BACTERIAL DIVERSITY IN A DEEP-SUBSURFACE CLAY ENVIRONMENT

The presence of bacteria in a deep clay sediment was analyzed in a 20- m-long core horizontally drilled from a mine gallery at a depth of 224 m in the Boom clay formation (Mol, Belgium), This clay deposit is the result of a marine sedimentary process that occurred 35 million years ago, Bacterial activities were estimated by measuring respiration on [C-14]glucose. Using the same samples, universal primers for the genes coding for eubacterial 16S rRNA were used to amplify extracted DNA. P CR products were then cloned, sequenced, and analyzed by molecular phy logeny, Our data showed a decrease in bacterial densities as a functio n of distance from the gallery, with few bacteria detectable by cultur e at more than 80 cm from the gallery wall, PCR experiments showed the presence of bacteria in all samples, and phylogenetic analyses were t hen used to tentatively identify these organisms. Because of low bacte rial densities in deep clay samples, direct counts and enumeration of viable bacteria on diverse culture media remained negative. All experi ments, both cultures and PCR, demonstrated the difficulty of analyzing samples that contain only a few poorly active bacteria as it is diffi cult to avoid a small contamination by active bacteria during sampling , Since the porosity of the Boom clay formation is less than the expec ted size of bacteria, it is possible that some of the bacteria present in this 35-million-year-old deep clay deposit derive from cells initi ally trapped during the sedimentation process.