Water Column and Sediment Microbial Ecology of a West Florida Shelf Blue Hole

Microbial communities mediate most oceanic biogeochemical cycles. Blue holes, or subsurface caverns, are underexplored marine environments that might harbor unique microbial niches. Their rims are “ecological hotspots” with important implications for local biodiversity, but their microbial composition and corresponding roles in nutrient cycling are unknown. We characterized the microbial composition of the water column and top 20 cm of sediment of a West Florida Shelf blue hole using 16S rRNA gene amplicon sequencing and compared these results with water chemistry and nutrient data. Oxygen concentration decreased with depth in the water column and became anoxic around 100 m, classifying the blue hole as an oxygen minimum zone (OMZ). Similar to other aquatic systems, the water column and sediment microbial communities were significantly different (p=0.001) and the sediments had higher microbial diversity than the water column. The sediment communities included taxa common in anoxic marine sediments, as well as novel sequences, and varied with core depth and with dissolved inorganic carbon. Similar to other OMZs, the microbial composition of the water column was distinct from overlying water and changed with depth, likely driven by oxygen concentration. Microbial composition separated into three distinct depth groupings: 0-32, 46-61, and 85-105 m (p=0.001) and the depth profile was notably different from that of other OMZs. Unlike open-ocean OMZs, the deep water column had the highest diversity compared to more oxygenated regions. Remarkably, we found high relative abundances of a recently described archaeal group, the Woesearchaeia, in the deep water column (36.8-38.9%) and sediments (1.19-6.41%). This group is poorly characterized and rarely seen in other OMZs, suggesting they play a unique biogeochemical role in blue hole environments.