UO 22+ speciation determines uranium toxicity and bioaccumulation in an environmental Pseudomonas sp. isolate

Abstract

In the present study, experiments were performed to investigate how representative cellulosic breakdown products, when serving as growth substrates under aerobic conditions, affect hexavalent uranyl cation (UO(2) (2+)) toxicity and bioaccumulation within a Pseudomonas sp. isolate (designated isolate A). Isolate A taken from the Cold Test Pit South (CTPS) region of the Idaho National Laboratory (INL), Idaho Falls, ID, USA. The INL houses low-level uranium-contaminated cellulosic material and understanding how this material, and specifically its breakdown products, affect U-bacterial interactions is important for understanding UO(2) (2+) fate and mobility. Toxicity was modeled using a generalized Monod expression. Butyrate, dextrose, ethanol, and lactate served as growth substrates. The potential contribution of bicarbonate species present in high concentrations was also investigated and compared with toxicity and bioaccumulation patterns seen in low-bicarbonate conditions. Isolate A was significantly more sensitive to UO(2) (2+) and accumulated significantly more UO(2) (2+) in low-bicarbonate concentrations. In addition, UO(2) (2+) growth inhibition and bioaccumulation varied depending on the growth substrate. In the presence of high bicarbonate concentrations, sensitivity to UO(2) (2+) inhibition was greatly mitigated, and did not vary between the four substrates tested. The extent of UO(2) (2+) accumulation was also diminished. The observed patterns were related to UO(2) (2+) aqueous complexation, as predicted by MINTEQ (ver. 2.52) (Easton, PA, USA). In the low- bicarbonate medium, the presence of positively charged and unstable UO(2) (2+)-hydroxide complexes explained both the greater sensitivity of isolate A to UO(2) (2+), and the ability of isolate A to accumulate significant amounts of UO(2) (2+). The exclusive presence of negatively charged and stable UO(2) (2+)-carbonate complexes in the high bi-carbonate medium explained the diminished sensitivity of isolate A to UO(2) (2+) toxicity, and limited ability of isolate A to accumulate UO(2) (2+).

Interaction of Uranium with Bacterial Cell Surfaces: Inferences from Phosphatase-Mediated Uranium Precipitation

“…Binding of U to bacterial cell surfaces is evidently the first step toward obtaining cellular access, which subsequently causes the disruption of metabolic processes, eventually leading to lethality (9,17). The predominantly negatively charged aqueous complexes of U formed under GC2 are repelled by the bacterial cell surface, making it difficult for U to acquire sufficient proximity to cells to cause significant damage.…”

“…The reported values are the means of the results from three replicates from a representative experiment, and the error bars correspond to 95% confidence intervals. Aqueous U speciation under GC1 or GC2 was determined using Visual MINTEQ version 3.1 (http: //vminteq.lwr.kth.se/download/) (9,17).…”

“…It is important to understand how these different U species interact with bacterial cellular surfaces, especially for designing biological wastewater treatment systems. However, studies evaluating the effect of aqueous U speciation have been largely limited to biosorption, bioaccumulation, and bioreduction (9,15,16,20).…”

“…It is important to understand how these different U species interact with bacterial cellular surfaces, especially for designing biological wastewater treatment systems. However, studies evaluating the effect of aqueous U speciation have been largely limited to biosorption, bioaccumulation, and bioreduction (9,15,16,20).Among the biological mechanisms involved in metal remediation, enzymatic bioprecipitation of heavy metals as metal phosphates is particularly attractive and is considered to be a promising approach for biological treatment of U effluents due to its high efficiency (14, 21). Bioprecipitation of metals as phosphates is mediated by phosphatases that cleave a phosphomonoester substrate (such as ␤-glycerophosphate) to release the phosphate moiety, which in turn precipitates heavy metals, such as U, Cd, Ni, Am, etc., from solutions (22, 23).…”

“…ioremediation strategies, such as bioreduction (1-3), biosorption (4)(5)(6)(7)(8), bioaccumulation (9,10), and bioprecipitation (5,11,12,13), have been studied for their potential to immobilize U from solutions. There is also a large body of work on microbial interactions with uranium relevant to environmental in situ bioremediation.…”