Diversity and characterization of sulfate-reducing bacteria in groundwaterat a uranium mill tailings site

Microbially mediated reduction and immobilization of U(VI) to U(TV) plays a role in both natural attenuation and accelerated bioremediation of uranium contaminated sites. To realize bioremediation potential and accurately pre dict natural attenuation, it is important to first understand the microbial diversity of such sites. In this paper, the distribution of sulfate-reduci ng bacteria (SRB) in contaminated groundwater associated with a uranium mil l tailings disposal site at Shiprock, N.Mex,, was investigated. Two culture -independent analyses were employed: sequencing of clone libraries of PCR-a mplified dissimilatory sulfite reductase (DSR) gene fragments and phospholi pid fatty acid (PLFA) biomarker analysis. A remarkable diversity among the DSR sequences was revealed, including sequences from F-Proteobacteria, gram -positive organisms, and the Nitrospira division. PLFA analysis detected at least,52 different mid-chain-branched saturate PLFA and included a high pr oportion of 10me16:0, Desulfotomaculum and Desulfotomaculum-like sequences were the most dominant DSR genes detected. Those belonging to SRB within F- Proteobacteria were mainly recovered from low-uranium (less than or equal t o 302 ppb) samples. One Desulfotomaculum like sequence cluster overwhelming ly dominated high-U (>1,500 ppb) sites. Logistic regression showed a signif icant influence of uranium concentration over the dominance of this cluster of sequences (P = 0.0001), This strong association indicates that Desulfot omaculum has remarkable tolerance and adaptation to high levels of uranium and suggests the organism's possible involvement in natural attenuation of uranium. The in situ activity level of Desulfotomaculum in uranium-contamin ated environments and its comparison to the activities of other SRB and oth er functional groups should be an important area for future research.