Microbes in ocean floor basalt: their association to alteration textures and influence on the geochemical composition of palagonite

Samples of recent lava flows from the rift valley of the Mohns Ridge and Knipovich Ridge have been studied by electron microscopy and geochemical analyses, aimed at describing the microbial community in the basalt and the influence of microbes on the dissolution and alteration of the basaltic glass (Thorseth et al., 1995; Torsvik et al., 1998). The samples were collected at 2000-3500 m below sea level and the ambient seawater temperature was measured to -0.7 ^oC. Along fractures in the glassy margins of the lava flows, yellow brown to red brown alteration rims (palagonite) are developed. The thickness of the alteration rims varies from 80 †m at the outer surface to <1 †m in the interior. The alteration rims have a zoned texture where porosity and chemical composition differ. In most fractures various types of coccoid, rods and stalked cells are associated with the alteration product. The dominant morphology has multiple stalks (up to 100 †m long), radiating out from a central spherical cell (1†m in diameter) in a star-shaped manner. In all samples fossilized cells are observed as hollow sub-spherical, rod-shaped and filamentous structures, due to precipitation of alteration products around the different morphological forms of microbes. An increased accumulation of Fe and Mn in some morphological forms compared to the surrounding palagonite, may indicate utilization of these elements in energy metabolic processes (Lysnes et al., 2001; Steinsbu et al., 2001). Generally, the microbes are more numerous in the outer part of the glassy margin than in the inner part. The microbes are observed both at the primary fracture surfaces, in high porosity palagonite zones and at the glass-alteration fronts, where they attach to the fresh glass. Within zones of dense palagonite it is difficult to distinguish microbial morphologies. However, alteration rims showing numerous microbial morphologies nearly throughout, indicate a high content of living andor fossilized cells. Preliminary analyses of the palagonite show an organic carbon content of about 0.25 wt.% and a dC13-value of -22%. This indicates 1010 cells per 1 g alteration product, assuming 10-13g of C per cell. Since 1% of the glass in these samples is altered and pillow basalt generally are 2 % glass, there are about 106 cells per cm3 in these ocean floor basalts. Microbes that colonise the basaltic surface layer at spreading ridges may be buried below younger volcanic flows. This may carry the textural and geochemical traces of life and organic carbon into the deeper part of the ocean crust. Burial of microbial communities that colonise the surface layer may thus be of importance in studies of the deep sub-surface biosphere. References: Lysnes, K., Thorseth, I. H., Pedersen, R. B., Steinsbu, B. O., and Torsvik, T. Microbial populations in basalt from the ocean crust. Abstract: The deep sub-seafloor biosphere: current status and future directions. Workshop 1-3 Oct., Bremen, Germany. Steinsbu, B. O., Lysnes K., Thorseth I. H., Pedersen, R. B., and Torsvik, T. Isolation of bacteria present in basalt from Arctic Ridges. Abstract: The deep sub-seafloor biosphere: current status and future directions. Workshop 1-3 Oct., Bremen, Germany. Thorseth, I. H., Torsvik, T., Furnes, H. & Muehlenbachs, K., Chem. Geol., 126, 137-146 (1995). Torsvik, T., Furnes, H., Muehlenbachs, K., Thorseth, I. H. & Tumyr, O., Earth Planet. Sci. Lett., 162, 165-176 (1998).