Microbial hotspots in anchialine blue holes: initial discoveries from the Bahamas

Inland blue holes of the Bahamas are anchialine ecosystems with distinct fresh and salt water layers, and anoxic or microoxic conditions at depth. Scientific cave diving and geomicrobiology exploration of blue holes are providing a first glimpse of the geochemistry and microbial life in these vertically stratified karst features. We hypothesized that two geographically adjacent, sunlit blue holes on Abaco Island would have comparable biogeochemistry and microbial life. Water samples were analyzed using in situ multiparameter dataloggers and field and laboratory tests, and diver-retrieved microbial samples were analyzed using nucleic acid analysis. Microbial 16S rRNA genes were dominated by members of the anoxygenic phototroph clade Chlorobi, with smaller numbers of Deltaproteobacteria, in both blue holes. However, spatial distributions of microbial biomass and species present within these major clades were significantly different. We also found that differences in the intensity of solar insolation, terrestrial and marine inputs, water residence time, depth to the halo/chemocline, and cave passage geometry strongly influence geochemical changes with depth. The biogeochemical diversity of inland blue holes in the Bahamas make them valuable as natural laboratories, repositories of microbial diversity, and analogs for stratified and sulfidic oceans present early in Earth's history.