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

Artificial lakes formed from former mining activities represent unique and underexplored environments with diverse microbial communities. This study aimed to characterize and understand the effects of seasonal variation and depth gradients on bacterial, archaeal, and micro-eukaryotic communities in a stratified water column. The Blackburn Mine in the Outaouais region (Quebec, Canada), an intriguing site due to its mining history and microbial ecology remains largely unknown. Water and biofilm samples were collected by technical divers from the surface to 52 m depth during spring, summer, and autumn of 2021 and 2022, and analyzed using 16S and18S rRNA gene sequencing. The results revealed that while seasonal changes did not significantly impact environmental variables, they strongly influenced microbial community composition, along with depth gradients. Archaeal genera displayed stability across depths compared to bacterial and eukaryotic communities. The mine was stratified into three distinct zones based on oxygen concentration. In the oxic zone, the bacterial phylum Actinobacteria, a core member of freshwater ecosystems, dominated, along with the methanogenic archaeon Methanosarcina. The suboxic zone was characterized by a high abundance of the bacterial genus Chlorobium, with methanogens such as Methanospirillum and Methanosaeta also prevalent. The anoxic zone hosted sulfur-reducing bacteria (Desulfomonile and Desulfobacca), sulfur-oxidizing bacteria (Sulfuricurvum), and methanogenic and methanotrophic archaea. Eukaryotic communities exhibited a notable presence of algae, particularly from the Chrysophyceae class, and 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.