Person: Kai-Uwe Hinrichs

Abstract

Abstract

Microbial life in marine sediment contributes substantially to global biomass and is a crucial component of the Earth system. Subseafloor sediment includes both aerobic and anaerobic microbial ecosystems, which persist on very low fluxes of bioavailable energy over geologic time. However, the taxonomic diversity of the marine sedimentary microbial biome and the spatial distribution of that diversity have been poorly constrained on a global scale. We investigated 299 globally distributed sediment core samples from 40 different sites at depths of 0.1 to 678 m below the seafloor. We obtained ∼47 million 16S ribosomal RNA (rRNA) gene sequences using consistent clean subsampling and experimental procedures, which enabled accurate and unbiased comparison of all samples. Statistical analysis reveals significant correlations between taxonomic composition, sedimentary organic carbon concentration, and presence or absence of dissolved oxygen. Extrapolation with two fitted species–area relationship models indicates taxonomic richness in marine sediment to be 7.85 × 10³ to 6.10 × 10⁵ and 3.28 × 10⁴ to 2.46 × 10⁶ amplicon sequence variants for Archaea and Bacteria, respectively. This richness is comparable to the richness in topsoil and the richness in seawater, indicating that Bacteria are more diverse than Archaea in Earth’s global biosphere.

Abstract

The most recent decadal phase of scientific ocean drilling through the International Ocean Discovery Program (IODP) has resulted in paradigm-​shifting understanding of life below the seafloor. Enabled by new drilling and coring approaches, cutting-edge methodologies, and novel observatory science, IODP expeditions have significantly advanced understanding of the amount and diversity of subseafloor life, the metabolic strategies that this life uses to survive under extreme energy limitation, and consequences of this life for the Earth system. Here, we summarize highlights from recent IODP expeditions focused on life beneath the seafloor and emphasize remaining major science challenges in investigating the form and function of life in this environment.

Abstract

The hydrothermal sediments of Guaymas Basin, an active spreading center in the Gulf of California (Mexico), are rich in porewater methane, short-chain alkanes, sulfate and sulfide, and provide a model system to explore habitat preferences of microorganisms, including sulfate-dependent, methane- and short chain alkane-oxidizing microbial communities. In this study, hot sediments (above 60°C) covered with sulfur-oxidizing microbial mats surrounding a hydrothermal mound (termed “Mat Mound”) were characterized by porewater geochemistry of methane, C₂–C₆ short-chain alkanes, sulfate, sulfide, sulfate reduction rate measurements, in situ temperature gradients, bacterial and archaeal 16S rRNA gene clone libraries and V6 tag pyrosequencing. The most abundantly detected groups in the Mat mound sediments include anaerobic methane-oxidizing archaea of the ANME-1 lineage and its sister clade ANME-1Guaymas, the uncultured bacterial groups SEEP-SRB2 within the Deltaproteobacteria and the separately branching HotSeep-1 Group; these uncultured bacteria are candidates for sulfate-reducing alkane oxidation and for sulfate-reducing syntrophy with ANME archaea. The archaeal dataset indicates distinct habitat preferences for ANME-1, ANME-1-Guaymas, and ANME-2 archaea in Guaymas Basin hydrothermal sediments. The bacterial groups SEEP-SRB2 and HotSeep-1 co-occur with ANME-1 and ANME-1Guaymas in hydrothermally active sediments underneath microbial mats in Guaymas Basin. We propose the working hypothesis that this mixed bacterial and archaeal community catalyzes the oxidation of both methane and short-chain alkanes, and constitutes a microbial community signature that is characteristic for hydrothermal and/or cold seep sediments containing both substrates.

Abstract

Microbial 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.

Abstract

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.

Abstract

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.