Metal-phytoplankton interactions: modeling the effect of competing ions (H+, Ca2+, and Mg2+) on uranium uptake

Characterization of a uranium-tolerant green microalga of the genus Coelastrella with high potential for the remediation of metal-polluted waters

Uranium (U) contamination of terrestrial and aquatic ecosystems poses a significant threat to the environment and human health due to the chemotoxicity of this actinide. The characterization of organisms that tolerate and accumulate U is crucial to decipher the mechanisms evolved to cope with the radionuclide and to propose new effective strategies for the bioremediation of U-contaminated environments. Here, we isolated a unicellular green microalga of the genus Coelastrella from U-contaminated wastewater. We showed that Coelastrella sp. PCV is much more tolerant to U than Chlamydomonas reinhardtii and Chlorella vulgaris. Coelastrella sp. PCV is able to accumulate U very rapidly and then gradually release it into the medium, behaving as an excluder to limit the toxic effects of U. The ability of Coelastrella sp. PCV to accumulate U is remarkably high, with up to 240 mg of tightly bound U per g of dry biomass. Coelastrella sp. PCV is able to grow and maintain high photosynthesis in natural metal-contaminated waters from a wetland near a reclaimed U mine. In a single one-week growth cycle, Coelastrella sp. PCV is able to capture 25-55 % of the U from the contaminated waters and shows lipid droplet accumulation. Coelastrella sp. PCV is a very promising microalga for the remediation of polluted waters with valorization of algal biomass that accumulates lipids.

A review of metal contamination in seagrasses with an emphasis on metal kinetics and detoxification

Seagrasses are important foundation species in coastal ecosystems, and they provide food and habitat that supports high biodiversity. However, seagrasses are increasingly subjected to anthropogenic disturbances such as metal pollution, which has been implicated as a significant factor driving seagrass losses. There have been several reviews synthesizing the metal concentrations in seagrasses and evaluating their utility as biomonitors for metal pollution in the coastal environment at the local scale. However, the interpretation of metal data in seagrass biomonitors requires a more mechanistic understanding of the processes governing metal bioaccumulation and detoxification. In this review, the progress and trends in metal studies in seagrasses between 1973 and 2022 were analyzed to identify frontier topics in this field. In addition, we tried to (1) analyze and assess the current status of metal contamination in seagrasses on a global scale by incorporating more metal data from tropical and Indo-Pacific seagrasses, (2) summarize the geochemical and biological factors governing metal uptake and loss in seagrasses, and (3) provide an up-to-date understanding of metals' effects on seagrasses and their physiological responses to metal challenges. This review improves our understanding of the highly variable metal concentrations observed in the field.

Aquatic insect accumulation of uranium at spring outflows in the Grand Canyon region as influenced by aqueous and sediment geochemistry and biological factors: implications for monitoring

Potential adverse ecological effects of expanded uranium (U) mining within the Grand Canyon region motivated studies to better understand U exposure and risk to endemic species. This study documents U exposures and analyzes geochemical and biological factors affecting U bioaccumulation at spring-fed systems within the Grand Canyon region. The principal objective was to determine if aqueous U was broadly indicative of U accumulated by insect larvae, a dominate fauna. Analyses focused on three widely distributed taxa: Argia sp. (a predatory damselfly), Culicidae (suspension feeding mosquitos), and Limnephilus sp. (a detritivorous caddisfly). The study showed that U accumulated by aquatic insects (and periphyton) generally correlated positively with total dissolved U, although correlations were strongest when based on modeled concentrations of the U-dicarbonato complex, UO₂(CO₃)₂^-2, and UO₂(OH)₂. Sediment metal concentration was a redundant indicator of U bioaccumulation. Neither insect size or U in the gut content of Limnephilus sp. substantially affected correlations between aqueous U and whole-body U concentrations. However, in Limnephilus sp., the gut and its content contained large quantities of U. Estimates of the sediment burden in the gut indicated that sediment was a minor source of U mass but contributed substantially to the total insect weight. As a result, whole-body U concentration would tend to vary inversely with the sediment burden of the gut. The correlations between aqueous U and bioaccumulated U provide an initial relational baseline against which newly acquired data could be evaluated for changes in U exposure during and after mining operations.

Calcium affects uranium responses in Arabidopsis thaliana: From distribution to toxicity

Uranium, a heavy metal and primordial radionuclide, is present in surface waters and soils both naturally and due to industrial activities. Uranium is known to be toxic to plants and its uptake and toxicity can be influenced by multiple factors such as pH and the presence of different ions. However, the precise role of the different ions in uranium uptake is not yet known. Here we investigated whether calcium influences uranium uptake and toxicity in the terrestrial plant Arabidopsis thaliana. To this end, A. thaliana plants were exposed to different calcium and uranium concentrations and furthermore, calcium channels were blocked using the calcium channel blocker lanthanum chloride (LaCl₃). Fresh weight, relative growth rate, concentration of nutrients and uranium and gene expression of oxidative stress-related genes and calcium transporters were determined in roots and shoots. Calcium affected plant growth and oxidative stress in both control (no uranium) and uranium-exposed plants. In shoots, this was influenced by the total calcium concentration, but not by the different tested uranium concentrations. Uranium in turn did influence calcium uptake and distribution. Uranium-exposed plants grown in a medium with a higher calcium concentration showed an increase in gene expression of NADPH oxidases RBOHC and RBOHE and calcium transporter CAX7 after uranium exposure. In roots, these calcium-dependent responses in gene expression were not observed. This indicates that calcium indeed affects uranium toxicity, but only in shoots. In addition, a clear influence of uranium and LaCl₃ (separately and combined) on the expression of calcium transporters was observed.

Best practices for predictions of radionuclide activity concentrations and total absorbed dose rates to freshwater organisms exposed to uranium mining/milling

Predictions of radionuclide dose rates to freshwater organisms can be used to evaluate the radiological environmental impacts of releases from uranium mining and milling projects. These predictions help inform decisions on the implementation of mitigation measures. The objective of this study was to identify how dose rate modelling could be improved to reduce uncertainty in predictions to non-human biota. For this purpose, we modelled the activity concentrations of ²¹⁰Pb, ²¹⁰Po, ²²⁶Ra, ²³⁰Th, and ²³⁸U downstream of uranium mines and mills in northern Saskatchewan, Canada, together with associated weighted absorbed dose rates for a freshwater food chain using measured activity concentrations in water and sediments. Differences in predictions of radionuclide activity concentrations occurred mainly from the different default partition coefficient and concentration ratio values from one model to another and including all or only some ²³⁸U decay daughters in the dose rate assessments. Consequently, we recommend a standardized best-practice approach to calculate weighted absorbed dose rates to freshwater biota whether a facility is at the planning, operating or decommissioned stage. At the initial planning stage, the best-practice approach recommend using conservative site-specific baseline activity concentrations in water, sediments and organisms and predict conservative incremental activity concentrations in these media by selecting concentration ratios based on species similarity and similar water quality conditions to reduce the uncertainty in dose rate calculations. At the operating and decommissioned stages, the best-practice approach recommends relying on measured activity concentrations in water, sediment, fish tissue and whole-body of small organisms to further reduce uncertainty in dose rate estimates. This approach would allow for more realistic but still conservative dose assessments when evaluating impacts from uranium mining projects and making decision on adequate controls of releases.

Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana

Uranium (U) is a non-essential and toxic element that is taken up by plants from the environment. The assimilation pathway of U is still unknown in plants. In this study, we provide several evidences that U is taken up by the roots of Arabidopsis thaliana through Ca²⁺-permeable cation channels. First, we showed that deprivation of Arabidopsis plants with calcium induces a 1.5-fold increase in the capacity of roots to accumulate U, suggesting that calcium deficiency promotes the radionuclide import pathway. Second, we showed that external calcium inhibits U accumulation in roots, suggesting a common route for the uptake of both cations. Third, we found that gadolinium, nifedipine and verapamil inhibit the absorption of U, suggesting that different types of Ca²⁺-permeable channels serve as a route for U uptake. Last, we showed that U bioaccumulation in Arabidopsis mutants deficient for the Ca²⁺-permeable channels MCA1 and ANN1 is decreased by 40%. This suggests that MCA1 and ANN1 contribute to the absorption of U in different zones and cell layers of the root. Together, our results describe for the first time the involvement of Ca²⁺-permeable cation channels in the cellular uptake of U.

Chronic toxicity of uranium to three benthic organisms in laboratory spiked sediment

Due to mining activities, concentration of uranium (U) in the environment nearby former and operating sites can be higher than in other areas. The derivation of quality criteria for U in freshwater ecosystems, rivers and lakes includes the consideration of contaminated sediments and the associated risk to the benthic life. Therefore, the derivation of a quality criteria for sediment has been viewed as a logical and necessary extension of the work already done to establish water quality criteria. In order to contribute to the determination of a Quality Standard for sediment (QS_sediment) according to the European recommendations, this study focuses on the acquisition of a new toxicity dataset, to enrich the few rare existing data, most often unsuitable. A basic set of organisms, including three complementary benthic organisms (Chironomus riparius, Hyalella azteca, Myriophyllum aquaticum), was chronically exposed to U spiked to a standard laboratory-formulated sediment, according to the related bioassay guidelines (ISO/FDIS16303, OECD 218/9, ISO/DIS 16191). We looked to determine when possible both NOEC and EC₁₀ values for each organism. For C. riparius, a NOEC (emergence rate) value was estimated at 62 mgU, kg^-1, dm and the EC₁₀ value reached 188 mgU, kg^-1, dm (CI_95% 40-885 mgU kg^-1, dm). For H. azteca, a NOEC (survival rate) value of 40 mgU kg^-1, dm was observed while the EC₁₀ value at 296 mgU kg^-1, dm (CI_95% = 155-436 mgU kg^-1, dm) was slightly higher than for growth at 199 mgU kg^-1, dm (CI_95% = 107-291 mgU kg^-1 dm). Finally, the less sensitive organism seemed to be the plant, M. aquaticum, for which we determined a NOEC value of 100 mgU kg^-1, dm. Results obtained regarding the toxicity of U made it possible to suggest a preliminary QS_sediment value of 4 mgU kg^-1, dry mass. This value was shown conservative compared to U sediment quality criteria derived by other jurisdictions.

Uranium Bioaccumulation Dynamics in the Mayfly Neocloeon triangulifer and Application to Site-Specific Prediction

Little is known about the underlying mechanisms governing the bioaccumulation of uranium (U) in aquatic insects. We experimentally parameterized conditional rate constants for aqueous U uptake, dietary U uptake, and U elimination for the aquatic baetid mayfly Neocloeon triangulifer. Results showed that this species accumulates U from both the surrounding water and diet, with waterborne uptake prevailing. Elevated dietary U concentrations decreased feeding rates, presumably by altering food palatability or impairing the mayfly's digestive processes, or both. Nearly 90% of the accumulated U was eliminated within 24 h after the waterborne exposure ceased, reflecting the desorption of weakly bound U from the insect's integument. To examine whether the experimentally derived rate constants for N. triangulifer could be generalized to baetid mayflies, mayfly U concentrations were predicted using the water chemistry and U measured in periphyton from springs in Grand Canyon (United States) and were compared to U concentrations in spring-dwelling mayflies. Predicted and observed mayfly U concentrations were in good agreement. Under the modeled site-specific conditions, waterborne U uptake accounted for 52-93% of the bioaccumulated U. U accumulation was limited in these wild populations due to a combination of factors including low concentrations of bioavailable dissolved U species, slow U uptake rates from food, and fast U elimination.

Aqueous inorganic uranium speciation in European stream waters from the FOREGS dataset using geochemical modelling and determination of a U bioavailability baseline

The concentration of the bioavailable uranium fraction (U_bio) at the European scale was deduced by geochemical modelling considering several definitions found in the literature and the FOREGS European stream waters geochemical atlas dataset to produce a U_bio baseline. A sensitivity analysis was performed using three thermodynamic databases. We also investigated the link between total dissolved uranium (U_aq) concentrations, speciation and global stream water chemistry on the one hand, and the lithology and ages of the surrounding rocks on the other. The more U-enriched the stream sediments or rock type contexts are, which tends to be the case with rocks containing silicates (4.1 mg/kg), the less U-concentrated the stream waters are (0.15 μg/L). Sedimentary rocks lead to slightly higher U_aq concentrations (0.34 μg/L) even if the concentration in sediment (U_sed) is relatively low (1.6 mg/kg). This trend is reversed for U_bio, with higher concentrations in a crystalline context. The mean estimated U_bio value ranges from 1.5.10^-3 to 65.3 ng/L and can fluctuate by 3 orders of magnitude depending on the considered definition as opposed to by 2 orders of magnitude accountable to differences between thermodynamic databases. The classification of the water in relation to the two surrounding rock lithologies makes it possible to reduce the mean variability for the U_bio concentrations. Irrespective of the definition of U_bio considered, in 59% of cases the U_bio fraction represents less than 1% of U_aq. Several threshold values relating to U_bio were proposed, assuming knowledge only of the aqueous concentrations of the major elements and U_aq.

Uranium Isotope Fractionation (238U/235U) during U(VI) Uptake by Freshwater Plankton

Uranium contamination in the environment is a serious public health concern. Biotic U(VI) reduction and nonreductive U(VI) uptake by microorganisms (e.g., U(VI) biosorption by cyanobacteria) are effective U remediation techniques. Variations of ²³⁸U/²³⁵U have been extensively explored to track biotic U(VI) reduction in laboratory experiments and field applications. However, U isotope fractionation during nonreductive U(VI) uptake by microorganisms is poorly constrained. To investigate U isotope fractionation in this process, we cultured freshwater plankton in the presence of U(VI) and measured ²³⁸U/²³⁵U in the culture media and biomass. We found that nonreductive U(VI) uptake by freshwater plankton fractionated U isotopes in the opposite direction compared to biotic U(VI) reduction. δ²³⁸U values in freshwater plankton were consistently ∼0.23 ± 0.06‰ lighter than those in dissolved U in the culture medium at various fractions of U removal (12-30%), consistent with equilibrium isotope fractionation in a closed system. The equilibrium isotope fractionation observed in our experiments possibly results from changes in coordination geometry between dissolved U(VI) in the culture media and adsorbed U(VI) on cell surfaces. Our experimental results highlight the need to consider U isotope fractionation during nonredox U(VI) uptake by microorganisms and organic matter when applying variations of ²³⁸U/²³⁵U to track biogeochemical processes and evaluate U remediation.

Competitive interactions among H, Cu, and Zn ions moderate aqueous uptake of Cu and Zn by an aquatic insect

The absorption of aqueous copper (Cu) and zinc (Zn) by aquatic insects, a group widely used to assess water quality, is unresolved. This study examined interactions among Cu, Zn, and protons that potentially moderate Cu and Zn uptake by the acid-tolerant stonefly Zapada sp. Saturation uptake kinetics were imposed to identify competitive mechanisms. Decreasing pH reduced the maximum transport capacity, J_max, in both metals, had little effect on the Cu dissociation constant, K_D, and increased the Zn K_D. Partial noncompetitive (Cu) and partial mixed competitive (Zn) inhibitor models most closely tracked the observed Cu and Zn influx rates across pH treatments. The estimated values for acid dissociation constants for the binary (proton-receptor) and ternary (proton-metal-receptor) complexes indicated the strong inhibitory effect of protons on Cu and Zn. In neutral pH water, Cu inhibited Zn influx, but Zn had little effect on Cu influx. The mechanism of Cu-Zn interaction was not identified. Results from separate Zn experiments suggested that the insect's developmental stage may affect the apparent J_max. The study underscores some of the challenges of modeling metal bioaccumulation and informs future research directions.

Use of fish otoliths as a temporal biomarker of field uranium exposure

This study aimed to determine uranium (U) pollution over time using otoliths as a marker of fish U contamination. Experiments were performed in field contamination (~20 μg L^-1: encaged fish: 15d, 50d and collected wild fish) and in laboratory exposure conditions (20 and 250 μg L^-1, 20d). We reported the U seasonal concentrations in field waterborne exposed roach fish (Rutilus rutilus), in organs and otoliths. Otoliths were analyzed by ICPMS and LA-ICP SF MS of the entire growth zone. Concentrations were measured on transects from nucleus to the edge of otoliths to characterize environmental variations of metal accumulation. Results showed a spatial and temporal variation of U contamination in water (from 51 to 9.4 μg L^-1 at the surface of the water column), a high and seasonal accumulation in fish organs, mainly the digestive tract (from 1000 to 30,000 ng g^-1, fw), the gills (from 1600 to 3200 ng g^-1, fw) and the muscle (from 144 to 1054 ng g^-1, fw). U was detected throughout the otolith and accumulation varied over the season from 70 to 350 ng g^-1, close to the values measured (310 ng g^-1) after high exposure levels in laboratory conditions. U in otoliths of encaged fish showed rapid and high U accumulation from 20 to 150 ng g^-1. The U accumulation signal was mainly detected on the edge of the otolith, showing two U accumulation peaks, probably correlated to fish age, i.e. 2 years old. Surprisingly, elemental U and Zn signatures followed the same pattern therefore using the same uptake pathways. Laboratory, caging and field experiments indicated that otoliths were able to quickly accumulate U on the surface even for low levels and to store high levels of U. This study is an encouraging first step in using otoliths as a marker of U exposure.

Toxicity of tailing leachates from a niobium mine toward three aquatic organisms

The aim of this research was to assess the ecotoxicity of leachates originating from a niobium mine located in Canada. These tailings contain considerable amounts of carbonates and phosphates and could potentially be used as fertilizer for agriculture. However, the presence of different contaminants linked with the ores mined, including rare earth elements and daughter elements of the uranium disintegration chain is of concern. Bioassays have been used to determine if the tailings leachates could be harmful. The assessment of the toxicity of progressive dilutions of five tailing leachates (808, 809, 810, 811 and 897) was performed on different organisms: phytoplankton Raphidocelis subcapitata and duckweed Lemna minor, based on their growth and chlorophyll a content, and water flea Daphnia magna based on their mobility, mortality and reproduction. Overall, the leachates showed higher toxicity to Raphidocelis subcapitata and Lemna minor, than toward Daphnia magna. Leachate 808 showed no toxicity to all organisms while leachate 810 showed significant effects to all species. The results can be explained by the leachate dissolved metal or nutrient concentrations, but also by the metal bioavailability which depends on pH and hardness. Generally, toxicity was observed in undiluted samples tested, which is not representative of the conditions that could occur in the environment. This supports the idea that these tailings could be used as fertilizer albeit more studies may be required, particularly to assess the toxicity of the tailings leachate for benthic organisms, the toxicity of the tailings for terrestrial organisms and the variations of soil and sediment physicochemical properties after tailing treatments.

Assessment of Diatom Assemblages in Close Proximity to Mining Activities in Nunavik, Northern Quebec (Canada)

Nunavik (Northern Quebec, Canada) is experiencing a mining boom. While several studies have been conducted in the region in relation to climate change, the effects of mining have received much less attention. In this study, we explored the use of biofilms in natural streams as an indicator of potential stress on living organisms caused by metal contamination from nickel mining activities. More specifically, we assessed diatom assemblages and the presence of teratologies (morphologic abnormalities) as a response to metals in streams located in close proximity to mining sites. Metal concentrations (as well as other cations), anions and pH values varied markedly among stations. Different diatom assemblage structures (four biotypes, i.e., groups of samples with similar diatom species composition) were observed depending on the level and the type of contamination. The frequency of diatom teratologies was higher in metal contaminated sites. The present study lays the foundation for bioassessment of metal contamination in low Arctic streams using diatom-based approaches, and will serve as a point in time reference for future evaluation of ecosystems degradation or recovery in Nunavik.

Uranium transfer and accumulation in organs of Danio rerio after waterborne exposure alone or combined with diet-borne exposure

Uranium (U) toxicity patterns for fish have been mainly determined under laboratory-controlled waterborne exposure conditions. Because fish can take up metals from water and diet under in situ exposure conditions, a waterborne U exposure experiment (20 μg L^-1 , 20 d) was conducted in the laboratory to investigate transfer efficiency and target organ distribution in zebrafish Danio rerio compared with combined waterborne exposure (20 μg L^-1 ) and diet-borne exposure (10.7 μg g^-1 ). ²³³ Uranium was used as a specific U isotope tracer for diet-borne exposure. Bioaccumulation was examined in the gills, liver, kidneys, intestine, and gonads of D. rerio. Concentrations in the organs after waterborne exposure were approximately 500 ng g^-1 fresh weight, except in the intestine (> 10 μg g^-1 fresh wt) and the kidneys (200 ng g^-1 fresh wt). No significant difference was observed between waterborne and diet-borne conditions. Trophic U transfer in organs was found but at a low level (< 10 ng g^-1 fresh wt). Surprisingly, the intestine appeared to be the main target organ after both tested exposure modalities. The gonads (57% at 20 d) and the liver (41% at 20 d) showed the highest accumulated relative U burdens. Environ Toxicol Chem 2019;38:90-98. © 2018 SETAC.

Effect of Calcium on the Bioavailability of Dissolved Uranium(VI) in Plant Roots under Circumneutral pH

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO₃^- solutions at circumneutral pH. The accumulation of U in plants (3.1-21.3 mg kg^-1) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30-700 μg L^-1) and Ca (0-240 mg L^-1) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V₀) ranged from 4.4 to 62 μg g^-1 h^-1 in experiments with no added Ca and from 0.73 to 2.07 μg g^-1 h^-1 in experiments with 12 mg L^-1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L^-1 Ca. Ternary Ca-U-CO₃ complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy-energy-dispersive spectrometry detected U-P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Bioaccumulation and toxicity of uranium, arsenic, and nickel to juvenile and adult Hyalella azteca in spiked sediment bioassays

Uranium (U) mining and milling release arsenic (As), nickel (Ni) and U to receiving waters, which accumulate in sediments. The objective of the present study was to investigate if As, Ni, and U concentrations in tissue residue of Hyalella azteca, overlying water, sediment porewater, and solids could predict juvenile and adult survival and growth in conditions similar to lake sediments downstream of U mines and mills. We conducted 14-d static sediment toxicity tests spiked with U, As, and Ni salts. For U, we spiked uranyl nitrate with sodium bicarbonate to limit U precipitation once in contact with circumneutral sediment. The median lethal concentrations for As, Ni, and U of juveniles and adults based on measured concentrations in sediments were 134 and 165 μg/g, 370 and 787 μg/g, and 48 and 214 μg/g, respectively. Adult survival and growth linearly decreased with increasing bioaccumulation. For juveniles, metal accumulation linearly predicted survival. We calculated median lethal body concentrations for juveniles and adults of 5 and 36 μg As/g, 14 and 49 μg Ni/g, and 0.4 and 1.0 μg U/g. The concentrations of As, Ni, and U in tissue residue leading to a 20% decrease in adult growth were 32 μg As/g, 44 μg Ni/g, and 1 μg U/g. Overall, the present study showed that U was the most toxic element, followed by As and Ni; that juveniles were more sensitive to the 3 metals tested than adults; and that threshold body concentrations can support assessment of benthic invertebrate community impairment. Environ Toxicol Chem 2018;37:2340-2349. © 2018 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Predicting the bioavailability of sediment-bound uranium to the freshwater midge (Chironomus dilutus) using physicochemical properties

Assessment of uranium (U)-contaminated sediment is often hindered by the inability to accurately account for the physicochemical properties of sediment that modify U bioavailability. The present goal was to determine whether sediment-associated U bioavailability could be predicted over a wide range of conditions and sediment properties using simple regressions and a geochemical speciation model, the Windermere Humic Aqueous Model (WHAM7). Data from a U-contaminated field sediment bioaccumulation test, along with previously published bioaccumulation studies with U-spiked field and formulated sediments, were used to examine the models. Observed U concentrations in Chironomus dilutus larvae exposed to U-spiked and U-contaminated sediments correlated well (r²  > 0.74, p < 0.001) with the WHAM-calculated concentration of U bound to humic acid, indicating that humic acid may be a suitable surrogate for U binding sites (biotic ligands) in C. dilutus larvae. Subsequently, the concentration of U in C. dilutus was predicted with WHAM7 by numerically optimizing the equivalent mass of humic acid per gram of organism. The predicted concentrations of U in C. dilutus larvae exposed to U-spiked and U-contaminated field sediment compared well with the observed values, where one of the regression models provided a slightly better fit (mean absolute error = 18.1 mg U/kg dry wt) than WHAM7 (mean absolute error = 34.2 mg U/kg dry wt). The regression model provides a predictive capacity with a minimal number of variables, whereas WHAM7 provides additional complementary insight into the chemical variables influencing the speciation, sorption, and bioavailability of U in sediment. The present results indicate that physicochemical properties of sediment can be used to account for variability in U bioavailability as measured through bioaccumulation in chironomids exposed to U-contaminated sediments. Environ Toxicol Chem 2018;37:1146-1157. © 2017 SETAC.

Improvement of the Uranium Sequestration Ability of a Chlamydomonas sp. (ChlSP Strain) Isolated From Extreme Uranium Mine Tailings Through Selection for Potential Bioremediation Application

The extraction and processing of uranium (U) have polluted large areas worldwide, rendering anthropogenic extreme environments inhospitable to most species. Noticeably, these sites are of great interest for taxonomical and applied bioprospection of extremotolerant species successfully adapted to U tailings contamination. As an example, in this work we have studied a microalgae species that inhabits extreme U tailings ponds at the Saelices mining site (Salamanca, Spain), characterized as acidic (pH between 3 and 4), radioactive (around 4 μSv h⁻¹) and contaminated with metals, mainly U (from 25 to 48 mg L⁻¹) and zinc (from 17 to 87 mg L⁻¹). After isolation of the extremotolerant ChlSP strain, morphological characterization and internal transcribed spacer (ITS)-5.8S gene sequences placed it in the Chlamydomonadaceae, but BLAST analyses identity values, against the nucleotide datasets at the NCBI database, were very low (<92%). We subjected the ChlSP strain to an artificial selection protocol to increase the U uptake and investigated its response to selection. The ancestral strain ChlSP showed a U-uptake capacity of ≈4.30 mg U g⁻¹ of dry biomass (DB). However, the artificially selected strain ChlSG was able to take up a total of ≈6.34 mg U g⁻¹ DB, close to the theoretical maximum response (≈7.9 mg U g⁻¹ DB). The selected ChlSG strain showed two possible U-uptake mechanisms: the greatest proportion by biosorption onto cell walls (ca. 90%), and only a very small quantity, ~0.46 mg g⁻¹ DB, irreversibly bound by bioaccumulation. Additionally, the kinetics of the U-uptake process were characterized during a microalgae growth curve; ChlSG cells removed close to 4 mg L⁻¹ of U in 24 days. These findings open up promising prospects for sustainable management of U tailings waters based on newly evolved extremotolerants and outline the potential of artificial selection in the improvement of desired features in microalgae by experimental adaptation and selection.

Effect of cadmium accumulation on green algae Chlamydomonas reinhardtii and acid-tolerant Chlamydomonas CPCC 121

Cadmium is one of the most dangerous metals found in wastewater since exposure to low concentrations are highly toxic for cellular functions. In this study, the effect of cadmium accumulation on Chlamydomonas reinhardtii and acid-tolerant strain CPCC 121 was investigated during 48 h under 100-600 μM of Cd and two pH conditions (4 and 7). The toxicity of accumulated Cd was determined by the change of cellular and photosynthetic parameters. Obtained results showed that the maximum capacity of Cd accumulation in algal biomass was reached for both strains at 24 h of exposure to 600 μM of Cd. Under this condition, C. reinhardtii showed a higher uptake of Cd compared to the strain CPCC 121, inducing a stronger cellular toxic impact. Chlamydomonas CPCC 121 showed a tolerance for Cd due to the exclusion of Cd at the cell wall surface, which was higher at pH 4 than pH 7. TEM images and EDX spectrum of Cd distribution within the cell confirmed the role of the cell wall as a barrier for Cd uptake. Although Cd²⁺ concentration was the highest in the medium, CPCC 121 was the most tolerant at pH 4, but was not enough efficient to be considered for the phycoremediation of Cd. At neutral pH, the efficiency of C. reinhardtii for the removal of Cd was limited by its toxicity, which was dependent to the concentration of Cd in the medium and the time of exposure.

Biogeochemical Controls of Uranium Bioavailability from the Dissolved Phase in Natural Freshwaters

To gain insights into the risks associated with uranium (U) mining and processing, we investigated the biogeochemical controls of U bioavailability in the model freshwater species Lymnaea stagnalis (Gastropoda). Bioavailability of dissolved U(VI) was characterized in controlled laboratory experiments over a range of water hardness, pH, and in the presence of complexing ligands in the form of dissolved natural organic matter (DOM). Results show that dissolved U is bioavailable under all the geochemical conditions tested. Uranium uptake rates follow first order kinetics over a range encompassing most environmental concentrations. Uranium uptake rates in L. stagnalis ultimately demonstrate saturation uptake kinetics when exposure concentrations exceed 100 nM, suggesting uptake via a finite number of carriers or ion channels. The lack of a relationship between U uptake rate constants and Ca uptake rates suggest that U does not exclusively use Ca membrane transporters. In general, U bioavailability decreases with increasing pH, increasing Ca and Mg concentrations, and when DOM is present. Competing ions did not affect U uptake rates. Speciation modeling that includes formation constants for U ternary complexes reveals that the aqueous concentration of dicarbonato U species (UO2(CO3)2(-2)) best predicts U bioavailability to L. stagnalis, challenging the free-ion activity model postulate.

Using biofilms for monitoring metal contamination in lotic ecosystems: The protective effects of hardness and pH on metal bioaccumulation

Biofilms can make good bioindicators and biomarkers, offering a convenient tool to monitor metal contamination in streams that results from mine tailing sites. Biofilm metal content (Cu, Zn, Cd, Pb) as well as diatom diversity and the presence of teratologies (diatom abnormalities) were determined for biofilms from rivers with a variety of physicochemical properties across a metal contamination gradient. The results of metal accumulation were highly consistent from year to year, with significant relationships between calculated free metal ion concentrations and biofilm metal contents for samples from different rivers. This indicates the "universal nature" of the metal accumulation process in biofilms. The authors observed that protons and major cations protected against metal accumulation. A very low number of diatom taxa were found at the most contaminated sites, and the highest proportions of deformities were observed at these sites. However, it was difficult to distinguish the effect of metal contamination from the effect of other parameters, especially pH. The results suggest that the development of biofilm-based proxies for metal bioavailability is useful and that incorporation of the effects of hardness and pH in this metal contamination monitoring tool is important. Environ Toxicol Chem 2016;35:1489-1501. © 2015 SETAC.

Influence of nutrient medium composition on uranium toxicity and choice of the most sensitive growth related endpoint in Lemna minor

Uranium (U) toxicity is known to be highly dependent on U speciation and bioavailability. To assess the impact of uranium on plants, a growth inhibition test was set up in the freshwater macrophyte Lemna minor. First growth media with different compositions were tested in order to find a medium fit for testing U toxicity in L. minor. Following arguments were used for medium selection: the ability to sustain L. minor growth, a high solubility of U in the medium and a high percentage of the more toxic U-species namely UO2(2+). Based on these selection criteria a with a low phosphate concentration of 0.5 mg L(-1) and supplemented with 5 mM MES (2-(N-morpholino)ethanesulfonic acid) to ensure pH stability was chosen. This medium also showed highest U toxicity compared to the other tested media. Subsequently a full dose response curve for U was established by exposing L. minor plants to U concentrations ranging from 0.05 μM up to 150 μM for 7 days. Uranium was shown to adversely affect growth of L. minor in a dose dependent manner with EC10, EC30 and EC50 values ranging between 1.6 and 4.8 μM, 7.7-16.4 μM and 19.4-37.2 μM U, respectively, depending on the growth endpoint. Four different growth related endpoints were tested: frond area, frond number, fresh weight and dry weight. Although differences in relative growth rates and associated ECx-values calculated on different endpoints are small (maximal twofold difference), frond area is recommended to be used to measure U-induced growth effects as it is a sensitive growth endpoint and easy to measure in vivo allowing for measurements over time.

XAS and TRLIF spectroscopy of uranium and neptunium in seawater

Seawater contains radionuclides at environmental levels; some are naturally present and others come from anthropogenic nuclear activity. In this report, the molecular speciation in seawater of uranium(VI) and neptunium(V) at a concentration of 5 × 10(-5) M has been investigated for the first time using a combination of two spectroscopic techniques: Time-resolved laser-induced fluorescence (TRLIF) for U and extended X-ray absorption fine structure (EXAFS) for U and Np at the LIII edge. In parallel, the theoretical speciation of uranium and neptunium in seawater at the same concentration is also discussed and compared to spectroscopic data. The uranium complex was identified as the neutral carbonato calcic complex UO2(CO3)3Ca2, which has been previously described in other natural systems. In the case of neptunium, the complex identified is mainly a carbonato complex whose exact stoichiometry is more difficult to assess. The knowledge of the actinide molecular speciation and reactivity in seawater is of fundamental interest in the particular case of uranium recovery and more generally regarding the actinide life cycle within the biosphere in the case of accidental release. This is the first report of actinide direct speciation in seawater medium that can complement inventory data.

Assessment of co-contaminant effects on uranium and thorium speciation in freshwater using geochemical modelling

Speciation modelling of uranium (as uranyl) and thorium, in four freshwaters impacted by mining activities, was used to evaluate (i) the influence of the co-contaminants present on the predicted speciation, and (ii) the influence of using nine different model/database combinations on the predictions. Generally, co-contaminants were found to have no significant effects on speciation, with the exception of Fe(III) in one system, where formation of hydrous ferric oxide and adsorption of uranyl to its surface impacted the predicted speciation. Model and database choice on the other hand clearly influenced speciation prediction. Complexes with dissolved organic matter, which could be simulated by three of the nine model/database combinations, were predicted to be important in a slightly acidic, soft water. Model prediction of uranyl and thorium speciation needs to take account of database comprehensiveness and cohesiveness, including the capability of the model and database to simulate interactions with dissolved organic matter. Measurement of speciation in natural waters is needed to provide data that may be used to assess and improve model capabilities and to better constrain the type of predictive modelling work presented here.

Linking the chemical speciation of cerium to its bioavailability in water for a freshwater alga

Over the past decade, researchers have begun to use metals of the lanthanide family for numerous applications, including liquid crystal display (LCD) screens, optical fibers, and laser technology. Unfortunately, little is presently known about their bioavailability or the mechanisms by which they might cause toxicity. The present study focuses on cerium (Ce), one of the most widely used lanthanides, and on validating the biotic ligand model as a means to predict Ce bioaccumulation. Short-term exposures to Ce were performed using the unicellular alga Chlamydomonas reinhardtii, to better relate Ce bioavailability to its chemical speciation in solution. Maximum uptake fluxes (Jmax ) and affinity constants for the binding of Ce to the biological uptake sites (KS ) were established at pH 5.0 and pH 7.0. An apparent affinity constant of 1.8 × 10(7) M(-1) was observed at pH 5.0, with a larger value obtained at pH 7.0 (6 × 10(7) M(-1) ), albeit under conditions where equilibrium could not be confirmed. By evaluating Ce speciation using centrifugal ultrafiltration and single-particle inductively coupled plasma spectrometry, it could be concluded that very little (∼30%) Ce was truly dissolved at pH 7.0, with the majority of the metal being present in colloidal species. Speciation was also monitored by fluorescence to evaluate Ce complexation by natural organic matter (NOM). The presence of NOM decreased Ce bioaccumulation in line with free Ce concentrations. Finally, competition with calcium for the metal uptake sites was shown to result in a decrease in Ce uptake by C. reinhardtii.

Impact of environmentally based chemical hardness on uranium speciation and toxicity in six aquatic species

Treated effluent discharge from uranium (U) mines and mills elevates the concentrations of U, calcium (Ca), magnesium (Mg), and sulfate (SO4 (2-) ) above natural levels in receiving waters. Many investigations on the effect of hardness on U toxicity have been experiments on the combined effects of changes in hardness, pH, and alkalinity, which do not represent water chemistry downstream of U mines and mills. Therefore, more toxicity studies with water chemistry encountered downstream of U mines and mills are necessary to support predictive assessments of impacts of U discharge to the environment. Acute and chronic U toxicity laboratory bioassays were realized with 6 freshwater species in waters of low alkalinity, circumneutral pH, and a range of chemical hardness as found in field samples collected downstream of U mines and mills. In laboratory-tested waters, speciation calculations suggested that free uranyl ion concentrations remained constant despite increasing chemical hardness. When hardness increased while pH remained circumneutral and alkalinity low, U toxicity decreased only to Hyalella azteca and Pseudokirchneriella subcapitata. Also, Ca and Mg did not compete with U for the same uptake sites. The present study confirms that the majority of studies concluding that hardness affected U toxicity were in fact studies in which alkalinity and pH were the stronger influence. The results thus confirm that studies predicting impacts of U downstream of mines and mills should not consider chemical hardness. Environ Toxicol Chem 2015;34:562-574. © 2014 The Authors. Published by Wiley Periodicals, Inc. on behalf of SETAC.

The relationship between metal toxicity and biotic ligand binding affinities in aquatic and soil organisms: a review

The biotic ligand model (BLM) is a theoretical, potentially mechanistic approach to assess metal bioavailability in soil and aquatic systems. In a BLM, toxicity is linked to the fraction of biotic ligand occupied, which in turn, depends on the various components of the solution, including activity of the metal. Bioavailability is a key factor in determining toxicity and uptake of metals in organisms. In this study, the present status of BLM development for soil and aquatic organisms is summarized. For all species and all metals, toxicity was correlated with the conditional biotic ligand binding constants. For almost all organisms, values for Ag, Cu, and Cd were higher than those for Zn and Ni. The constants derived for aquatic systems seem to be equally valid for soil organisms, but in the case of soils, bioavailability from the soil solution is greatly influenced by the presence of the soil solid phase.

Exploring ecotoxicological fish bioassay for the evaluation of uranium reprotoxicity

Although reproduction in fish is known to be sensitive to metal exposure, few ecotoxicological studies have focused on the toxicological effects of metals. Because uranium (U) is naturally present in aquatic ecosystems (0.6-2 mg/L), freshwater organisms are subjected to chronic U exposure. Although new standardized assays are currently being developed to mimic realistic exposure conditions, they could be improved by taking into account the contamination that occurs throughout the life cycle of fish. The authors initially evaluated the effect of food (commercial flakes vs pure Spirulina) and ionic composition of the exposure medium on the reproductive performance of Danio rerio. The effects of U exposure on reproduction then were assessed 1) for the F0 adult stage at short exposure times (5 d, 20 d, and 40 d), and 2) for the F0 stage and the F1 generation after 200 d of exposure to control, low (20 µg U/L), and moderate (250 µg U/L) waterborne levels of U. Reproductive endpoints (reproductive success, fecundity, number of spawns, egg and larvae viability, and hatching) were measured mainly after the first spawn and after 10 d of cumulative spawns. The authors evaluated the plasticity of these endpoints and compared the effect of exposure conditions to identify the most relevant markers of the effect of U exposure on reproductive performance of D. rerio.

Uranium accumulation and toxicity in the green alga Chlamydomonas reinhardtii is modulated by pH

The effects of pH on metal uptake and toxicity in aquatic organisms are currently poorly understood and remain an evolving topic in studies about the biotic ligand model (BLM). In the present study, the authors investigated how pH may influence long-term (4 d) uranium (U) accumulation and chronic toxicity in batch cultures of the freshwater green alga Chlamydomonas reinhardtii. The toxicity expressed as a function of the free uranyl ion was much greater at pH 7 (effective concentration, 50% [EC50] = 1.8 × 10(-9)  M UO2 (2+) ) than at pH 5 (EC50 = 1.2 × 10(-7)  M UO2 (2+) ). The net accumulation rate of U in algal cells was much higher at pH 7 than at pH 5 for the same free [UO2 (2+) ], but the cells exposed at pH 5 were also more sensitive to intracellular U than the cells at pH 7 with EC50s of 4.0 × 10(-15) and 7.1 × 10(-13)  mol of internalized U cell(-1) , respectively. The higher cellular sensitivity to U at pH 5 than at pH 7 could be explained partly by the increase in cytosolic U binding to algal soluble proteins or enzymes at pH 5 as observed by subcellular fractionation. To predict U accumulation and toxicity in algae accurately, the important modulating effects of pH on U accumulation and U cellular sensitivity should be considered in the BLM.

In situ bioremediation of surface waters by periphytons

Environmentally benign and sustainable biomeasures have become attractive options for the in situ remediation of polluted surface waters. In this paper, we review the current state of reported experiments utilizing naturally occurring periphyton. These are microbial communities consisting of heterotrophic and photoautotrophic microorganisms that are reportedly capable of remediating surface waters which suffer from pollution due to a variety of contaminants. In our review, we focus on four aspects of bioremediation: multiple contaminant removal, the processes involved in contaminant removal, successful cell immobilization technologies and finally, the consideration of safety in aquaculture. It has been noted that recent developments in immobilization technologies offer a fresh approach facilitating the application of periphyton. The use of periphyton biofilm overcomes several disadvantages of single species microbial aggregates. The inclusion of periphyton, as a stable micro-ecosystem, is a promising in situ strategy to restore decimated surface water ecosystems.

Insights into the interactions of cyanobacteria with uranium

Due to various activities associated with nuclear industry, uranium is migrated to aquatic environments like groundwater, ponds or oceans. Uranium forms stable carbonate complexes in the oxic waters of pH 7-10 which results in a high degree of uranium mobility. Microorganisms employ various mechanisms which significantly influence the mobility and the speciation of uranium in aquatic environments. Uranyl bioremediation studies, this far, have generally focussed on low pH conditions and related to adsorption of positively charged UO2 (2+) onto negatively charged microbial surfaces. Sequestration of anionic uranium species, i.e. [UO2(CO3) 2 (2-) ] and [UO2(CO3) 3 (4-) ] onto microbial surfaces has received only scant attention. Marine cyanobacteria are effective metal adsorbents and represent an important sink for metals in aquatic environment. This article addresses the cyanobacterial interactions with toxic metals in general while stressing on uranium. It focusses on the possible mechanisms employed by cyanobacteria to sequester uranium from aqueous solutions above circumneutral pH where negatively charged uranyl carbonate complexes dominate aqueous uranium speciation. The mechanisms demonstrated by cyanobacteria are important components of biogeochemical cycle of uranium and are useful for the development of appropriate strategies, either to recover or remediate uranium from the aquatic environments.

Water hardness reduces the accumulation and toxicity of uranium in a freshwater macrophyte (Ceratophyllum demersum)

There is a lack of good quality data and mechanistic understanding on the effects of true water hardness (calcium (Ca) and magnesium (Mg)) on the bioavailability and toxicity of uranium (U) to freshwater biota. This study determined the effect of true water hardness (20, 75, 150, 275 and 400 mg CaCO(3) L(-1)) on the cell surface binding affinity (log K), accumulation and toxicity (growth inhibition) of U in a submerged, rootless, macrophyte (Ceratophyllum demersum) in a synthetic freshwater with constant alkalinity (13 mg CaCO(3) L(-1)) and pH (6.2) over 7 days. A 20-fold increase in water hardness resulted in a 4-fold decrease in U toxicity (median effect concentration (EC50)=134 μg L(-1)U at 20 mg CaCO(3 )L(-1) hardness, increasing to 547 μg L(-1) U at 400 mg CaCO(3) L(-1) hardness), cell surface binding affinity (log K=6.25 at 20 mg CaCO(3) L(-1) hardness, decreasing to log K=5.64 at 400 mg CaCO(3) L(-1) hardness) and accumulation (the concentration factor decreased from 63 at 20 mg CaCO(3) L(-1) hardness to 15 at 400 mg CaCO(3) L(-1) hardness) of U. Calcium provided a 4-fold greater protective effect against U accumulation and toxicity compared to Mg. Speciation calculations indicated negligible differences in the percentages of key U species (UO(2)(2+), UO(2)OH(+), UO(2)(OH)(2)) over the range of water hardness tested. The inhibition of U binding at the cell surface, and subsequent uptake, by C. demersum, with increasing Ca and/or Mg concentration, may be explained in terms of (i) competition between Ca(2+)/Mg(2+) and UO(2)(2+) (and/or UO(2)OH(+)) for physiologically active sites at the cell surface, and/or (ii) reduced negative charge (electrical potential) at the cell surface, resulting in a decrease in the activity of UO(2)(2+) (and/or UO(2)OH(+)) at the plant/water interface (boundary layer), and consequently, less U bound to physiologically active cell surface sites. In the absence of a biotic ligand model for U, the results of this study (together with previous work) reinforce the need for a more flexible, hardness-dependent, U guideline for the protection of selected freshwater biota.

Effects of uranium on the metabolism of zebrafish, Danio rerio

The increasing demand for nuclear energy results in heightened levels of uranium (U) in aquatic systems which present a potential health hazard to resident organisms. The aim of this study was to mechanistically assess how chronic exposure to environmentally relevant concentrations of U perturbs the complex interplay between feeding, growth, maintenance, maturation and reproduction throughout the life-cycle of an individual. To this end we analysed literature-based and original zebrafish toxicity data within a same mass and energy balancing conceptual framework. U was found to increase somatic maintenance leading to inhibition of spawning as well as increase hazard rate and costs for growth during the early life stages. The fish's initial conditions and elimination through reproduction greatly affected toxico-kinetics and effects. We demonstrate that growth and reproduction should be measured on specific individuals since mean values were hardly interpretable. The mean food level differed between experiments, conditions and individuals. This last 'detail' contributed substantially to the observed variability by its combined effect on metabolism, toxic effects and toxico-kinetics. The significance of this work is that we address exactly how these issues are related and derive conclusions which are independent of experimental protocol and coherent with a very large body of literature on zebrafish eco-physiology.

Interaction of uranium with a filamentous, heterocystous, nitrogen-fixing cyanobacterium, Anabaena torulosa

The filamentous, heterocystous, diazotrophic cyanobacterium, Anabaena torulosa was found to bind uranium from aqueous solutions containing 100 μM uranyl carbonate at pH 7.8. The uranyl sequestration kinetics exhibited (a) an initial rapid phase, binding 48% uranium within 30 min resulting in a loading of 56 mg U g(-1) of dry wt, followed by (b) a slower phase, binding 65% uranium with resultant loading of 77.35 mg U g(-1) in 24h. Energy Dispersive X-ray fluorescence spectroscopy of uranium loaded biomass revealed all components of UL X-rays (UL(l), UL(α), UL(β1) and UL(β2)). Heat killed cells or extracellular polysaccharides derived from live cells exhibited limited uranyl binding (~26%) highlighting the importance of cell viability for optimum uranyl binding. The present study revealed the involvement of acid soluble polyphosphates in uranium accumulation by this brackish water cyanobacterium.

Divalent base cations hamper Hg(II) uptake

Despite the alarming trends of declining base cation concentrations in boreal lakes, no studies have attempted to predict the consequences of this decline on the geochemical cycle of mercury, a top priority contaminant worldwide. In this study, we used a whole-cell gram-negative bioreporter to evaluate the direction and magnitude of changes in net accumulation of Hg(II) by bacteria in response to changing base cation concentrations. We show that regardless of the speciation of Hg(II) in solution, increasing divalent base cation concentrations decrease net Hg(II) accumulation by the bioreporter, suggesting a protective effect of these cations. Our work suggests that the complexity of the cell wall of gram-negative bacteria must be considered when modeling Hg uptake pathways; we propose that base divalent cations contribute to hamper net Hg(II) accumulation by decreasing outer membrane permeability and, therefore, the passive diffusion of Hg(II) species to the periplasmic space. This work points to an unsuspected and likely harmful consequence of a delay in recovering from acidification in boreal lakes, in that uptake of Hg(II) by bacteria is not only enhanced by the reduced pH but can also be enhanced by a decline in base cation levels.

Are antioxidant and transcriptional responses useful for discriminating between chemo- and radiotoxicity of uranium in the crayfish Procambarus clarkii?

The main objectives of this study were to evaluate uranium (U) toxicity in the crayfish Procambarus clarkii at a low dose of exposure and to discriminate between the chemotoxicity and radiotoxicity of U. We conducted two sets of experiments using either 30 μg L(-1) of depleted uranium (DU) or (233)U, which differ from each other only in their specific activity (DU=1.7×10(4)Bqg(-1), (233)U=3.57×10(8)Bqg(-1)). The endpoints were oxidative stress responses and mitochondrial functioning in the gills and hepatopancreas, which were measured in terms of enzyme activities and gene expression levels. U accumulation levels were measured in different organs (gills, hepatopancreas, stomach, intestine, green gland, muscles, and carapace), and internal dose rates in the hepatopancreas were compared after DU and (233)U exposures. Significant U accumulation occurred in the organs of P. clarkii, and mitochondrial damage and antioxidant responses were detected. Despite the huge difference (21,000×) in the specific activities of DU and (233)U, few significant differences in biological responses were detected in P. clarkii exposed to these two pollutants. This finding indicates that the radiotoxicity was low compared to the chemotoxicity under our exposure conditions. Finally, genes expression levels were more sensitive markers of U toxicity than enzyme activities.

Effects of uranium on crayfish Procambarus clarkii mitochondria and antioxidants responses after chronic exposure: what have we learned?

We examined the impacts of Uranium (U) on mitochondria and on the response of antioxidants in the gills and the hepatopancreas of crayfish Procambarus clarkii after long-term exposure (30 and 60 days) to an environmentally relevant concentration (30 μg U/L). The expression of mitochondrial genes (12s, atp6, and cox1), as well as the genes involved in oxidative stress responses (sod(Mn) and mt) were evaluated. The activities of antioxidant enzymes (SOD, CAT, GPX and GST) were also studied. U accumulation in organs induced changes in genes' expression. The evolution of these transcriptional responses and differences between gene expression levels at high and low doses of exposure were also discussed. This study demonstrated that, after long-term exposure, U caused a decrease in antioxidant activities and induced oxidative stress. A possible ROS-mediated U cytotoxic mechanism is proposed. Expression levels of the investigated genes can possibly be used as a tool to evaluate U toxicity and seem to be more sensitive than the enzymatic activities. However a multiple biomarker approach is recommended as the perturbed pathways and the mode of action of this pollutant are not completely understood.

Effects of uranium uptake on transcriptional responses, histological structures and survival rate of the crayfish Procambarus clarkii

This work aims to investigate the accumulation levels and effects (transcriptional responses, histopathology and survival rate) associated with a wide range of dissolved uranium (U) concentrations (0, 0.03, 0.6, 4 and 8 mg/L of U) on adult male crayfish Procambarus clarkii during 4 (T4) and 10 (T10) days of exposure. The follow-up of the crayfish mortality showed that P. clarkii was highly resistant to U. Increasing waterborne U concentrations led to increasing bioaccumulation in key crayfish organs and increasing histological damages. U distribution in tissues was also evaluated using transmission electron microscopy and showed the presence of a detoxified form of U in the gill's epithelium in the shape of flakes. Expression levels of mitochondrial genes (cox1, atp6 and 12S gene) and genes involved in oxidative stress (sod(Mn) and mt) were examined together with the housekeeping gene 18S. atp6 and mt genes of P. clarkii were cloned and sequenced before analysis. Significant correlations were observed between U bioaccumulation and the down-regulation of both cox1 and sod(Mn) genes. This work provides a first U toxicogenomic and histopathological pattern of P. clarkii, identify U biomarkers and associate gene expression endpoints to accumulation levels. It also provides new insights into the mechanisms involved in U stress.

Isolation and analyses of uranium tolerant Serratia marcescens strains and their utilization for aerobic uranium U(VI) bioadsorption

Enrichment-based methods targeted at uranium-tolerant populations among the culturable, aerobic, chemo-heterotrophic bacteria from the subsurface soils of Domiasiat (India's largest sandstone-type uranium deposits, containing an average ore grade of 0.1 % U(3)O(8)), indicated a wide occurrence of Serratia marcescens. Five representative S. marcescens isolates were characterized by a polyphasic taxonomic approach. The phylogenetic analyses of 16S rRNA gene sequences showed their relatedness to S. marcescens ATCC 13880 (≥99.4% similarity). Biochemical characteristics and random amplified polymorphic DNA profiles revealed significant differences among the representative isolates and the type strain as well. The minimum inhibitory concentration for uranium U(VI) exhibited by these natural isolates was found to range from 3.5-4.0 mM. On evaluation for their uranyl adsorption properties, it was found that all these isolates were able to remove nearly 90-92% (21-22 mg/L) and 60-70% (285-335 mg/L) of U(VI) on being challenged with 100 μM (23.8 mg/L) and 2 mM (476 mg/L) uranyl nitrate solutions, respectively, at pH 3.5 within 10 min of exposure. his capacity was retained by the isolates even after 24 h of incubation. Viability tests confirmed the tolerance of these isolates to toxic concentrations of soluble uranium U(VI) at pH 3.5. This is among the first studies to report uranium-tolerant aerobic chemoheterotrophs obtained from the pristine uranium ore-bearing site of Domiasiat.

Internal distribution of uranium and associated genotoxic damages in the chronically exposed bivalve Corbicula fluminea

Uranium (U) internal distribution and involved effects in the bivalve Corbicula fluminea have been studied after direct chronic exposure (90 d, 10 μg.L-1). U distribution was assessed at the subcellular level (Metal Rich Granules -MRG-, pellets and cytosol fractions) in two main organs of the bivalve (gills and visceral mass). Micro-localisation was investigated by TEM-EDX analysis in the gills epithelium. DNA damage in gill and hemolymph samples was measured by the Comet assay. The 90-d exposure period led to a significant increase of U concentration in gills over time (× 5) and a large U quantity in subcellular granules in gills. Finally, a significant increase (× 2) in DNA damage was noted in exposed gills and haemocytes. This study shows that the accumulation levels and consequently the potential toxicity cannot be successfully predicted only on the basis of concentration in water or in tissues and subcellular fractions after chronic exposure.

Effects of dietary uranium on reproductive endpoints--fecundity, survival, reproductive success--of the fish Danio rerio

Exposure to metal-contaminated water has been shown to result in a number of reproductive abnormalities in adult and larvae fish, such as failure of oocyte maturation and teratogenic effects. Recently, dietary uptake of metals by fish has been recognized as a critical route of exposure, however, the mechanisms of metal uptake and toxicity are poorly understood and in need of further investigation. The objectives of the present study are to quantify uranium (U dietary transfers from spiked artificial diets) in Danio rerio tissues and embryos, as well as establish its effect on reproduction and embryonic development. Uranium's environmental prominence is currently increasing because of new mining and milling activities. Uranium concentrations range from 0.02 µg/L in natural waters to 2 mg/L. The focus of this study was to examine the trophic transfer and effects of U following exposure modalities (dose, exposure duration 1 to 20 d). Two different isotopes were used to distinguish between chemical and radioactivity toxicity of U. Results showed that U trophic transfer was low (0.52%). Uranium tissue distributions showed that accumulation occurred in digestive organs (liver, digestive tract) following dietary exposure. High levels of U were measured in the gonads (female in particular, >20% of relative burden). High U accumulation levels in eggs indicated maternal transfer of the contaminant. Moreover, U trophic exposure led to a reduction in reproduction success as a function of U accumulated levels. High U exposure conditions strongly reduced the total number of eggs (50%) and their viability at 10 d (reduction of the clutch number, low quality of eggs).

Uranium toxicity and speciation during chronic exposure to the tropical freshwater fish, Mogurnda mogurnda

The effects of chronic uranium (U) exposure on larval Northern trout gudgeon, Mogurnda mogurnda, were assessed in two experiments using a newly-developed 28d survival and growth toxicity test. Significant effects were observed in both tests, but toxicity was markedly higher in Test 2 than Test 1. The LC50s for Tests 1 and 2 were 2090microgL(-1) and 1070microgL(-1), respectively. Larval growth IC10s for Tests 1 and 2 were 860microgL(-1) and 660microgL(-1) (dry weight), and 1160microgL(-1) and 850microgL(-1) (length), respectively. Uranium speciation modelling showed that a lower pH in Test 2 (mean of 6.0) compared to Test 1 (mean of 6.7) resulted in a greater proportion of free uranyl ion (UO(2)(2+)), the predominant bioavailable form of U. A higher dissolved organic carbon concentration (DOC) in Test 2 (4.2mgL(-1)) compared to Test 1 (2.1mgL(-1)) resulted in a higher proportion of U-DOC in Test 2, but this was insufficient to counter the effect of pH on the proportion of UO(2)(2+). The difference in U toxicity between the two tests could be explained by normalising for UO(2)(2+); the concentrations of UO(2)(2+) at the LC50s for Tests 1 and 2 were calculated to be 13.3 and 13.7microgL(-1), respectively. Finally, the results of this study, and comparisons with other studies suggest that U toxicity to M. mogurnda appears to be as much, if not more, a function of exposure water quality and feeding regime, as exposure duration.

Effects of Mg(2+) and H(+) on the toxicity of Ni(2+) to the unicellular green alga Pseudokirchneriella subcapitata: model development and validation with surface waters

In this study, increasing Mg concentrations and decreasing pH were observed to decrease Ni toxicity to the green alga Pseudokirchneriella subcapitata. To investigate to what extent the original biotic ligand model (BLM) concept could explain Ni toxicity as a function of water chemistry, the protective effects of Mg(2+) and H(+) were modeled as BLM-type single-site competition effects. The model parameters representing these effects were log K(MgBL)=3.3 and log K(HBL)=6.5. The BLM was capable of predicting Ni toxicity by an error of less than a factor of 2 in most synthetic and natural waters used in this study. However, since the relationship between 72-h E(r)C50(Ni(2+)) (i.e. the 72-h E(r)C50 expressed as Ni(2+) activity) and H(+) activity was not linear over the entire tested pH range, only the 'linear part' between pH 6.45 and 7.92 was used for derivation of log K(HBL). This nonlinearity indicates that the effect of pH can probably not be attributed to H(+) competition with Ni(2+) for a single site alone. When modeling the effect of pH as a linear relation between 72-h E(r)C50(pNi(2+)) (=-log (72-h E(r)C50(Ni(2+)) corrected for the presence of Mg)) and pH, the applicability of the model was successfully extended to pH levels as low as 6.01. This type of empirical model has also been used in our previous studies on the development of a chronic Ni bioavailability model for Daphnia magna and a long-term Ni bioavailability model for rainbow trout. Finally, we could not detect a statistically significant interactive effect of pH and Mg on the toxicity of Ni(2+) to P. subcapitata and this is in line with the formulation of our empirical model.

Phototrophic biofilms and their potential applications

Phototrophic biofilms occur on surfaces exposed to light in a range of terrestrial and aquatic environments. Oxygenic phototrophs like diatoms, green algae, and cyanobacteria are the major primary producers that generate energy and reduce carbon dioxide, providing the system with organic substrates and oxygen. Photosynthesis fuels processes and conversions in the total biofilm community, including the metabolism of heterotrophic organisms. A matrix of polymeric substances secreted by phototrophs and heterotrophs enhances the attachment of the biofilm community. This review discusses the actual and potential applications of phototrophic biofilms in wastewater treatment, bioremediation, fish-feed production, biohydrogen production, and soil improvement.

Water-sediment interactions for Hyalella azteca exposed to uranium-spiked sediment

Data on the toxicity of uranium in sediments to Hyalella azteca and the effect of overlying water chemistry are limited. This study exposed H. azteca to sediments spiked with U (0-10,000 microg U/g dry weight) and five different overlying waters, which varied independently in hardness and alkalinity. Water pH had a major effect on U bioavailability and uptake by H. azteca. Uranium toxicity was higher when overlying water pH was low, while desorption of U into the overlying water increased with increasing pH. There appears to be little effect of Ca on U uptake, other than its influence on U speciation. Experiments with caged animals indicate that U accumulation and toxicity occur mainly through the dissolved phase rather than the solid phase. Uranium bioaccumulation is a more reliable indicator of U toxicity than U concentration in water or sediment. Uranium bioaccumulation in the H. azteca and U adsorption to sediment can be satisfactorily explained using saturation models.

Effects of waterborne uranium on survival, growth, reproduction and physiological processes of the freshwater cladoceran Daphnia magna

Acute uranium toxicity (48 h immobilisation test) for Daphnia magna was determined in two different exposure media, differing in pH and alkalinity. LC(50) varied strongly between media, from 390+/-40 microgL(-1)U at pH 7 to 7.8+/-3.2 mgL(-1)U at pH 8. According to the free ion activity model uranium toxicity varies as a function of free uranyl concentration. This assumption was examined by calculating uranium speciation in our water conditions and in those reported in the literature. Predicted changes in free uranyl concentration could not solely explain observed differences in toxicity, which might be due to a competition or a non-competitive inhibition of H(+) for uranium transport and/or the involvement of other bioavailable chemical species of uranium. Chronic effects of uranium at pH 7 on mortality, ingestion and respiration, fecundity and dry mass of females, eggs and neonates were investigated during 21-day exposure experiments. A mortality of 10% was observed at 100 microgL(-1)U and EC(10) for reproduction was 14+/-7 microgL(-1)U. Scope for growth was affected through a reduction in feeding activity and an increase in oxygen consumption at 25 microgL(-1)U after 7 days of exposure. This had strong consequences for somatic growth and reproduction, which decreased, respectively, by 50% and 65% at 50 microgL(-1)U after 7 days and at 25 microgL(-1)U after 21 days. Uranium bioaccumulation was quantified and associated internal alpha dose rates from 2.1 to 13 microGyh(-1) were estimated. Compared to the toxicity of other alpha-emitting radionuclides and stable trace metals, our results confirmed the general assumption that uranium chemical toxicity predominates over its radiotoxicity.