During the recent Integrated Ocean Drilling Program (IODP) Expedition 357 Atlantis Massif, Serpentinization and Life (26 October-11 December, 2015), rock samples were collected by drilling at shallow depths (up to 16.4 meters below the seafloor) at different sites located on the Atlantis Massif (820-1570 meters below sea level next to the Mid-Atlantic Ridge). The hydration by seawater of the outcropping rocks of the Earth's mantle forming the Atlantis Massif (i.e. serpentinization), produces large amounts of hydrogen, heat and methane, as well as, small organic molecules (as exemplified by the famous Lost City hydrothermal system), which can sustain the development of a wide range of chemolithoautotrophic microorganisms in the absence of sunlight. The main goal for our team of microbiologists is to cultivate and study the physiology of anaerobic microorganisms capable of growing in this deep environment associated with active serpentinization and characterized by high pH and high amounts of hydrogen. Here, we propose to identify the microorganisms(Bacteria and Archaea) that grew in the first anaerobic shipboard enrichment cultures from the core samples of the deepest holes drilled in the Atlantis Massif during IODP expedition 357, using V4-V5 16S deep-sequencing. PI Marianne Quemeneur marianne.quemeneur@mio.osupytheas.fr