Multiple mechanisms of uranium immobilization by Cellulomonas sp. strain ES6

Abstract

Removal of hexavalent uranium (U(VI)) from aqueous solution was studied using a Gram-positive facultative anaerobe, Cellulomonas sp. strain ES6, under anaerobic, non-growth conditions in bicarbonate and PIPES buffers. Inorganic phosphate was released by cells during the experiments providing ligands for formation of insoluble U(VI) phosphates. Phosphate release was most probably the result of anaerobic hydrolysis of intracellular polyphosphates accumulated by ES6 during aerobic growth. Microbial reduction of U(VI) to U(IV) was also observed. However, the relative magnitudes of U(VI) removal by abiotic (phosphate-based) precipitation and microbial reduction depended on the buffer chemistry. In bicarbonate buffer, X-ray absorption fine structure (XAFS) spectroscopy showed that U in the solid phase was present primarily as a non-uraninite U(IV) phase, whereas in PIPES buffer, U precipitates consisted primarily of U(VI)-phosphate. In both bicarbonate and PIPES buffer, net release of cellular phosphate was measured to be lower than that observed in U-free controls suggesting simultaneous precipitation of U and PO₄³⁻. In PIPES, U(VI) phosphates formed a significant portion of U precipitates and mass balance estimates of U and P along with XAFS data corroborate this hypothesis. High-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) of samples from PIPES treatments indeed showed both extracellular and intracellular accumulation of U solids with nanometer sized lath structures that contained U and P. In bicarbonate, however, more phosphate was removed than required to stoichiometrically balance the U(VI)/U(IV) fraction determined by XAFS, suggesting that U(IV) precipitated together with phosphate in this system. When anthraquinone-2,6-disulfonate (AQDS), a known electron shuttle, was added to the experimental reactors, the dominant removal mechanism in both buffers was reduction to a non-uraninite U(IV) phase. Uranium immobilization by abiotic precipitation or microbial reduction has been extensively reported; however, the present work suggests that strain ES6 can remove U(VI) from solution simultaneously through precipitation with phosphate ligands and microbial reduction, depending on the environmental conditions. Cellulomonadaceae are environmentally relevant subsurface bacteria and here, for the first time, the presence of multiple U immobilization mechanisms within one organism is reported using Cellulomonas sp. strain ES6.

Uranium speciation and stability after reductive immobilization in aquifer sediments

“…While lactate biostimulation led to a rapid shift in the augmented microbial community from seeded Shewanella sp. to indigenous Firmicutes, this and other recent reports (Bernier-Latmani et al, 2010;Fletcher et al, 2010;Lee et al, 2010;Sivaswamy et al, 2011;Veeramani et al, 2011) suggest that the geochemical environment may play a greater role in influencing U(IV) products than does biological variability. It remains unclear whether this is due to purely chemical effects such as complexation and solute competition or rather to geochemical influences on microbial processes such as the production of exudates.…”

“…and even S. oneidensis sp. incubated in the presence of higher ionic strength media (Bernier-Latmani et al, 2010;Fletcher et al, 2010;Ray et al, 2011;Sivaswamy et al, 2011;Veeramani et al, 2011). Ningyoite, a Ca-U(IV)-phosphate precipitate, has also been observed during U reduction in pure cultures (Bernier-Latmani et al, 2010;Lee et al, 2010).…”

“…This last study identified U(VI)-phosphates, U(VI)-organic complexes, and U(VI) sorbed onto minerals, in addition to reduced U(IV), but did not resolve more detailed properties of the U(IV) phases. There is also increasing evidence that U(IV) species other than uraninite can form as a result of enzymatic (Bernier-Latmani et al, 2010;Ray et al, 2011;Sivaswamy et al, 2011) and abiotic U(VI) reduction , even in the absence of sediments.…”

Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

“…[6,8,9] Under anoxic conditions, U can be present as molecular U IV , nano-uraninite, or the most stable and least soluble of the reduced forms, crystalline uraninite (UO 2 ). [10][11][12][13] Although U is less mobile in its reduced forms than in its oxidised forms, As is generally more mobile in its reduced form as As III . [14][15][16] Arsenic can adsorb to iron oxides under environmental pH as both As III (H 2 AsO 3 0 ) and As V (H 2 AsO 4 -).…”

“…dissolved phosphate. [1,12,40,41] Arsenic mobility at a former U mill site has also been investigated independently of U mobility. For example, Stucker et al discovered that As was released to Rifle, CO, groundwater during bioremediation of U as a result of, in part, the formation of thioarsenic species.…”