Microbial community of Greenland Thermokarst Lakes

Arctic warming accelerates permafrost thaw, driving the formation of thermokarst lakes. These ecosystems alter the mercury (Hg) biogeochemical cycle, possibly leading to the formation of highly toxic methylmercury (MMHg). This process is mainly conducted by microorganisms possessing hgcA and hgcB genes. To investigate the Hg dynamics and identify Hg-related microbiome, two thermokarst lakes (ZAC1 and ZAC2) were sampled in the Zackenberg Valley, Northeast Greenland. Total Hg (THg) and MMHg concentrations were measured in the water column and sediments, where methylation and demethylation rates were also calculated along with the potential impact of known Hg-methylating microorganisms. Sediment DNA was isolated to assess the presence and abundance of marker genes associated with Hg-related processes. While no differences were found in water column Hg concentrations, ZAC2 THg and MMHg sediment concentrations were 2-fold greater than ZAC1. The demethylation potential was similar in the sediments of both lakes. However, ZAC2 exhibited higher methylation rates, supported by a greater availability of organic substrate and higher abundance of hgcA and hgcB sequences. The microbial community also outlines potential for methane (CH4) production, sulfate (SO42-) reduction, and conversion of divalent Hg (Hg2+) in elemental Hg (Hg0). No evidence of merB-mediated MMHg degradation was found. This study presents the first characterization of Hg-related microbial communities in Greenland's thermokarst lakes. Moreover, it emphasizes the lake development stage as an important determinant of sediment methylation potential and the critical role of sulfate-reducing bacteria (SRB) in Hg methylation, integrating evidence from Hg stable isotope incubation experiments and gene-centric metagenomic analysis.