Metabolic flexibility of aerobic methanotrophs under anoxic conditions in Arctic lake sediments

Methane (CH4) emissions from Arctic lakes are a large and growing source of greenhouse gas to the atmosphere with critical implications for global climate. Because Arctic lakes are ice covered for much of the year, understanding the metabolic flexibility of methanotrophs under anoxic conditions would aid in characterizing the mechanisms responsible for limiting CH4 emissions from high-latitude regions. Using sediments from an active CH4 seep in Lake Qalluuraq, Alaska, we conducted DNA-based stable isotope probing (SIP) in anoxic mesocosms and found that aerobic Gammaproteobacterial methanotrophs dominated in assimilating CH4. Aerobic methanotrophs were also detected down to 70 cm deep in sediments at the seep site, where anoxic conditions persist. Metagenomic analyses of the heavy DNA from (CH4)-C-13-SIP incubations showed that these aerobic methanotrophs had the capacity to generate intermediates such as methanol, formaldehyde, and formate from CH4 oxidation and to oxidize formaldehyde in the tetrahydromethanopterin (H4MPT)-dependent pathway under anoxic conditions. The high levels of Fe present in sediments, combined with Fe and CH4 profiles in the persistent CH4 seep site, suggested that oxidation of CH4, or, more specifically, its intermediates such as methanol and formaldehyde might be coupled to iron reduction. Aerobic methanotrophs also possessed genes associated with nitrogen and hydrogen metabolism, which might provide potentially alternative energy conservation options under anoxic conditions. These results expand the known metabolic spectrum of aerobic methanotrophs under anoxic conditions and necessitate the re-assessment of the mechanisms underlying CH4 oxidation in the Arctic, especially under lakes that experience extended O-2 limitations during ice cover.