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Ultramafic rocks are hypothesized to support a subseafloor hydrogen-driven biosphere because of extensive production of bioavailable energy sources like H2 or CH4 from fluid-rock interactions. Hence, the apparent lack of microbial remains in subseafloor ultramafic rocks, in contrast to their frequent observation in subseafloor basalts, is somewhat of a paradox. Here we report fossilized microbial remains in aragonite veins in ultramafic rocks from the 15°20′N Fracture Zone area on the Mid-Atlantic Ridge (MAR), collected during Ocean Drilling Program (ODP) Leg 209. The microbial remains consist of filamentous structures associated with biofilms. The young age (<1 Myr) and absence of diagenesis result in fossilized microbial communities with a pristine composition characterized by carbonaceous matter (CM) and the enrichment in trace elements such as Ni, Co, Mo and Mn. Our study confirms the presence of the hypothesized deep subseafloor biosphere hosted in ultramafic rocks. We further show that host rock composition may influence the microbial elemental composition, which is recorded during the fossilization. [ABSTRACT FROM AUTHOR]

The deep biosphere of the subseafloor crust is believed to contain a significant part of Earth's biomass, but because of the difficulties of directly observing the living organisms, its composition and ecology are poorly known. We report here a consortium of fossilized prokaryotic and eukaryotic micro-organisms, occupying cavities in deep-drilled vesicular basalt from the Emperor Seamounts, Pacific Ocean, 67.5 m below seafloor (mbsf). Fungal hyphae provide the framework on which prokaryote-like organisms are suspended like cobwebs and iron-oxidizing bacteria form microstromatolites ( Frutexites). The spatial inter-relationships show that the organisms were living at the same time in an integrated fashion, suggesting symbiotic interdependence. The community is contemporaneous with secondary mineralizations of calcite partly filling the cavities. The fungal hyphae frequently extend into the calcite, indicating that they were able to bore into the substrate through mineral dissolution. A symbiotic relationship with chemoautotrophs, as inferred for the observed consortium, may be a pre-requisite for the eukaryotic colonization of crustal rocks. Fossils thus open a window to the extant as well as the ancient deep biosphere. [ABSTRACT FROM AUTHOR]

Abstract: The combination of fluid inclusion analyses and microfossil analyses is an excellent method to study the preservation process of deep sub-seafloor microorganisms. By studying fluid inclusions in the same mineral phases as microfossils, it is possible to reconstruct the conditions that prevailed when the microorganisms where entombed and to put them in a geological and environmental context. This study has been performed on carbonate and gypsum veins in drilled basalt samples from three seamounts belonging to the Emperor Seamounts in the Pacific Ocean: Detroit, Nintoku and Koko Seamounts. The study show that variations in salt composition (MgCl2, NaCl, KCl and CaCl2) and salinity (2.1 and 10.5eq. wt% NaCl) of the hydrothermal fluids do not have an influence on the occurrence of microfossils throughout the samples. The microorganisms were trapped and entombed at minimum temperatures of ∼130°C which implies that the microorganisms could have existed at temperatures of ∼130°C for shorter periods of time. The microorganisms were entrapped at shallow-marine to submarine conditions and the entrapment of the microorganisms occurred relatively late compared to the volcanic activity. [Copyright &y& Elsevier]