14. Genetic Evidence for Endolithic Microbial Life Colonizing Basaltic Glass/sea Water Interfaces

The majority of the Earth's shallow crust is composed of basalt that erupted on the seafloor and was subsequently altered by chemical exchange with seawater. One aspect of this alteration is the replacement of glass by secondary minerals, including clays. Petrographic thin sections from ODP Holes 504B and 896A revealed characteristic patterns of pitting (channels) at the interface of fresh basalt glass and secondary clay. Fluorescent dyes that bind specifically to nucleic acids (Hoechst 33342, PO-PRO-3, and Sytol 1) were used to examine thin sections for evidence of cellular life. Independent experiments with the three dyes indicated the presence of paniculate nucleic acids at the interface of altered and unaltered glass, particularly at the distal tips of channels. Organic material was extracted from crushed basalts from Holes 504B and 896A and examined for the presence of microbial DNA by a polymerase chain reaction (PCR) assay. This technique detects specific ribosomal RNA genes that are present in all cellular life-forms. The PCR assays demonstrated the presence of prokaryotic genomic DNA in the rock extracts. Among the geochemical reactions that could provide energy for biological processes are the oxidation of iron, manganese, and sulfur. Electron microprobe analyses of areas of glass that appear to be affected by microbial action have low iron relative to the fresh glass. Also, elements that are consistent with the presence of cellular life, phosphate and potassium, were elevated in the channels (P is 0.02 wt% in glass and up to 1.4 wt% in channels and K is 0.01 wt% in glass and up to 2.0 wt% in channels). These data raise the possibility that chemical transformations of basalt on the seafloor are mediated by microorganisms.