Microbial Populations Stimulated for Hexavalent Uranium Reduction in Uranium Mine Sediment

Abstract

Uranium-contaminated sediment and water collected from an inactive uranium mine were incubated anaerobically with organic substrates. Stimulated microbial populations removed U almost entirely from solution within 1 month. X-ray absorption near-edge structure analysis showed that U(VI) was reduced to U(IV) during the incubation. Observations by transmission electron microscopy, selected area diffraction pattern analysis, and energy-dispersive X-ray spectroscopic analysis showed two distinct types of prokaryotic cells that precipitated only a U(IV) mineral uraninite (UO 2 ) or both uraninite and metal sulfides. Prokaryotic cells associated with uraninite and metal sulfides were inferred to be sulfate-reducing bacteria. Phylogenetic analysis of 16S ribosomal DNA obtained from the original and incubated sediments revealed that microbial populations were changed from microaerophilic Proteobacteria to anaerobic low-G؉C gram-positive sporeforming bacteria by the incubation. Forty-two out of 94 clones from the incubated sediment were related to sulfate-reducing Desulfosporosinus spp., and 23 were related to fermentative Clostridium spp. The results suggest that, if in situ bioremediation were attempted in the uranium mine ponds, Desulfosporosinus spp. would be a major contributor to U(VI) and sulfate reduction and Clostridium spp. to U(VI) reduction.Uranium-bearing wastes from nuclear weapons production (22) and mining (31) have generally been disposed of in nearsurface environments. Dispersion of toxic aqueous uranium species through groundwater is of great environmental concern (30). In situ stimulation of the growth of microorganisms capable of immobilizing dissolved uranium has been proposed as a potentially cost-effective remediation method (23,24).In the laboratory, it has been demonstrated that microorganisms can reduce hexavalent uranium [U(VI)] to tetravalent uranium [U(IV)] and precipitate a U(IV) mineral called uraninite (UO 2 ) (27,40). Microorganisms that reduce U(VI) in pure culture include a hyperthermophilic archaeon (15), a thermophilic bacterium (19), mesophilic Fe(III)-and sulfatereducing bacteria (4,5,34,25,27,28), and fermentative bacteria (9). Thus, the ability to reduce U(VI) occurs in phylogenetically diverse organisms. In laboratory studies, U(VI) is also reduced by microbes in solutions that contain organic or inorganic ligands or other cations (13,26,33) or that contain other electron acceptors such as Fe(III) oxides, sulfate, or selenate (12,24,40,45).Microbial U(VI) reduction in uranium-contaminated settings has been studied by incubating field-collected sediment and water with organic substrates to stimulate the growth of indigenous microorganisms in the laboratory (1, 2, 15). Although previous studies showed uranium removal from solution during laboratory incubation, the mechanisms by which uranium was removed from solution and the microbial species responsible remain unclear.In this study, we attempted to better understand the bioremediation process through integration of results obtaine...

Bacterial community succession during in situ uranium bioremediation: spatial similarities along controlled flow paths

“…Other field studies observed bacterial population shifts during short-term push-pull stimulations, which were predominated by iron-reducing populations (North et al, 2004) or a mix of sulfate-reducing and/or iron-reducing populations (Abdelouas et al, 2000;Vrionis et al, 2005), whereas others observed predominance by sulfate reducing bacteria (SRBs), namely Desulfosporosinus (Nevin et al, 2003), Desulfosporosinus and Clostridium (Suzuki et al, 2003), Desulfobacter species (Vrionis et al, 2005). Over the tested period in our study, the communities became dominated by sulfate-reducing populations closely related to Desulfovibrio species.…”

U(VI) reduction by spores of Clostridium acetobutylicum

“…Environmental microbial community analyses have shown that Gram-positive bacteria are represented in several U(VI)-contaminated sites, suggesting the potential relevance of these strains in the remediation process (Chang et al, 2001;Suzuki et al, 2002, N'Guessan et al, 2008, Dong et al, 2006. Therefore, recent research has focused on the characterization of U(VI) reduction by Clostridium spp.…”

Geomicrobiological factors that control uranium mobility in the environment: Update on recent advances in the bioremediation of uranium-contaminated sites

“…Accordingly, U contaminated settings such as mine sites, mill tailings, and sites where nuclear weapons have been manufactured have been studied directly by field observations or indirectly in laboratory experiments in which sediment and water samples collected from the field have been used to simulate in situ biogeochemical processes. The laboratory amendment of field sediments with organic substrates as electron donors to stimulate anaerobic microbial respiration typically results in significant removal of U from the pore water within a short period (< 1 month) (Abdelouas et al, 1999;Abdelouas et al, 2000;Finneran et al, 2002a;Finneran et al, 2002b;Suzuki et al, 2002;Elias et al, 2003a;Nevin et al, 2003;Suzuki et al, 2003;North et al, 2004;Gu et al, 2005a;Senko et al, 2005b;Tokunaga et al, 2005;Wan et al, 2005). No U(VI) reduction has been observed in the abiological controls, which strongly suggests that U(VI) reduction is biologically mediated.…”

“…Under sulfate rich conditions, sulfate reducing Desulfosporosinus spp. appear to be associated with the stimulated U(VI) reduction (Nevin et al, 2003;Suzuki et al, 2003). In slightly acidic sediments, Anaeromycobacter spp.…”

“…In all examples, U(VI) reduction was accompanied by the complete depletion of O 2 and nitrate (Senko et al, 2005b). Microbial populations associated with the stimulated U(VI) reduction have been characterized in a number of studies using molecular biological techniques (Holmes et al, 2002;Nevin et al, 2003;Petrie et al, 2003;Suzuki et al, 2003;North et al, 2004;Vrionis et al, 2005). U(VI) reduction is correlated with the enrichment of microorganisms related to known Fe(III) and U(VI) reducing Geobacter spp.…”