Bacterial abundance and composition in marine sediments beneath the R oss I ce S helf, A ntarctica

Marine sediments of the R oss S ea, A ntarctica, 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 R oss S ea 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 × 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 δ 13 C measurements of PLFA s and sedimentary and dissolved carbon sources, in combination with ribosomal RNA ( SSU r RNA ) gene pyrosequencing, were used to infer microbial metabolic pathways. The δ 13 C values of dissolved inorganic carbon ( DIC ) in porewaters ranged downcore from −2.5‰ to −3.7‰, while δ 13 C values for the corresponding sedimentary particulate OC ( POC ) varied from −26.2‰ to −23.1‰. The δ 13 C values of PLFA s ranged between −29‰ and −35‰ throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU r RNA gene pyrosequencing revealed that members of this microbial community were dominated by β‐, δ‐, and γ‐ P roteobacteria, A ctinobacteria, C hloroflexi and B acteroidetes. 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 δ 13 C PLFA analysis. Integrating phospholipids analyses with porewater chemistry, δ 13 C DIC and δ 13 C POC values and SSU r RNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.