspace to quantify the effect of stimulation of aerobic methane oxidation on microbial community changes and iron reduction. The exposure of oxygen levels up to 1% promoted aerobic methanotrophy but surprisingly went along with considerable net iron reduction, changing our traditional perspective on iron reduction in the presence of oxygen. The increase in iron reduction might result from iron recycling and reactivation, along with leakage of a metabolic substrate for iron-reducing bacteria from the aerobic methanotrophs, or the use of iron as an electron acceptor by the methanotrophs. The process is microbially mediated and is performed by the iron-reducing bacteria Geobacter or Desulfuromonas or by the methanotrophic bacteria Methylomonas and Methylobacter. We additionally explored the involvement of methanol oxidation in the iron reduction or even potentially as a source for the 13C-enrichments in fatty acids of methanotrophs, as suggested during previous anoxic slurry experiments. Microbial and lipid isotope analyses of experiments with the addition of 13C-labeled methanol showed only the stimulation of methylotrophic methanogenesis and not simple organic matter oxidation. When methanogenesis was inhibited, the anaerobic heterotrophic community members, including iron-reducing bacteria, performed the highest methanol turnover.  Our experiments show a link between aerobic methanotrophy and iron reduction, and how iron reduction metabolism could be the surviving mechanism of methanotrophs under hypoxia.