Abstract:
Mid-ocean ridge black smokers are recognized as oases of life that support diverse and densely populated communities of organisms. But the diversity and total biomass of organisms drops suddenly away from these oases into the relative desert of the ocean floor. Recent ocean drilling has opened a window into the deep biosphere and has led to the discovery of microbial activity in the upper oceanic crust characterized by endolithic microbial alteration of basaltic glass. We report microscopic textural, geochemical, isotopic, and biomolecular evidence for microbial alteration of volcanic glass from pillow basalt margins and shards preserved in poorly sorted tuffs from the Early Cretaceous ( approximately 120 Ma) Ontong Java Plateau drilled during ODP Leg 192. Petrographic analysis of the glasses reveals an astounding density and variety of exceptionally preserved granular and tubular microbial alteration textures. Both textures are observed to extend away from fractures and shard boundaries, where liquid water was once present, into unaltered glass. It is likely the glass shards were initially exposed to high water-rock ratios that may make glass in the tuffs more susceptible to microbial alteration than glassy seafloor pillow basalts. Detailed SEM imaging of these features reveals complex channels, delicate filaments, and desiccated biofilm. Microbial alteration features are associated with elevated levels of C, N, S, P, and K in X-ray element maps. Carbon isotope ratios of disseminated carbonates in bioaltered glass are depleted by as much as -16 per mil, suggesting biologic fractionation. Temperatures calculated from oxygen isotope ratios of carbonate, assuming equilibrium with seawater, suggest formation at elevated temperatures, between 25 degrees and 80 degrees C, possibly due to passive geothermal heat flow. The results of nucleic acid staining and detailed laser scanning confocal microscopy confirm the presence of DNA/RNA localized within the microbial alteration features along channel walls, probably within desiccated biofilms. The presence of DNA/RNA suggests the biogenic features are relatively recent and that the microbes may currently be active. We believe the oceanic crust is not a desert but instead hosts a still unquantified flourishing deep biosphere that may contribute significantly to global geochemical fluxes.