Microbial community structure across biogeochemical gradients in a perennially stratified Arctic lake

Meromictic lakes with deep anoxic saline waters and fresh oxygenated surface waters are useful biogeochemical models because of their persistent stratified chemical gradients and potential separation of redox reactions down the water column. Perennially ice-covered meromictic lakes in both the Arctic and Antarctic are particularly stable and are thought to change little over time. As part of the International Polar Year (IPY) in 2008, we sampled Lake A, a deep meromictic lake on Ellesmere Island, Canada (lat. 83°N), in early May and again in August. Using high-throughput 16S rRNA gene tag pyrosequencing we identified Bacteria and Archaea communities in the surface waters, the hypoxic chemocline and the deep anoxic zone. Both common and rarer organisms that could contribute to key biogeochemical functions were stratified by depth and differed at a fine-scale taxonomic level between sampling dates. Sequences of cyanobacteria and photosynthetic anoxygenic Chlorobi were more abundant in August when the exceptionally warm summer resulted in complete ice out and higher than normal light penetration into the water column. The proportion of Actinobacteria and Verrucomicrobia sequences decreased between May and August, while Bacteroidetes increased in the surface after ice out. The archaeal populations in the deep anoxic layer were mostly Group C3 Crenarchaeota on both sampling dates, suggesting seasonal insensitivity to change at depth. Finally, we matched taxon-specific biogeochemical functions using 16S rRNA gene identities and found a close correspondence between the depth distribution of functional specialists and chemical features of the water column, indicating the potential of taxonomic surveys to trace biogeochemical processes.