Structure and function of the subsurface microbiome in oligotrophic ocean regions
In contrast to ocean margins, the seafloor underneath open-ocean gyres receives far less organic matter from the water column. Oxygen consequently penetrates much deeper into the sediment. The western flank of the Mid-Atlantic Ridge underlies the oligotrophic waters of the central Atlantic. The seafloor is characterized by sediment-filled depressions that are surrounded by steep basaltic outcrops. Cooling of the crust continues beyond the ridge, resulting in a low-temperature hydrothermal circulation, where the outcrops provide permeable conduits for seawater flow within the crust underneath the sediment. During Ocean Drilling Expedition 336 to ‘North Pond’, four boreholes were drilled throughout the sediment. Oxygen penetrated deeply into the sediment, was completely consumed at mid-depth, and increased towards the basaltic basement due to an upward diffusive supply from seawater circulating in the basalt. The oxygen profile resembles that of an oxygen minimum zone. Pore-water nitrate concentrations increased with depth and accumulated at mid-depth far above seawater levels, reflecting an active subsurface nitrogen cycle.
The goal of our study was to identify the microbial communities with a focus on a comparison between different oxygen regimes. We successfully extracted intact cells from different depth horizons, from which shotgun metagenome libraries were prepared. The taxonomic compositions of the upper and lower oxic layers were very similar and differed significantly from the anoxic mid-section. A total of 36 metagenome-assembled genomes (MAGs) were used to gain insight into the metabolic potential of the different communities and revealed insight into the functional role of these communities in subsurface element cycling.