Bacterial abundance and composition in marine sediments beneath the Ross Ice Shelf, Antarctica

Marine sediments of the Ross Sea, Antarctica, harbor microbial communities that play a significant role in the decomposition, mineralization, and recycling of organic carbon (OC). In this study, the cell densities within a 153-cm sediment core from the Ross Sea were estimated based on microbial phospholipid fatty acid (PLFA) concentrations and acridine orange direct cell counts. The resulting densities were as high as 1.7 9 10 7 cells mL ! 1 in the top ten centimeters of sediments. These densities are lower than those calculated for most near-shore sites but consistent with deep-sea locations with comparable sedimentation rates. The d 13 C measurements of PLFAs and sedimentary and dissolved carbon sources, in combination with ribosomal RNA (SSU rRNA) gene pyrosequencing, were used to infer microbial metabolic pathways. The d 13 C values of dissolved inorganic carbon (DIC) in porewaters ranged downcore from ! 2.5& to ! 3.7&, while d 13 C values for the corresponding sedimentary particulate OC (POC) varied from ! 26.2& to ! 23.1&. The d 13 C values of PLFAs ranged between ! 29& and ! 35& throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU rRNA gene pyrosequencing revealed that members of this microbial community were dominated by b-, d-, and c-Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Among the sequenced organisms, many appear to be related to known heterotrophs that utilize OC sources such as amino acids, oligosaccharides, and lactose, consistent with our interpretation from d 13 CPLFA analysis. Integrating phospholipids analyses with porewater chemistry, d 13 CDIC and d 13 CPOC values and SSU rRNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.