Terrestrial-aquatic connectivity structures microbial communities during the formation of thermokarst lakes

Rising air temperatures and permafrost degradation drive the erosion of palsas (permafrost mounds mainly composed of frozen peat and ice layers) and lead to the formation of thermokarst ponds and lakes, known for their high greenhouse gas (GHG) emissions. This study investigates the impact of permafrost soil erosion during thermokarst lake formation on microbial community structure and its implications for GHG dynamics in a highly degraded permafrost valley (Nunavik, northern Quebec, Canada). Samples were collected from a palsa, an emerging lake connected to the palsa, surrounding peat and soil pore water, and two mature lakes which are older, stratified, and less connected to the palsa. Analysis of total and potentially active microbial communities, based on 16S rRNA gene amplicon sequence variants revealed significant changes in taxonomic and phylogenetic diversity during thermokarst lake formation. We found distinct assembly processes depending on the stage of formation. Firstly stochastics, they became more deterministic as lakes mature. Distinct methanogens/trophs communities in emerging lake led to lower CO₂:CH₄ ratio compared to the surface of mature lakes. Which presented a greater diversity of methanogens and distinct methanotrophic communities, with acetogenic, hydrogenotrophic and methylotrophic methanogens along anaerobic an aerobic methanotrophs. Multivariate analyses revealed that selection processes were primarily driven by concentrations of CH₄, CO₂, and NO₃ ^-. The interplay between the nitrogen and carbon cycles appears to be pivotal in these assemblages, with nitrogen playing key roles on community structure. These findings underscore the significance of terrestrial-aquatic connectivity in shaping microbial communities and GHG emissions in thermokarst lakes.