Methane production in the oxygenated water column of a perennially ice-covered Antarctic lake

Abstract

Aerobic methane production in aquatic ecosystems impacts the global atmospheric budget of methane, but the extent, mechanism, and taxa responsible for producing this greenhouse gas remain unclear. Lake Bonney (LB), a perennially ice-covered Antarctic lake, has cold hypersaline waters underlying an oxygenated freshwater layer. We present temporal methane concentration profiles in LB indicating methane production in the oxygenated (>200% air saturation) water. Experiments amended with methylphosphonate (MPn) yielded methane generation, suggesting in situ methanogenesis via the carbonphosphorus (C-P) lyase pathway. Enrichment cultures from the lake were used to isolate five bacterial strains capable of generating methane when supplied with MPn as the sole P source. Based on 16S rRNA gene sequencing, the isolates belong to the Proteobacteria (closely related to Marinomonas, Hoeflea and Marinobacter genera) and Bacteroidetes (Algoriphagus genus). 16S rRNA metagenomic sequencing confirms the presence of these taxa in LB. None of the isolated species were reported to be capable to produce methane. In addition, orthologs of the phosphoenolpyruvate mutase gene (PepM) and methylphosphonate synthase (MPnS), enzymes involved in phosphonate and methylphosphonate biosynthesis, were widely spread in the LB shotgun metagenomic libraries; genes related to C-P lyase pathways (phn gene clusters) were also abundant. Anaerobic methanogenic 16S rRNA and mcrA genes were absent in both 16S rRNA and metagenomics libraries. These data reveal that in situ aerobic biological methane production is likely a significant source of methane in LB.