AbstractMicrobial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40° to 60°C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~104 cells cm−3. Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.
Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane- and sulfur-cycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host δ13C- and δ34S-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic δ13C signatures locally attenuated by heterotrophic metabolism.
The immediate objective is to learn techniques of intact polar and apolar lipid analysis (IPL and AL, respectively) in the Hinrichs Lab (MARUM, Bremen, Germany). Analysis will be conducted in several sections from five sediment cores collected in a cross-shelf transect during the LARISSA 2012 research expedition in the Weddell Sea. This work would be undertaken in a three-month graduate student research exchange. The overarching research objective is to assess the changes in sedimentary microbial community structure and the input of organic matter as the Larsen A embayment (Weddell Sea) transitioned from an oligotrophic sub-ice shelf, dark system to a photosynthetic open ocean system. These changes will be verified and related to the ice shelf collapse in space and time. The results will be merged with geochemical (concentrations of methane, sulfate, sulfide, dissolved inorganic carbon, and nutrients), geological (Lead 210 dating, diatom counts and chlorophyll concentration) and genetic profiles (i.e. bacterial and archaeal 16S rDNA sequencing and metageomics data) generated by collaborators in the NSF-funded LARISSA Program.