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The role of prokaryotic mercury methylators and demethylators in Canadian Arctic thermokarst lakesNicola Gambardella1,2 , Joana Costa 1 , Beatriz Malcata Martins 3 , Diogo Folhas 3 , Ana Patrícia Ribeiro1,2 , Holger Hintelmann 3,4 , João Canário 3 & Catarina Magalhães1,2?Permafrost soils are critical reservoirs for mercury (Hg), with the thawing process leading to the release of this element into the environment, posing signif i cant environmental risks. Of particular concern is the methylated form of mercury, monomethylmercury (MMHg), known for its adverse ef f ects on Human health. Microbial communities play a pivotal role in the formation of MMHg by facilitating Hg methylation and in the demethylation of MMHg, slowing the crossing of toxic threshold concentration in the environment. However, the specif i c microbes involved still need to be understood. This study aimed to identify the microbial drivers behind changes in Hg speciation (MMHg and Hg) in permafrost thaw lakes and assess the signif i cance of the biotic component in Hg biogeochemistry. Sediment samples from two thermokarst lakes in the Canadian sub-Arctic were collected during the winter and summer of 2022. Gene-centric metagenomics using whole-genome sequencing (WGS) was employed to identify key genes involved in mercury methylation (hgcA and hgcB) and demethylation (merA and merB), supported by qPCR analyses. A seasonal decline in microbial diversity, involved in the Hg methylation, and hgcA gene coverage was observed from winter to summer, mirroring patterns in mercury methylation rates. Notably, hgcA sequences were signif i cantly more abundant than merAB sequences, with contrasting seasonal trends. These results indicate a seasonal shift in the microbial community, transitioning from a dominance of mercury methylation in winter to a predominance of mercury demethylation in summer. Environmental drivers of these dynamics were integrated into a conceptual model. This study provide new insights on the microbial processes inf l uencing the Hg cycle in Arctic permafrost undergoing degradation.T h e International Permafrost Association def i nes permafrost as ground (soil or rock and including ice or organic material) that remains at or below 0 °C for at least two consecutive years 1 . Th e surface soil layer on top of the permafrost thaws in summer and refreezes in winter, and it is def i ned as an active layer 2 ). Th e present-day distribution of permafrost and ground ice is a combined ef f ect of the historical development of permafrost during the glacial period and the present conditions of heat exchange at the Earth’s surface and in the ground 3 . Permafrost thaw is critical evidence of the impact of climate change, particularly manifesting in Arctic regions where the warming ef f ect is easily detectable 4 . Permafrost accounts for nearly half of all organic carbon stored within the planet’s soil 5 . Permafrost thawing causes the organic matter present to become available for microbes that decompose and transform it, leading to the release of carbon dioxide and methane into the atmosphere 6 . Th e production of these greenhouse gases from the permafrost creates a positive feedback loop thereby amplifying the thawing process in a dif f i cult-to-stop escalation. Additionally, the release of biological, chemical and radioactive materials that have been sequestered for tens to hundreds of thousands of years is also a concern 6 . Recent research shows that a warming climate that leads to the rapid thawing of permafrost has transformed the region from a carbon sink to a carbon source 7 . Regarding Hg stored in permafrost regions, it is estimated that Northern Hemisphere permafrost regions contain 1,656 ± 962 Gg Hg, of which 793 ± 461 Gg Hg is frozen in permafrost 2 . Th ese values suggest that permafrost soils store nearly twice as much Hg as all other soils, the ocean, and the atmosphere combined 2 . Th e thawing causes the release of the previously stored 1 Interdisciplinary Centre of Marine and Environmental Research (CIIMAR/CIMAR), University of Porto, Porto, Portugal. 2 Faculty of Sciences, University of Porto, Porto, Portugal. 3 Centro de Química Estrutural, Institute of Molecular Sciences and Department of Chemical Engineering, Instituto Superior Técnico, University of Lisbon, Lisbon, Portugal. 4 Water Quality Centre, Trent University, Peterborough, Canada. ? email: catarina.magalhaes@fc.up.ptOPENScientif i c Reports | (2025) 15:7173 1 | https://doi.org/10.1038/s41598-025-89438-7www.nature.com/scientificreports