Microbial communities involved in aerobic and anaerobic methane cycling in a meromictic ferruginous subarctic lake

Since boreal lakes are now considered an important source of atmospheric methane, new information concerning the activity and diversity of methane cycle microorganisms is critical for understanding the causes of methane emission from these lakes. This study investigated the diversity of microbial communities in the water column of subarctic Lake Svetloe, belonging to a rare type of freshwater sulfate-poor meromictic basins with high concentrations of Fe2+ and methane in the permanently anoxic hypolimnion. A combination of physicochemical and radiotracer analysis, high-throughput sequencing of the 16S rRNA genes and incubation experiments was used to link microbial community profile and methane cycle processes. It was shown that methane was produced by aceticlastic Methanothrix and hydrogenotrophic Methanoregula, which were also detected in the oxygenated epilimnion, together with a small increase in methane concentration. Radiotracer analysis revealed methane oxidation (MO) in oxic and anoxic zones with 2 maxima at the chemocline. The first MO peak was attributed to aerobic Methylobacter trophically interacting with cyanobacteria, which was confirmed by obtaining light-dependent MO. The highest MO activity matched the lower chemocline layer where aerobic methanotrophs were less abundant; this suggested that other microorganisms contributed to MO together with Methylobacter. Known anaerobic methanotrophs were not detected, and incubations with Fe3+ did not reveal methane consumption under anoxic conditions. Thus, further investigations are re quired to determine the microorganisms and electron acceptors driving anaerobic MO. Although some questions remain open, our study may provide insight into the methane cycle microbial communities in boreal lakes.