Microbial biosignatures in the lower oceanic crust at Atlantis Bank, Indian Ocean

Abstract:

The lithified lower oceanic crust is one of Earth’s last biological frontiers. To survive, microbiota in gabbroic rocks must obtain sufficient carbon and energy to support growth or to meet basal power requirements during periods of resource scarcity. At Atlantis Bank, Indian Ocean, Earth’s lower crust is exposed at the seafloor. Analyses of rock samples spanning 10-750 meters below the seafloor reveal heterogeneously distributed, but detectable enzyme activities, lipid biomarkers, and marker genes. Microscopy indicates viable biomass with ultra-low cell densities (<2000 cells/cm³). Expression of genes for heterotrophic processes include degradation of polyaromatic hydrocarbons, use of polyhydroxyalkanoates as carbon storage molecules, and recycling of amino acids to produce compounds that can participate in redox reactions and energy production and storage. Microbial cells appear to survive primarily within fractures or porous substrates by coupling sources of energy and organic and inorganic carbon likely delivered via circulation of seawater-derived fluids. Further exploration of the lower crust at Atlantis Bank is required to evaluate the potential for life in zones deeper than drilled during IODP Expedition 360. Exploration of other sites where the lower crust is accessible will determine the extent to which our findings are unique to Hole U1473A.