The recesses of the oceanic crust harbour microbes that influence geochemical fluxes between the solid Earth and the hydrosphere(1,2). In the roots of the crust, mantle-derived rocks are progressively hydrated by hydrothermal circulation, a process known as serpentinization. The associated release of molecular hydrogen could provide metabolic energy for microbes(3). Phylogenetic analyses of chimneys associated with seafloor hydrothermal systems have provided direct but spatially restricted evidence for the existence of active microbial communities in these hydrated rocks(4); indirect evidence comes from isotopic analyses of drill cores(5). Here, we examine fully serpentinized peridotites recovered from the Mid-Atlantic Ridge, using Raman microspectroscopy and electron microscopy. We detect high concentrations of organic matter, of two types, intimately associated with serpentine-hosted hydrogarnets. One type contains a complex mixture of aliphatic and aromatic compounds and functional groups such as amides, usually associated with biopolymers such as proteins, lipids and nucleic acids. The other corresponds to dense aggregates of thermally evolved carbonaceous matter, with a weak structural organization, which we attribute to the maturation of carbon compounds present in the other type of organic matter identified. We suggest that the observed endogenic accumulations of organic matter result from past microbial activity within the serpentinized oceanic crust, potentially supported by the by-products of serpentinization. We further suggest that the proposed crustal community mediates elemental fluxes from the Earth's mantle to the oceans.
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