Seafloor topography structures subseafloor microbial communities in the Puerto Rico Trench
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The influence of seafloor topography on subseafloor microbial communities is poorly understood. In this study, we systematically sampled subseafloor sedimentary microbial communities and pore-water geochemistry at bathyal, abyssal, and hadal water depths of the Puerto Rico Trench. The Puerto Rico Trench exhibits vertical relief of over 8 km with seafloor topography shaped by active plate tectonics and characterized by a very steep southern wall and active mass wasting. Sampling across these features, we find that subseafloor microbial communities are influenced by topography in the following two ways. First, subseafloor sediment in the deepest part of the trench is anoxic, enriched in organic matter due to mass transport of sediment from shallower water depths, and contains an anaerobic subseafloor community that is quite distinct from the anaerobic subseafloor community in anoxic sediment at bathyal depths. The difference between these bathyal and hadal subseafloor anaerobic communities is consistent with influence of water-depth-related physical properties on subseafloor community composition. Second, subseafloor sediment remains oxygenated for meters below seafloor on steep slopes within the bathyal zone, an abyssal topographic high within the trench, and the abyssal plain seaward of the trench subseafloor, likely due to limited sediment accumulation, which selects for the presence of aerobic microbial communities. The difference between these bathyal and abyssal aerobic subseafloor communities and the anaerobic communities at bathyal and hadal depths reflects the role of seafloor topography in structuring subseafloor availability of redox reactants, which in turn drives the activities and composition of subseafloor microbial communities.
Significance Statement
Energy limitation is a major factor structuring microbial communities that persist deep below the seafloor. Seafloor topography has the potential to shape energy sources available to the subseafloor biosphere, but this relationship remains poorly understood. We addressed this topic by systematically analyzing subseafloor communities across depth zones and spatial features of the Puerto Rico Trench down to 20 meters below the seafloor. The diversity and metabolic potential of subseafloor communities were indirectly structured by seafloor topography due to its control on sediment accumulation rates, porewater geochemical profiles, and the availability of high-energy terminal electron acceptors for respiration. These findings provide new insights into how seafloor topography can structure microbial communities in the deep biosphere.
