EXAFS investigation of uranium(VI) complexes formed at Bacillus cereus and Bacillus sphaericus surfaces

Exploring uranium bioaccumulation in the brown alga Ascophyllum nodosum: insights from multi-scale spectroscopy and imaging

Legacy radioactive waste can be defined as the radioactive waste produced during the infancy of the civil nuclear industry's development in the mid-20th Century, a time when, unfortunately, waste storage and treatment were not well planned. The marine environment is one of the environmental compartments worth studying in this regard because of legacy waste in specific locations of the seabed. Comprising nearly 70% of the earth's service, the oceans are the largest and indeed the final destination for contaminated fresh waters. For this reason, long-term studies of the accumulation biochemical mechanisms of metallic radionuclides in the marine ecosystem are required. In this context the brown algal compartment may be ecologically relevant because of forming large and dense algal beds in coastal areas and potential important biomass for contamination. This report presents the first step in the investigation of uranium (U, an element used in the nuclear cycle) bioaccumulation in the brown alga Ascophyllum nodosum using a multi-scale spectroscopic and imaging approach. Contamination of A. nodosum specimens in closed aquaria at 13 °C was performed with a defined quantity of U(VI) (10-5 M). The living algal uptake was quantified by ICP-MS and a localization study in the various algal compartments was carried out by combining electronic microscopy imaging (SEM), X-ray Absorption spectroscopy (XAS) and micro X-ray Florescence (μ-XRF). Data indicate that the brown alga is able to concentrate U(VI) by an active bioaccumulation mechanism, reaching an equilibrium state after 200 h of daily contamination. A comparison between living organisms and dry biomass confirms a stress-response process in the former, with an average bioaccumulation factor (BAF) of 10 ± 2 for living specimens (90% lower compared to dry biomass, 142 ± 5). Also, these results open new perspectives for a potential use of A. nodosum dry biomass as uranium biosorbent. The different partial BAFs (bioaccumulation factors) range from 3 (for thallus) to 49 (for receptacles) leading to a compartmentalization of uranium within the seaweed. This reveals a higher accumulation capacity in the receptacles, the algal reproductive parts. SEM images highlight the different tissue distributions among the compartments with a superficial absorption in the thallus and lateral branches and several hotspots in the oospheres of the female individuals. A preliminary speciation XAS analysis identified a distinct U speciation in the gametes-containing receptacles as a pseudo-autunite phosphate phase. Similarly, XAS measurements on the lateral branches (XANES) were not conclusive with regards to the occurrence of an alginate-U complex in these tissues. Nonetheless, the hypothesis that alginate may play a role in the speciation of U in the algal thallus tissues is still under consideration.

First Isolation and Characterization of Bacteria from the Core's Cooling Pool of an Operating Nuclear Reactor

Microbial life can thrive in the most inhospitable places, such as nuclear facilities with high levels of ionizing radiation. Using direct meta-analyses, we have previously highlighted the presence of bacteria belonging to twenty-five different genera in the highly radioactive water of the cooling pool of an operating nuclear reactor core. In the present study, we further characterize this specific environment by isolating and identifying some of these microorganisms and assessing their radiotolerance and their ability to decontaminate uranium. This metal is one of the major radioactive contaminants of anthropogenic origin in the environment due to the nuclear and mining industries and agricultural practices. The microorganisms isolated when sampling was performed during the reactor operation consisted mainly of Actinobacteria and Firmicutes, whereas Proteobacteria were dominant when sampling was performed during the reactor shutdown. We investigated their tolerance to gamma radiation under different conditions. Most of the bacterial strains studied were able to survive 200 Gy irradiation. Some were even able to withstand 1 kGy, with four of them showing more than 10% survival at this dose. We also assessed their uranium uptake capacity. Seven strains were able to remove almost all the uranium from a 5 µM solution. Four strains displayed high efficiency in decontaminating a 50 µM uranium solution, demonstrating promising potential for use in bioremediation processes in environments contaminated by radionuclides.

Uranium adsorption – a review of progress from qualitative understanding to advanced model development

Surface adsorption has a major influence on the environmental mobility of radionuclides, including uranium. Six decades ago, the description of the sorption process relied predominantly on simple descriptive parameters of solid–liquid partitioning (such as Kd values). There have since been numerous systematic investigations of the processes controlling U adsorption, including the affinity of U for different types of geologic materials, the influence of factors such as pH, the effects of complexing ligands, and the role of microorganisms. Mathematical descriptions of sorption processes have adopted various models – including sorption isotherms, surface complexation models and other types of modelling approaches, aided by advances in computational and analytical techniques. In recent years, mechanistic models have incorporated structural insights gained from spectroscopic techniques (such as EXAFS and TRLFS). Throughout the period, the nuclear waste community has sought to develop models for U sorption in complex systems associated with radioactive waste disposal, involving a range of mineral surfaces and incorporating numerous interactions and processes. To some extent, the ongoing questions concerning U adsorption can be considered as being common to many environmental metal contaminants. However, uranium is a unique and significant case, particularly for the radiochemical community, where the long-term behaviour of actinides is a central issue.

Cadmium-Bacteria Complexation and Subsequent Bacteria-Facilitated Cadmium Transport in Saturated Porous Media

Cadmium (Cd) contamination is becoming a significant environmental concern due to its persistency in soil. In the subsurface, the fate and transport of Cd are significantly affected by the presence of organic carriers, including bacteria, which are ubiquitous. In this research, facilitated Cd transport by four different bacterial strains in zeolite was investigated by column experiments under three different Cd introduction scenarios (i.e., a Cd and bacteria mixture, Cd and bacteria introduced separately but simultaneously, and Cd added to the column, followed by bacterial flushing), through which Cd-bacteria complexes formed. In turn, Cd-bacteria complexation affected bacterial transport. Bacteria were least retarded when Cd was pre-deposited, with mass recovery>90% for all strains. More Cd was recovered when introduced as a mixture with bacteria (i.e., Cd mass recovery ranging from 16 to 25%). Obtained bacteria and Cd breakthrough curves were simulated by the attachment-detachment model in Hydrus-1D. Damkohler number and reversibility were both found to be suitable to control the mass recovery of Cd and bacteria in all investigated scenarios.

Metaschoepite Dissolution in Sediment Column Systems-Implications for Uranium Speciation and Transport

Metaschoepite is commonly found in U-contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO₃·nH₂O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield Ltd. site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO₂²⁺ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially mediated sulfate reduction, significant amounts of UO₂-like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), noncrystalline U(IV), and biogenic UO₂, with UO₂ becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

Metabolism-dependent bioaccumulation of uranium by Rhodosporidium toruloides isolated from the flooding water of a former uranium mine

Remediation of former uranium mining sites represents one of the biggest challenges worldwide that have to be solved in this century. During the last years, the search of alternative strategies involving environmentally sustainable treatments has started. Bioremediation, the use of microorganisms to clean up polluted sites in the environment, is considered one the best alternative. By means of culture-dependent methods, we isolated an indigenous yeast strain, KS5 (Rhodosporidium toruloides), directly from the flooding water of a former uranium mining site and investigated its interactions with uranium. Our results highlight distinct adaptive mechanisms towards high uranium concentrations on the one hand, and complex interaction mechanisms on the other. The cells of the strain KS5 exhibit high a uranium tolerance, being able to grow at 6 mM, and also a high ability to accumulate this radionuclide (350 mg uranium/g dry biomass, 48 h). The removal of uranium by KS5 displays a temperature- and cell viability-dependent process, indicating that metabolic activity could be involved. By STEM (scanning transmission electron microscopy) investigations, we observed that uranium was removed by two mechanisms, active bioaccumulation and inactive biosorption. This study highlights the potential of KS5 as a representative of indigenous species within the flooding water of a former uranium mine, which may play a key role in bioremediation of uranium contaminated sites.

Extractable Bacterial Surface Proteins in Probiotic-Host Interaction

Some Gram-positive bacteria, including probiotic ones, are covered with an external proteinaceous layer called a surface-layer. Described as a paracrystalline layer and formed by the self-assembly of a surface-layer-protein (Slp), this optional structure is peculiar. The surface layer per se is conserved and encountered in many prokaryotes. However, the sequence of the corresponding Slp protein is highly variable among bacterial species, or even among strains of the same species. Other proteins, including surface layer associated proteins (SLAPs), and other non-covalently surface-bound proteins may also be extracted with this surface structure. They can be involved a various functions. In probiotic Gram-positives, they were shown by different authors and experimental approaches to play a role in key interactions with the host. Depending on the species, and sometime on the strain, they can be involved in stress tolerance, in survival within the host digestive tract, in adhesion to host cells or mucus, or in the modulation of intestinal inflammation. Future trends include the valorization of their properties in the formation of nanoparticles, coating and encapsulation, and in the development of new vaccines.

Unusual Versatility of the Filamentous, Diazotrophic Cyanobacterium Anabaena torulosa Revealed for Its Survival during Prolonged Uranium Exposure

Reports on interactions between cyanobacteria and uranyl carbonate are rare. Here, we present an interesting succession of the metabolic responses employed by a marine, filamentous, diazotrophic cyanobacterium, Anabaena torulosa for its survival following prolonged exposure to uranyl carbonate extending up to 384 h at pH 7.8 under phosphate-limited conditions. The cells sequestered uranium (U) within polyphosphates on initial exposure to 100 μM uranyl carbonate for 24 to 28 h. Further incubation until 120 h resulted in (i) significant degradation of cellular polyphosphates causing extensive chlorosis and cell lysis, (ii) akinete differentiation followed by (iii) extracellular uranyl precipitation. X-ray diffraction (XRD) analysis, fluorescence spectroscopy, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy established the identity of the bioprecipitated uranium as a U(VI) autunite-type mineral, which settled at the bottom of the vessel. Surprisingly, A. torulosa cells resurfaced as small green flakes typical of actively growing colonies on top of the test solutions within 192 to 240 h of U exposure. A consolidated investigation using kinetics, microscopy, and physiological and biochemical analyses suggested a role of inducible alkaline phosphatase activity of cell aggregates/akinetes in facilitating the germination of akinetes leading to substantial regeneration of A. torulosa by 384 h of uranyl incubation. The biomineralized uranium appeared to be stable following cell regeneration. Altogether, our results reveal novel insights into the survival mechanism adopted by A. torulosa to resist sustained uranium toxicity under phosphate-limited oxic conditions.IMPORTANCE Long-term effects of uranyl exposure in cyanobacteria under oxic phosphate-limited conditions have been inadequately explored. We conducted a comprehensive examination of the metabolic responses displayed by a marine cyanobacterium, Anabaena torulosa, to cope with prolonged exposure to uranyl carbonate at pH 7.8 under phosphate limitation. Our results highlight distinct adaptive mechanisms harbored by this cyanobacterium that enabled its natural regeneration following extensive cell lysis and uranium biomineralization under sustained uranium exposure. Such complex interactions between environmental microbes such as Anabaena torulosa and uranium over a broader time range advance our understanding on the impact of microbial processes on uranium biogeochemistry.

Uranium Binding Mechanisms of the Acid-Tolerant Fungus Coniochaeta fodinicola

The uptake and binding of uranium [as (UO2)(2+)] by a moderately acidophilic fungus, Coniochaeta fodinicola, recently isolated from a uranium mine site, is examined in this work in order to better understand the potential impact of organisms such as this on uranium sequestration in hydrometallurgical systems. Our results show that the viability of the fungal biomass is critical to their capacity to remove uranium from solution. Indeed, live biomass (viable cells based on vital staining) were capable of removing ∼16 mg U/g dry weight in contrast with dead biomass (autoclaved) which removed ∼45 mg U/g dry weight after 2 h. Furthermore, the uranium binds with different strength, with a fraction ranging from ∼20-50% being easily leached from the exposed biomass by a 10 min acid wash. Results from X-ray absorption spectroscopy measurements show that the strength of uranium binding is strongly influenced by cell viability, with live cells showing a more well-ordered uranium bonding environment, while the distance to carbon or phosphorus second neighbors is similar in all samples. When coupled with time-resolved laser fluorescence and Fourier transformed infrared measurements, the importance of organic acids, phosphates, and polysaccharides, likely released with fungal cell death, appear to be the primary determinants of uranium binding in this system. These results provide an important progression to our understanding with regard to uranium sequestration in hydrometallurgical applications with implications to the unwanted retention of uranium in biofilms and/or its mobility in a remediation context.

Decrease of U(VI) immobilization capability of the facultative anaerobic strain Paenibacillus sp. JG-TB8 under anoxic conditions due to strongly reduced phosphatase activity

Interactions of a facultative anaerobic bacterial isolate named Paenibacillus sp. JG-TB8 with U(VI) were studied under oxic and anoxic conditions in order to assess the influence of the oxygen-dependent cell metabolism on microbial uranium mobilization and immobilization. We demonstrated that aerobically and anaerobically grown cells of Paenibacillus sp. JG-TB8 accumulate uranium from aqueous solutions under acidic conditions (pH 2 to 6), under oxic and anoxic conditions. A combination of spectroscopic and microscopic methods revealed that the speciation of U(VI) associated with the cells of the strain depend on the pH as well as on the aeration conditions. At pH 2 and pH 3, uranium was exclusively bound by organic phosphate groups provided by cellular components, independently on the aeration conditions. At higher pH values, a part (pH 4.5) or the total amount (pH 6) of the dissolved uranium was precipitated under oxic conditions in a meta-autunite-like uranyl phosphate mineral phase without supplying an additional organic phosphate substrate. In contrast to that, under anoxic conditions no mineral formation was observed at pH 4.5 and pH 6, which was clearly assigned to decreased orthophosphate release by the cells. This in turn was caused by a suppression of the indigenous phosphatase activity of the strain. The results demonstrate that changes in the metabolism of facultative anaerobic microorganisms caused by the presence or absence of oxygen can decisively influence U(VI) biomineralization.

Uranium interaction with two multi-resistant environmental bacteria: Cupriavidus metallidurans CH34 and Rhodopseudomonas palustris

Depending on speciation, U environmental contamination may be spread through the environment or inversely restrained to a limited area. Induction of U precipitation via biogenic or non-biogenic processes would reduce the dissemination of U contamination. To this aim U oxidation/reduction processes triggered by bacteria are presently intensively studied. Using X-ray absorption analysis, we describe in the present article the ability of Cupriavidus metallidurans CH34 and Rhodopseudomonas palustris, highly resistant to a variety of metals and metalloids or to organic pollutants, to withstand high concentrations of U and to immobilize it either through biosorption or through reduction to non-uraninite U(IV)-phosphate or U(IV)-carboxylate compounds. These bacterial strains are thus good candidates for U bioremediation strategies, particularly in the context of multi-pollutant or mixed-waste contaminations.

Coordination of uranium(VI) with functional groups of bacterial lipopolysaccharide studied by EXAFS and FT-IR spectroscopy

The complexation of uranyl ions with lipopolysaccharide (LPS), the main component of the cell wall of Gram-negative bacteria, was investigated on a molecular level with U L(III)-edge extended X-ray absorption fine structure (EXAFS) and attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy over a wide pH range (2.6 to 7.0). For the first time, structural determinations of uranyl complexes with cell wall compounds were extended from acidic up to neutral pH. The main functionalities responsible for uranyl binding are phosphoryl and carboxyl groups. At an excess of LPS, related to environmental conditions, the uranyl ion is mainly complexed by phosphoryl groups four-fold monodentately coordinated in the equatorial plane of the uranyl dioxo cation UO(2)(2+) showing great homologies to the uranyl mineral phase meta-autunite in the EXAFS spectra. At equimolar ratios of uranyl and functional groups of LPS, according to a slight deficit of phosphoryl groups, additional carboxyl coordination in a bidentate manner becomes important as it is shown by IR spectroscopy. From the vibrational spectra, a mixed coordination of UO(2)(2+) with both phosphoryl and carboxyl groups is derived. The coordination of uranyl ions to the LPS molecule is obviously mainly controlled by the U/LPS concentration ratio, and the influence of pH is only of minor significance at the investigated range.

Bioaccumulation of U(VI) by Sulfolobus acidocaldarius under moderate acidic conditions

U(VI) accumulation by the acidothermophilic archaeon Sulfolobus acidocaldarius at a moderate acidic pH of 4.5 was investigated. This pH value is relevant for some heavy metal and uranium polluted environments where populations of S. acidocaldarius were found to persist. We demonstrate that U(VI) is rapidly complexed by the archaeal cells. A combination of X-ray absorption spectroscopy and time-resolved laser-induced fluorescence spectroscopy revealed that at pH 4.5 organic phosphate and carboxylic groups are involved in the U(VI) complexation. These results are in contrast to those published for most bacteria which at this pH precipitate U(VI) mainly in inorganic uranyl phosphate phases. As demonstrated by TEM only a limited part of the added U(VI) was biomineralized extracellularly in the case of the studied archaeon. Most of the U(VI) accumulates were localized in a form of intracellular deposits which were associated with the inner side of the cytoplasma membrane. Observed differences in U(VI) bioaccumulation between the studied archaeon and bacteria can be explained by the significant differences in their cell wall structures as well as by their different physiological characteristics.

Surface complexation of Neptunium(V) onto whole cells and cell components of Shewanella alga: modeling and experimental study

We systematically quantified surface complexation of Np(V) onto whole cells, cell wall, and extracellular polymeric substances (EPS) of Shewanella alga strain BrY. We first performed acid and base titrations and used the mathematical model FITEQL to estimate the concentrations and deprotonation constants of specific surface functional groups. Deprotonation constants most likely corresponded to a carboxyl group not associated with amino acids (pK(a) approximately 5), a phosphoryl site (pK(a) approximately 7.2), and an amine site (pK(a) > 10). We then carried out batch sorption experiments with Np(V) and each of the S. alga components as a function of pH. Since significant Np(V) sorption was observed on S. alga whole cells and its components in the pH range 2-5, we assumed the existence of a fourth site: a low-pK(a) carboxyl site (pK(a) approximately 2.4) that is associated with amino acids. We used the SPECIATE submodel of the biogeochemical model CCBATCH to compute the stability constants for Np(V) complexation to each surface functional group. The stability constants were similar for each functional group on S. alga bacterial whole cells, cell walls, and EPS, and they explain the complicated sorption patterns when they are combined with the aqueous-phase speciation of Np(V). For pH < 8, the aquo NpO(2)(+) species was the dominant form of Np(V), and its log K values for the low-pK(a) carboxyl, mid-pK(a) carboxyl, and phosphoryl groups were 1.8, 1.8, and 2.5-3.1, respectively. For pH greater than 8, the key surface ligand was amine >XNH(3)(+), which complexed with NpO(2)(CO(3))(3)(5-). The log K for NpO(2)(CO(3))(3)(5-) complexed onto the amine groups was 3.1-3.9. All of the log K values are similar to those of Np(V) complexes with aqueous carboxyl and N-containing carboxyl ligands. These results help quantify the role of surface complexation in defining actinide-microbiological interactions in the subsurface.

Interactions of Sulfolobus acidocaldarius with uranium

Interactions of the acidothermophilic archaeon Sulfolobus acidocaldarius DSM 639 with U(VI) were studied by using a combination of batch experiments, X-ray absorption spectroscopy (XAS), and time-resolved laser-induced fluorescence spectroscopy (TRLFS). We demonstrated that at pH 2 this archaeal strain possesses a low tolerance to U(VI) and that its growth is limited to a uranium concentration below 1.1 mM. At similarly high acidic conditions (pH 1.5 and 3.0), covering the physiological pH growth optimum of S. acidocaldarius, at which U(VI) is soluble and highly toxic, rapid accumulation of the radionuclide by the cells of the strain occurred. About half of the uranium binding capacity was reached by the strain after an incubation of five minutes and nearly total saturation of the binding sites was achieved after 30 min. Both, EXAFS- and TRLF-spectroscopic analyses showed that the accumulated U(VI) was complexed mainly through organic phosphate groups. The EXAFS measurements revealed that U(VI) is coordinated to the organic phosphate ligands of the archaeal cells in a monodentate binding mode with an average U–P bond distance of 3.60±0.02 Å.

Uranium(VI) complexes with sugar phosphates in aqueous solution

The complex formation in the aqueous systems of uranium(VI) with glucose 6-phosphate (G6P) and fructose 6-phosphate (F6P) were studied using potentiometric titration, TRLFS, and EXAFS. Two complexes with a uranyl-to-ligand ratio of 1:1 and 1:2 for both sugars were observed. Complex stability constants were determined by potentiometric titration for both complexes to be log β11(G6P)=5.89±0.40, log β12(G6P)=9.45±0.08, log β11(F6P)=5.72±0.21, and log β12(F6P)=9.54±0.09. Stability constants could be determined with TRLFS under the specific experimental conditions only for the 1:1 complexes: log β11(G6P)=6.35±0.28, and log β11(F6P)=5.66±0.17.

The TRLFS measurements results show the UO2G6P complex to exhibit no fluorescence properties. For this system, only a decrease of the fluorescence intensity with increasing ligand concentration was observed. For the UO2F6P system, a red shift of the fluorescence emission bands of about 8 to 9 nm compared to the free uranyl ion was observed. The fluorescence emission wavelengths of the UO2F6P complex were determined to be 483, 496, 518, 542, and 567 nm, and the lifetime of this complex is 0.13±0.05 μs.

Uranium L III-edge EXAFS measurements at pH 3.5, 4.0, and 5.5 yielded a shortened U–Oeq bond distance (2.30±0.02 to 2.37±0.02 Å), compared to the UO2 2+(H2O)5 ion (2.40±0.02 Å), due to a monodentate coordination via the oxygen atoms of the phosphate group.

Uranium association with halophilic and non-halophilic bacteria and archaea

We determined the association of uranium with bacteria isolated from the Waste Isolation Pilot Plant (WIPP), Carlsbad, New Mexico, and compared this with known strains of halophilic and non-halophilic bacteria and archaea. Examination of the cultures by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) showed uranium accumulation extracellularly and/or intracellularly to a varying degree. In Pseudomonas fluorescens and Bacillus subtilis uranium was associated with the cell surface and in the latter it was present as irregularly shaped grains. In Halobacterium halobium, the only archeon studied here, uranium was present as dense deposits and with Haloanaerobium praevalens as spikey deposits. Halomonas sp. isolated from the WIPP site accumulated uranium both extracellularly on the cell surface and intracellularly as electron-dense discrete granules. Extended X-ray absorption fine structure (EXAFS) analysis of uranium with the halophilic and non-halophilic bacteria and archaea showed that the uranium present in whole cells was bonded to an average of 2.4±0.7 phosphoryl groups at a distance of 3.65±0.03 Å. Comparison of whole cells of Halomonas sp. with the cell wall fragments of lysed cells showed the presence of a uranium bidentate complex at 2.91±0.03 Å with the carboxylate group on the cell wall, and uranyl hydroxide with U-U interaction at 3.71±0.03 Å due to adsorption or precipitation reactions; no U-P interaction was observed. Addition of uranium to the cell lysate of Halomonas sp. resulted in the precipitation of uranium due to the inorganic phosphate produced by the cells. These results show that the phosphates released from bacteria bind a significant amount of uranium. However, the bacterially immobilized uranium was readily solubilized by bicarbonate with concurrent release of phosphate into solution.

Characterization of U(VI)-Acidithiobacillus ferrooxidans complexes using EXAFS, transmission electron microscopy, and energy-dispersive X-ray analysis

We used a combination of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Transmission Electron Microscopy (TEM) and Energy-Dispersive X-ray (EDX) analysis to conduct molecular scale studies on U(VI) interaction with three recently described eco-types of Acidithiobacillus ferrooxidans. On the basis of the information obtained by using these methods, we concluded that uranyl phosphate complexes were formed by the cells of the three eco-types studied. The uranium accumulated by A. ferrooxidans cells was located mainly within the extracellular polysaccharides, and on the cell wall. Smaller amounts were also observed in the cytoplasm.

Actinide uptake by transferrin and ferritin metalloproteins

In order to better understand the mechanisms of actinide uptake by specific biomolecules, it is essential to explore the intramolecular interactions between the cation and the protein binding site. Although this has long been done for widely investigated transition metals, very few studies have been devoted to complexation mechanisms of actinides by active chelation sites of metalloproteins. In this field, X-ray absorption spectroscopy has been extensively used as a structural and electronic metal cation probe. The two examples that are presented here are related to two metalloproteins in charge of iron transport and storage in eukaryote cells: transferrin and ferritin. U(VI)O2 2+, Np(IV) and Pu(IV) have been selected because of their possible role as contaminant from the geosphere.

Complex formation between uranium(VI) and α-D-glucose 1-phosphate

The complex formation of uranium(VI) with α-D-glucose 1-phosphate (C6H11O6PO3 2-, G1P) was determined by time-resolved laser-induced fluorescence spectroscopy (TRLFS) at pH 4 and potentiometric titration in the pH range from 3 to 10. Both measurements show the formation of a 1:1 complex at lower pH values. The formation constant of UO2(C6H11O6PO3) was calculated from TRLFS measurements to be log β 11=5.72±0.12, and from potentiometric titration log β 11=5.40±0.25, respectively.It was found by potentiometric titration that at higher pH values the complexation changes to a 1:2 complex. The stability constant for this complex was calculated to be log β 12 = 8.96±0.18.

The interaction ofDesulfovibrio äspöensisDSM 10631Twith plutonium

Microbes are widely distributed in nature and they can strongly influence the migration of actinides in the environment. This investigation describes the interaction of plutonium in mixed oxidation states (Pu(VI) and Pu(IV)-polymers) with cells of the sulfate-reducing bacterial (SRB) strainDesulfovibrio äspöensisDSM 10631T, which frequently occurs in the deep granitic rock aquifers at the Äspö Hard Rock Laboratory (Äspö HRL), Sweden. In this study, accumulation experiments were performed in order to obtain information about the amount of Pu bound by the bacteria in dependence on the contact time and the initial plutonium concentration. We used solvent extractions, UV-Vis absorption spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy to determine the speciation of Pu oxidation states. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to study the coordination of the Pu bound by the bacteria. In the first step, the Pu(VI) and Pu(IV)-polymers are bound to the biomass. Solvent extractions showed that 97% of the initially present Pu(VI) is reduced to Pu(V) due to the activity of the cells within the first 24 h of contact time. Most of the formed Pu(V) dissolves from the cell envelope back to the aqueous solution due to the weak complexing properties of this plutonium oxidation state. Indications were found for a penetration of Pu species inside the bacterial cells.

Uranium(VI) complexation in cell culture medium: influence of speciation on Normal Rat Kidney (NRK-52E) cell accumulation

Uranium bioavailability and toxicity are closely linked to the metal's speciation in solution. However in biological fluids or in media classically used for cell culture – and subsequently for in vitro cell exposure –, uranium is rarely present as free-ion since these media contain non-negligible concentrations of potential ligands such as phosphate and bicarbonate but also co-ions such as calcium which can cause U(VI) complexes precipitation. The chemical form of uranium that is internalized in cells and interferes with biological processes is of major concern. Uranium toxicity and accumulation were evaluated in vitro on NRK-52E cells, model for rat renal proximal tubule. Uranium intracellular accumulation begins after 12 h exposure to 600 μM U(VI); toxicity appears as soon as cells accumulated 25 to 30 mg U/g protein. Modification of uranium speciation in the exposure medium induces great changes in toxicity and cell accumulation. Comparison of toxicity and accumulation results to theoretical uranium speciation, calculated with the J-Chess computer program, shows that free-ion concentration can not explain the total uranium intracellular accumulation. Low molecular weight U(VI) complexes, such as UO2(CO3)3 4− but also UO2PO4 − could be implicated in U(VI) cellular accumulation and toxicity.

Uranium speciation in plants

Detailed knowledge of the nature of uranium complexes formed after the uptake by plants is an essential prerequisite to describe the migration behavior of uranium in the environment. This study focuses on the determination of uranium speciation after uptake of uranium by lupine plants. For the first time, time-resolved laser-induced fluorescence spectroscopy and X-ray absorption spectroscopy were used to determine the chemical speciation of uranium in plants. Differences were detected between the uranium speciation in the initial solution (hydroponic solution and pore water of soil) and inside the lupine plants. The oxidation state of uranium did not change and remained hexavalent after it was taken up by the lupine plants. The chemical speciation of uranium was identical in the roots, shoot axis, and leaves and was independent of the uranium speciation in the uptake solution. The results indicate that the uranium is predominantly bound as uranyl(VI) phosphate to the phosphoryl groups. Dandelions and lamb´s lettuce showed uranium speciation identical to lupine plants.

Interaction mechanisms of bacterial strains isolated from extreme habitats with uranium

This paper summarizes the effect of pH on the speciation and cellular localization of uranium bound by bacterial strains isolated from the S15 deep-well montoring site, located at the Siberian radioactive subsurface depository Tomsk-7, Russia. Microbiological methods in combination with extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX) were applied. EXAFS analysis showed that the cells of the two isolates, Microbacterium oxydans S15-M2 and Sphingomonas sp. S15-S1, precipitate U(VI) as m-autunite-like phase at pH 4.5, probably due to the release of inorganic phosphate from the cells as result of the microbial metabolism. However, at pH 2 uranium formed complexes with organically bound phosphates of the cell surface. The results of the EXAFS studies corroborate those found using TEM and EDX analysis. Hypotheses explaining the different coordination chemistry of uranium to bacteria as a function of pH of uranium solution in terms of solubility of m-autunite and/or microbial activity are discussed.

Interaction of uranium(VI) with lipopolysaccharide

Bacteria have a great influence on the migration behaviour of heavy metals in the environment. Lipopolysaccharides form the main part of the outer membrane of Gram-negative bacteria. We investigated the interaction of the uranyl cation (UO2(2+)) with lipopolysaccharide (LPS) from Pseudomonas aeruginosa by using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy (TRLFS) over a wide pH and concentration range. Generally, LPS consists of a high density of different functionalities for metal binding such as carboxyl, phosphoryl, amino and hydroxyl groups. The dissociation constants and corresponding site densities of these functional groups were determined using potentiometric titration. The combination of both methods, potentiometry and TRLFS, show that at an excess of LPS uranyl phosphoryl coordination dominates, whereas at a slight deficit on LPS compared to uranyl, carboxyl groups also become important for uranyl coordination. The stability constants of one uranyl carboxyl complex and three different uranyl phosphoryl complexes and the luminescence properties of the phosphoryl complexes are reported.

Interactions of uranium with polyphosphate

Inorganic polyphosphates (PolyP) are simple linear phosphate (PO(4)(3-)) polymers which are produced by a variety of microorganisms. One of their functions is to complex metals resulting in their precipitation. We investigated the interaction of phosphate and low-molecular-weight PolyP (1400-1900Da) with uranyl ion at various pHs. Potentiometric titration of uranyl ion in the presence of phosphate showed two sharp inflection points at pHs 4 and 8 due to uranium hydrolysis reaction and interaction with phosphate. Titration of uranyl ion and PolyP revealed a broad inflection point starting at pH 4 indicating that complexation of U-PolyP occurs over a wide range of pHs with no uranium hydrolysis. EXAFS analysis of the U-HPO(4) complex revealed that an insoluble uranyl phosphate species was formed below pH 6; at higher pH (> or = 8) uranium formed a precipitate consisting of hydroxophosphato species. In contrast, adding uranyl ion to PolyP resulted in formation of U-PolyP complex over the entire pH range studied. At low pH (< or = 6) an insoluble U-PolyP complex having a monodentate coordination of phosphate with uranium was observed. Above pH 6 however, a soluble bidentate complex with phosphate and uranium was predominant. These results show that the complexation and solubility of uranium with PO(4) and PolyP are dependent upon pH.

Microbacterium isolates from the vicinity of a radioactive waste depository and their interactions with uranium

Three oligotrophic bacterial strains were cultured from the ground water of the deep-well monitoring site S15 of the Siberian radioactive waste depository Tomsk-7, Russia. They were affiliated with Actinobacteria from the genus Microbacterium. The almost fully sequenced 16S rRNA genes of two of the isolates, S15-M2 and S15-M5, were identical to those of cultured representatives of the species Microbacterium oxydans. The third isolate, S15-M4, shared 99.8% of 16S rRNA gene identity with them. The latter isolate possessed a distinct cell morphology as well as carbon source utilization pattern from the M. oxydans strains S15-M2 and S15-M5. The three isolates tolerated equal amounts of uranium, lead, copper, silver and chromium but they differed in their tolerance of cadmium and nickel. The cells of all three strains accumulated high amounts of uranium, i.e. up to 240 mg U (g dry biomass)(-1) in the case of M. oxydans S15-M2. X-ray absorption spectroscopy (XAS) analysis showed that this strain precipitated U(VI) at pH 4.5 as a meta-autunite-like phase. At pH 2, the uranium formed complexes with organically bound phosphate groups on the cell surface. The results of the XAS studies were consistent with those obtained by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX).

Study of the interaction between europium (III) and Bacillus subtilis: fixation sites, biosorption modeling and reversibility

In order to elucidate the underlying mechanisms involved in the biosorption of metal ions, potentiometric titrations, complexation studies, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) measurements were used to characterize the interaction between Eu(III) and Bacillus subtilis. The reversibility of the interaction between Eu(III) and Bacillus subtilis was studied by a cation-exchange technique using the Chelex resin. For complexation studies in the presence of 0.15 mol/l of NaCl, the metal ion, the biomass, concentrations and the pH were varied. The adsorption data were quantified by a surface complexation model without electrostatic term. The data on the Eu(III)/B.subtilis system at pH 5 were satisfactorily described by one site at which Eu(III) was bound through one carboxylic function of the bacteria. With increasing pH, another site should be considered, involving a phosphate-bound environment. This was partially confirmed by time-resolved laser-induced fluorescence spectroscopy. In addition to this, it was evidenced that the site availability was dependent on the nature of the cation, i.e., a proton or Eu(III). Finally, it was shown that, at pH 5, the Eu(III)/Bacillus subtilis equilibrium was reversible.

Metal binding by bacteria from uranium mining waste piles and its technological applications

Uranium mining waste piles, heavily polluted with radionuclides and other toxic metals, are a reservoir for bacteria that have evolved special strategies to survive in these extreme environments. Understanding the mechanisms of bacterial adaptation may enable the development of novel bioremediation strategies and other technological applications. Cell isolates of Bacillus sphaericus JG-A12 from a uranium mining waste pile in Germany are able to accumulate high amounts of toxic metals such as U, Cu, Pb, Al, and Cd as well as precious metals. Some of these metals, i.e. U, Cu, Pd(II), Pt(II) and Au(III), are also bound by the highly orderd paracrystalline proteinaceous surface layer (S-layer) that envelopes the cells of this strain. These special capabilities of the cells and the S-layer proteins of B. sphaericus JG-A12 are highly interesting for the clean-up of uranium contaminated waste waters, for the recovery of precious metals from electronic wastes, and for the production of metal nanoclusters. The fabricated nanoparticles are promising for the development of novel catalysts. This work reviews the molecular biology of the S-layer of the strain JG-A12 and the S-layer dependent interactions of the bacterial cells with metals. It presents future perspectives for their application in bioremediation and nanotechnology.

Complexation of uranium by cells and S-layer sheets of Bacillus sphaericus JG-A12

Bacillus sphaericus JG-A12 is a natural isolate recovered from a uranium mining waste pile near the town of Johanngeorgenstadt in Saxony, Germany. The cells of this strain are enveloped by a highly ordered crystalline proteinaceous surface layer (S-layer) possessing an ability to bind uranium and other heavy metals. Purified and recrystallized S-layer proteins were shown to be phosphorylated by phosphoprotein-specific staining, inductive coupled plasma mass spectrometry analysis, and a colorimetric method. We used extended X-ray absorption fine-structure (EXAFS) spectroscopy to determine the structural parameters of the uranium complexes formed by purified and recrystallized S-layer sheets of B. sphaericus JG-A12. In addition, we investigated the complexation of uranium by the vegetative bacterial cells. The EXAFS analysis demonstrated that in all samples studied, the U(VI) is coordinated to carboxyl groups in a bidentate fashion with an average distance between the U atom and the C atom of 2.88 +/- 0.02 A and to phosphate groups in a monodentate fashion with an average distance between the U atom and the P atom of 3.62 +/- 0.02 A. Transmission electron microscopy showed that the uranium accumulated by the cells of this strain is located in dense deposits at the cell surface.

Adsorption of aqueous uranyl complexes onto Bacillus subtilis cells

In oxygenated, CO2-rich systems, negatively charged uranyl complexes dominate the aqueous uranium speciation, and it is commonly assumed that these complexes exhibit negligible adsorption onto negatively charged surfaces such as bacteria. We measured the adsorption of 4.2 x 10(-6) M aqueous uranium onto Bacillus subtilis from pH 1.5 to 9 and with wet weight bacterial concentrations from 0.125 to 0.5 g/L. Experiments were performed in the presence and absence of dissolved CO2, and additional experiments were performed in the presence of dissolved CO2 and Ca. We observed extensive uranium adsorption onto the bacterial surface under all conditions. Thermodynamic modeling of the data suggests that uranylhydroxide, uranyl-carbonate, and calcium-uranylcarbonate species each can form stable surface complexes on the bacterial cell wall. These results could dramatically alter predictions of uranium mobility in near-surface environments.

The impact of ionic strength on the adsorption of protons, Pb, Cd, and Sr onto the surfaces of Gram negative bacteria: testing non-electrostatic, diffuse, and triple-layer models

Bacterial surface adsorption reactions are influenced by electric field effects caused by changes in ionic strength; however, existing datasets are too sparse to definitively constrain these differences or to determine the best way to account for them using thermodynamic models. In this study, we examine the ionic strength dependence of proton and metal adsorption onto the surfaces of Pseudomonas mendocina and Pseudomonas putida by conducting proton, Cd(II), Pb(II), and Sr(II) adsorption experiments over the ionic strength range of 0.001 to 0.6 M. Chosen experimental results are thermodynamically modeled using a non-electrostatic approach, a diffuse layer model (DLM), and a triple-layer model (TLM). The results demonstrate that bacterial surface electric field effects are negligible for proton, Cd, and Pb adsorption onto P. putida and P. mendocina, and that the discrete site non-electrostatic model developed in this study is adequate for describing these reactions. The extent of Sr adsorption is influenced by changes in the bacterial surface electric field; however, the non-electrostatic model better describes Sr adsorption behavior than the DLM or TLM. The DLM and TLM greatly overpredict the effect of the electric field for all adsorption reactions at all ionic strengths tested.