Uranium (U) is a wide spread contaminant but effective means for uranium remediation are missing. We recently explored U bioremediation using a ubiquitous bacterium Caulobacter crescentus. Caulobacter is an attractive candidate for U(VI) bioremediation because of its ability to tolerate high U concentrations and survive in low-nutrient environment. We observed that U(VI) caused a temporary growth arrest, during which our culture did not accumulate U(VI) resistance mutations, but rather, a reduction in U(VI) toxicity occurred concomitantly with an increase in medium pH. Our data shows that C. crescentus recovers from U(VI)-induced growth arrest by reducing U(VI) toxicity through pH modulation, highlighting a possible mode of microbial activity in affecting aquatic chemistry and toxicity of U. During the active growth period, we observed that Caulobacter is able to facilitate U(VI) biomineralization through the formation of U-P precipitates via its native alkaline phosphatase activity. The U-Pi precipitates, deposited on the cell surface in the form of meta-autunite structures, have a lower U/Pi ratio than do chemically produced precipitates. The enzyme that is responsible for the biomineralization process is identified as PhoY, a periplasmic alkaline phosphatase with broad substrate specificity. Furthermore, PhoY is shown to confer a survival advantage on C. crescentus toward U(VI). Efforts are currently underway to characterize the morphological and structural properties of biogenic U-Pi minerals and the environmental factors that influence their production and stability. Preliminary AFM studies suggest that U-Pi minerals formed under biomineralization conditions appear morphologically distinct from those formed abiotically between U(VI) and inorganic phosphate. Additionally, we observed that biomineralization tolerates a wide pH range (pH 6-9). Our long-range goal is the development of a conceptual model for the role of microbes in U cycling under oxidizing conditions, and ultimately, provide a scientific basis to support decisions for the remediation of legacy sites worldwide.

Microbe-mediated uranium transformation and mineralization

251st National Meeting (2016)

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