Fig. 8. The proposed mechanisms of aerobic methanotrophic metabolism in the anoxic SIP microcosms, including CH4-derived carbon secretion and oxidation, as well as nitrogen and hydrogen metabolism for sustaining methanotrophy coupled to iron reduction.
The genes in red were not detected using metagenomic analysis in the heavy DNA from the 13CH4 incubation and red x-symbols (x) denote pathways was unlikely involved in anaerobic CH4 oxidation due to lack of detection in the SIP-metagenomics data. The red oval labeled e− shows extracellular electron transfer from the cell to Fe (III). The organic C released by methanotrophs may stimulate Fe (III) reduction via dissimilatory iron-reducing bacteria. Based on the metabolic characteristics of potential iron-cycling microbes in the heavy DNA, e.g., Chlorobium ferrooxidans, and the minimal light exposure during the SIP experiment, Fe (II) oxidation might be coupled to sulfur metabolism under the light or dark conditions, as shown on the left side of the figure. The SIP microcosm headspace consisted of 1–2% H2, 5% CH4, and 93–94% N2.