The product of microbial uranium reduction includes multiple species with U(IV)–phosphate coordination

Abstract

Until recently, the reduction of U(VI) to U(IV) during bioremediation was assumed to produce solely the sparingly soluble mineral uraninite, UO 2(s) . However, results from several laboratories reveal other species of U(IV) characterized by the absence of an EXAFS U-U pair correlation (referred to here as noncrystalline U(IV)). Because it lacks the crystalline structure of uraninite, this species is likely to be more labile and susceptible to reoxidation. In the case of single species cultures, analyses of U extended X-ray fine structure (EXAFS) spectra have previously suggested U(IV) coordination to carboxyl, phosphoryl or carbonate groups. In spite of this evidence, little is understood about the species that make up noncrystalline U(IV), their structural chemistry and the nature of the U(IV)-ligand interactions. Here, we use infrared spectroscopy (IR), uranium L III -edge X-ray absorption spectroscopy (XAS), and phosphorus K-edge XAS analyses to constrain the binding environments of phosphate and uranium associated with Shewanella oneidensis MR-1 bacterial cells. Systems tested as a function of pH included: cells under metal-reducing conditions without uranium, cells under reducing conditions that produced primarily uraninite, and cells under reducing conditions that produced primarily biomass-associated noncrystalline U(IV). P X-ray absorption near-edge structure (XANES) results provided clear and direct evidence of U(IV) coordination to phosphate. Infrared (IR) spectroscopy revealed a pronounced perturbation of phosphate functional groups in the presence of uranium. Analysis of these data provides evidence that U(IV) is coordinated to a range of phosphate species, including monomers and polymerized networks. U EXAFS analyses and a chemical extraction measurements support these conclusions. The results of this study provide new insights into the binding mechanisms of biomass-associated U(IV) species which in turn sheds light on the mechanisms of biological U(VI) reduction.

Cell surface acid-base properties of the cyanobacterium Synechococcus : Influences of nitrogen source, growth phase and N:P ratios

“…Irrespective of the incubation time and the contents of the growth media, the FT-IR spectra collected for Synechococccus cells were relatively similar to each other and consistent with those obtained by other studies examining bacterial cells and proteins (after Zeroual et al, 1994;Kansiz et al, 1999;Benning et al, 2004;Jiang et al, 2004;Omoike and Chorover, 2004;Yee et al, 2004;Dittrich and Sibler, 2005;Kong and Yu, 2007;Leone et al, 2007;Meade et al, 2007;Movasaghi et al, 2008;Castro et al, 2010;Quilès et al, 2010;Alessi et al, 2014). This either means that there is a change in the type of phospholipids produced under these growth conditions, or the very sharp nature of this peak may indicate that the material may be more crystalline, and therefore, may be a relic of the sample preparation and relate to some residual media in the sample which crystallized upon drying.…”

“…The assignments of principal vibrational bands (compiled after Zeroual et al, 1994;Kansiz et al, 1999;Benning et al, 2004;Jiang et al, 2004;Omoike and Chorover, 2004;Yee et al, 2004;Dittrich and Sibler, 2005;Kong and Yu, 2007;Leone et al, 2007;Meade et al, 2007;Movasaghi et al, 2008;Castro et al, 2010;Quilès et al, 2010;Alessi et al, 2014 …”

Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments

“…As shown in Figure 4a, 65 ± 5 % of the total uranium content was extracted from the three samples by this method, and could be assigned to mononuclear U(IV). Indeed, non-crystalline U(IV) species may include mononuclear and polymerised U(IV)-complexes, the latter being less extractable than the former (Alessi et al, 2014b). U(IV)-bearing minerals identified by SEM-EDXS, including ningyoite, rhabdophane, thorite and zircon, thus likely accounted at most for 35 ± 5 % of the total uranium in the sediment samples.…”

“…4) consisted of nano-crystalline or amorphous U(IV) phases. For instance, biogenic nano-crystalline U(IV) is not easily extracted by 1M NaHCO 3 (Alessi et al, 2014b). We thus infer that a fraction of the primary U(IV)-bearing uranium minerals identified by SEM-EDXS, as zircon and thorite are likely metamict because of their high U and Th contents (Table S- 2), which would make such phases difficult to detect by EXAFS as minor phases in a mixture with mononuclear U(IV) complexes.…”

“…Researchers addressing remediation of U-contaminated groundwaters have focused on in situ biostimulation strategies involving microbial reduction of U(VI) (Wu et al, 2007;Yabusaki et al, 2007) into biogenic uraninite (Suzuki et al, 2005;Bargar et al, 2008) as well as non-uraninite U(IV) phases (Kelly et al, 2008;Bargar et al, 2013;Alessi et al, 2014a;Newsome et al, 2014). Ex situ incubations of aquifer sediments under anoxic conditions have highlighted the importance of non-crystalline U(IV) species as major products of microbial reduction of U(VI) (Sharp et al, 2011;Alessi et al, 2014b). In laboratory bioassays, mononuclear U(IV)-phosphate complexes, in which a U(IV) ion coordinates to a PO 4 group, have been especially observed as products of microbial U(VI) reduction (Bernier-Latmani et al, 2010;Fletcher et al, 2010;Sivaswamy et al, 2011).…”