AME 81:257-276 (2018)  -  DOI: https://doi.org/10.3354/ame01874

ABSTRACT: Small oxygen-stratified humic lakes of the boreal zone are important sources of methane to the atmosphere. Although stable isotope profiling has indicated that a substantial part of methane is already oxidized in the anaerobic water layers in these lakes, the contributions of aerobic and anaerobic methanotrophs in the process are unknown. We used next-generation sequencing of mcrA and 16S rRNA genes to characterize the microbial communities in the water columns of 2 boreal lakes in Finland, Lake Alinen-Mustajärvi and Lake Mekkojärvi, and complemented this with a shotgun metagenomic analysis from Alinen-Mustajärvi and an analysis of pmoA genes and 16S rRNA, mcrA, and pmoA transcripts from Mekkojärvi. Furthermore, we tested the effect of various electron acceptors and light on methane oxidation (¹³C-CH₄ labeling) in incubations of water samples collected from the lakes. Aerobic gammaproteobacterial methanotrophs (order Methylococcales) exclusively dominated the methanotrophic community both above and below the oxycline in the lakes. A novel lineage within Methylococcales, Candidatus Methyloumidiphilus alinensis, defined here for the first time, dominated in Alinen-Mustajärvi, while methanotrophs belonging to Methylobacter were more abundant in Mekkojärvi. Light enhanced methane oxidation in the anoxic water layer, while alternative electron acceptors (SO₄^2-, Fe³⁺, Mn⁴⁺, and anthraquinone-2,6-disulfonate), except for NO₃^-, suppressed the process. Our results suggest that oxygenic photosynthesis potentially fuels methanotrophy below the aerobic water layers in methane-rich boreal lakes. Furthermore, incubation results, together with the detection of denitrification genes from metagenome-assembled genomes of gammaproteobacterial methanotrophs, imply that boreal lake methanotrophs may couple methane oxidation with NO_x^- reduction in hypoxic conditions.

KEY WORDS: Methanotroph · Methane oxidation · Boreal lake · Water column · Shotgun metagenomics · 16S rRNA · mcrA

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Cite this article as: Rissanen AJ, Saarenheimo J, Tiirola M, Peura S, Aalto SL, Karvinen A, Nykänen H (2018) Gammaproteobacterial methanotrophs dominate methanotrophy in aerobic and anaerobic layers of boreal lake waters. Aquat Microb Ecol 81:257-276. https://doi.org/10.3354/ame01874 Export citation Share:    Facebook - - linkedIn