The production of nuclear materials at Department of Energy (DOE) sites has resulted in uranium contamination of soil and ground water covering 2 million acres in 30 states across the U.S. Traditional remediation strategies, such as excavation and pump-and-treat are costly and impractical for large scale remediation efforts. Therefore, on-site containment of nuclear waste represents a more practical approach. Bioremediation of soluble U(VI) to insoluble U(IV) by dissimilatory metal-reducing bacteria (DMRB) in anoxic environments has received the most attention, however the potential for re-oxidation of U(IV) formed by the DMRB may compromise the long term stability of U(IV) minerals. The geochemical conditions at many uranium contamination sites such as acidic pH and elevated nitrate are also problematic for bacteria-mediated U(VI) reduction. In order to formulate a long-term strategy for U immobilization, we investigated U(VI)-mineralization by Caulobacter cresecetus under aerobic conditions. C. crescentus (strain NA1000) is an oligotrophic aquatic bacterium that grows in aerobic environments and exhibits tolerance to U(VI) (growth at pH &gt; 6). We have previously shown that inorganic phosphate release by C. cresecetus facilitates the precipitation of highly insoluble U-phosphate mineral phases (autunites), highlighting the potential usefulness of C. crescentus in remediation efforts and necessity of further investigation U remediation under environmentally relevant conditions. Given that U contaminated sites are commonly acidic, we expanded our investigation to also include an acid tolerant strain of Caulobacter crescentus (strain OR37) that was previously isolated from the Oak Ridge Field Research Center. We performed comparative studies of strains NA1000 and OR37 with respect to the effects of pH and phosphatase activity on U-biomineralization. We found that the OR37 strain exhibits acid phosphatase activity (vs alkaline phosphatase in strain NA1000), resulting in the precipitation of U-phosphates at pH as low as 4. Additionally, changes in membrane lipid composition and fluidity in OR37 may also contribute additional acid and metal resistance compared to strain NA1000. Results from our comparative studies of these two strains highlight the importance of environmental conditions that are likely the main driver for the evolution and adaptation of Caulobacter in U-contaminated environments.

Uranium biomineralization by the acid tolerant Caulobacter crescentus OR37

253rd National Meeting (2017)

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