MICROBIAL COMMUNITY DATA SUPPORTS BIOLOGICALLY MEDIATED CRYPTIC SULFUR CYCLING IN ARCTIC SEDIMENTS, VAN KEULENFJORDEN, SVALBARD (79°N)

The effects of climate warming are most notable near the north pole, where Arctic amplification results in glacial retreat. As shrinking glaciers become cut off from fjords, the direct conduit of terrestrially derived material is removed. Changes in fjord sediment delivery may upset biogeochemical cycling, ultimately altering the ecosystem balance of sediment microbial communities. Here, we investigate microbial community structure and interactions within sediments adjacent to a tidewater glacier in Van Keulenfjorden, Svalbard (79°N). Devonian iron-rich bedrock surrounding the glacier supplies significant amounts of iron to the fjord with seasonal glacial runoff. In these anoxic sediments, sulfate does not decrease with depth, despite active sulfate reduction, indicating that chemical and biological factors may facilitate the oxidation of sulfide. We hypothesized tight coupling between iron oxidizers and sulfate reducers that are engaged in microbially-mediated cryptic sulfur cycling. We extracted DNA from 1 cm intervals down to 20 cm depth and generated 16S rRNA gene libraries. The change in relative abundance of key taxa with depth was evaluated with respect to distance from the glacier and with organic geochemistry measurements. Desulfuromonadales, which include iron reducers, were more prevalent in sediments closest to the glacier. The majority of sulfate reducing sequences were from the genus Desulfobulbus, which peak in deeper sediment layers alongside the sulfur disproportionating Desulfocapsa. Microbial network analysis showed the iron-reducing Geobacteraceae co-occur with sulfate reducers. Nitrosomonadales, containing iron oxidizers, have negative relationships with both sulfate reducing and iron reducing taxa. These observations coupled to porewater geochemistry help us interpret how key microorganisms shape their geochemical environment and mediate transformations of sulfur species.