Seasons and vertical dynamics influence community composition in a flooded and abandoned mica mine

Artificial lakes formed from past mining activities represent unique but underexplored ecosystems that support diverse microbial communities. This study examined how seasonal variation and depth influence bacterial, archaeal, and microeukaryotic assemblages in the stratified water column of the Blackburn mine (Outaouais, Quebec, Canada). Water and biofilm samples were collected by technical divers from the surface to 52 m during spring, summer, and autumn of 2021-2022, and analysed by 16S/18S rRNA gene sequencing. Seasonal changes had little effect on physicochemical parameters but strongly shaped microbial community composition, together with depth. Archaeal taxa displayed greater stability across depths compared to bacteria and eukaryotes. Oxygen profiles defined three ecological zones: an oxic layer dominated by Actinobacteria and the methanogen Methanosarcina; a transition zone enriched in Chlorobium and methanogens such as Methanospirillum and Methanosaeta; and an anoxic layer containing sulfur-reducing (Desulfomonile and Desulfobacca), sulfur-oxidizing (Sulfuricurvum), and methane-cycling archaea. Eukaryotic communities included algae, particularly Chrysophyceae, and diverse protists. These findings suggest that microbial communities in the mine are integral to sulfur and carbon cycling, emphasizing the ecological significance of such stratified, mining-associated aquatic systems. The Blackburn mine provides valuable insight into how anthropogenic legacies shape microbial diversity and ecosystem functioning in artificial aquatic environments.