A deep sleep in coal beds Deep below the ocean floor, microorganisms from forest soils continue to thrive. Inagaki et al. analyzed the microbial communities in several drill cores off the coast of Japan, some sampling more than 2 km below the seafloor (see the Perspective by Huber). Although cell counts decreased with depth, deep coal beds harbored active communities of methanogenic bacteria. These communities were more similar to those found in forest soils than in other deep marine sediments. Science , this issue p. 420 ; see also p. 376

Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane-and sulfurcycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host delta C-13-and delta S-34-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic delta C-13 signatures locally attenuated by heterotrophic metabolism. Main text Subseafloor basaltic crust represents the largest habitable zone by volume on Earth (1). Chemical reactions of basalt with seawater flowing through fractures release energy that may support chemosynthetic communities. Microbes exploiting these reactions are known from basalt exposed at the seafloor, where the oxidation of reduced sulfur (S) and iron (Fe) from basalt with dissolved oxygen and nitrate from seawater supports high microbial biomass and diversity (2, 3). Multiple lines of indirect evidence that include textural alterations (4), depletions in δ 34 S-pyrite (FeS 2) (5) and δ 13 C-dissolved inorganic carbon (DIC) (6), and DNA sequences from

The coupling of subseafloor microbial life to oceanographic and atmospheric conditions is poorly understood. We examined diagenetic imprints and lipid biomarkers of past subseafloor microbial activity to evaluate its response to glacial-interglacial cycles in a sedimentary section drilled on the Peruvian shelf (Ocean Drilling Program Leg 201, Site 1229). Multiple and distinct layers of diagenetic barite and dolomite, i.e., minerals that typically form at the sulfate−methane transition (SMT), occur at much shallower burial depth than the present SMT around 30 meters below seafloor. These shallow layers co-occur with peaks of 13 C-depleted archaeol, a molecular fossil of anaerobic methane-oxidizing Archaea. Presentday, non-steady state distributions of dissolved sulfate also suggest that the SMT is highly sensitive to variations in organic carbon flux to the surface shelf sediments that may lead to shoaling of the SMT. Reaction-transport modeling substantiates our hypothesis that shallow SMTs occur in response to cyclic sediment deposition with a high organic carbon flux during interglacials and a low organic carbon flux during glacial stages. Long diffusion distances expectedly dampen the response of deeply buried microbial communities to changes in sediment deposition and other oceanographic drivers over relatively short geological time scales, e.g., glacial-interglacial periods. However, our study demonstrates how dynamically sediment biogeochemistry of the Peru Margin has responded to glacialinterglacial change and how these changes are now preserved in the geological record. Such changes in subsurface biogeochemical zonation need to be taken into account to assess the role of the subseafloor biosphere in global element and redox cycling.deep biosphere | paleodiagenetic | methane oxidation front | biogeochemical cycles M icrobial life beneath the ocean reacts to and alters the organic matter and sediment deposited on the seafloor and buried over geological time scales of millennia or more. This subseafloor biosphere mineralizes buried organic matter, changes the geochemical gradients, and affects the precipitation or dissolution of minerals (1-3). Discrete zones of microbial abundance and activity develop (4-7) where sulfate (SO 4 2− ) diffusing downward from the overlying seawater intersects with upward diffusing methane (CH 4 ). Here the anaerobic oxidation of methane (AOM) is coupled to sulfate reduction (5, 8), and both compounds become depleted in this sulfate methane transition (SMT). The SMT is typically located within the top few meters to tens of meters below seafloor in continental shelf and slope sediments. Under steady state conditions, i.e., when the rates of both organic and bulk sedimentation remain constant and the quality of the deposited organic matter is uniform over time, no significant change occurs in the fluxes of dissolved methane and sulfate, and the SMT remains at a constant depth beneath the seafloor (8, 9). Sedimentation and organic carbon flux, however, are seldom constant over tim...

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