| The microbial communities of the saturated subsurface play an important role in the quality of groundwater, a resource of incalculable value to human and ecological concerns. Further, although the magnitude of the effect that subsurface microbes have on global biogeochemical cycling is not known, estimates suggest that because of the sheer numbers of subsurface bacteria, they likely play a significant role in the cycling of bioactive elements. Despite these facts, subsurface microbial communities have been little studied until recently. In an effort to extend current knowledge in this area, a study of the microbial communities in clean and chemically-contaminated aquifer sediment was undertaken. The diversity and composition of the microbial communities at two sites was assessed using molecular techniques. At a pristine site in Oyster, Virginia, the members of microbial communities in oligotrophic sediment were characterized by phylogenetic methods. The bacterial community profile from oligotrophic sediment was compared to that of the communities in a contrasting geochemical environment in the same formation. Similarities were found to exist between the two communities. Bacterial community diversity in aquifer sediment was markedly lower than that observed in surface soils. A novel group of phylogenetically deeply-branching bacteria was detected at the site and found to exist in eight out of ten sediments tested. The microbial communities at a tetrachloroethylene-contaminated site in Oscoda, Michigan were also studied. Members of communities in clean and contaminated sediment were described phylogenetically, and found to be closely related to the β, γ, and δ Proteobacteria , the Acidobacteria, Leptospirillum/Nitrospira, and Green nonsulfur bacteria. Within the plume, 16S sequences closely related to the species Syntrophus were identified, suggesting that interspecies hydrogen or acetate transfer played a role in carbon cycling at the site. Bacterial diversity was found to be significantly higher in contaminated sediment than in sediment from upstream of the plume. Finally, the distribution and abundance of a sulfate-reducing bacterium which was detected within the plume was studied using the emerging method of real-time PCR. The 16S sequence of this organism was found to constitute only a small proportion of total bacterial 16S sequences at the site, and was highly over-represented in the 16S clone library. |
