Microbial Sediment Community Changes from the Last Glacial Maximum to Modern beneath the Ross Sea

Subseafloor microbial communities play an important role in carbon storage on continental shelves, but how these communities respond to changes in climate is not well characterized. Additionally, the Southern Ocean is an understudied environment for subsurface microbiology. Therefore, here we examine microbial community shifts from two sites in the Ross Sea collected during the International Ocean Discovery Program's Expedition 374. Site U1523 is on the edge of the continental shelf under 828 m of water while site U1524 is situated on the continental rise in 2394 m water depth. Both sites lie beneath the modern-day Antarctic Slope current and were likely ice proximal during the Last Glacial Maximum. We will present 16S rRNA amplicon data, cell counts, pore water and sediment geochemistry from each location. Cell counts show a higher microbial load at Site U1523 than Site U1524, likely driven by the differences in water depth, with Site U1523 being the shallower site. Amplicon data shows traditional subsurface microbial groups including Acidobacteria, Aerophobetes, Atribacteria, Chloroflexi, Deltaproteobacteria, Gammaproteobacteria and Planctomycetes. Significant shifts in the microbial communities occur at both sites, concurrent with changes in sediment lithology. During consistent lithologies at both sites we see a change in microbial percent abundances driven by Chloroflexi, concurrent with the shift during the Last Glacial Maximum to present, suggesting that benthic microbes are responding to external environmental factors. We explore how microbial community changes might be driven by alterations to bottom water chemistry and sediment deposition during this period of rapid climate change.