Radionuclide biogeochemistry: from bioremediation toward the treatment of aqueous radioactive effluents

Civilian and military nuclear programs of several nations over more than 70 years have led to significant quantities of heterogenous solid, organic, and aqueous radioactive wastes bearing actinides, fission products, and activation products. While many physicochemical treatments have been developed to remediate, decontaminate and reduce waste volumes, they can involve high costs (energy input, expensive sorbants, ion exchange resins, chemical reducing/precipitation agents) or can lead to further secondary waste forms. Microorganisms can directly influence radionuclide solubility, via sorption, accumulation, precipitation, redox, and volatilization pathways, thus offering a more sustainable approach to remediation or effluent treatments. Much work to date has focused on fundamentals or laboratory-scale remediation trials, but there is a paucity of information toward field-scale bioremediation and, to a lesser extent, toward biological liquid effluent treatments. From the few biostimulation studies that have been conducted at legacy weapon production/test sites and uranium mining and milling sites, some marked success via bioreduction and biomineralisation has been observed. However, rebounding of radionuclide mobility from (a)biotic scale-up factors are often encountered. Radionuclide, heavy metal, co-contaminant, and/or matrix effects provide more challenging conditions than traditional industrial wastewater systems, thus innovative solutions via indirect interactions with stable element biogeochemical cycles, natural or engineered cultures or communities of metal and irradiation tolerant strains and reactor design inspirations from existing metal wastewater technologies, are required. This review encompasses the current state of the art in radionuclide biogeochemistry fundamentals and bioremediation and establishes links toward transitioning these concepts toward future radioactive effluent treatments.

Potential of indigenous bacteria driven U(VI) reduction under relevant deep geological repository (DGR) conditions

Understanding the biogeochemical U redox processes is crucial for controlling U mobility and toxicity under conditions relevant to deep geological repositories (DGRs). In this study, we examined the microbial reduction of aqueous hexavalent uranium U(VI) [U(VI)aq] by indigenous bacteria in U-contaminated groundwater. Three indigenous bacteria obtained from granitic groundwater at depths of 44-60 m (S1), 92-116 m (S2), and 234-244 m (S3) were used in U(VI)aq bioreduction experiments. The concentration of U(VI)aq was monitored to evaluate its removal efficiency for 24 weeks under anaerobic conditions with the addition of 20 mM sodium acetate. During the anaerobic reaction, U(VI)aq was precipitated in the form of U(IV)-silicate with a particle size >100 nm. The final U(VI)aq removal efficiencies were 37.7%, 43.1%, and 57.8% in S1, S2, and S3 sample, respectively. Incomplete U(VI)aq removal was attributed to the presence of a thermodynamically stable calcium uranyl carbonate complex in the U-contaminated groundwater. High-throughput 16S rRNA gene sequencing analysis revealed the differences in indigenous bacterial communities in response to the depth, which affected to the U(VI)aq removal efficiency. Pseudomonas peli was found to be a common bacterium related to U(VI)aq bioreduction in S1 and S2 samples, while two SRB species, Thermodesulfovibrio yellowstonii and Desulfatirhabdium butyrativorans, played key roles in the bioreduction of U(VI)aq in S3 sample. These results indicate that remediation of U(VI)aq is possible by stimulating the activity of indigenous bacteria in the DGR environment.

Impact of the Fukushima Dai-ichi Nuclear Power Plant Accident on dolphin fishes in the Northwest Pacific

More than 9 years since the Fukushima Dai-ichi Nuclear Power Plant Accident (FDNPPA), the impact of FDNPPA on marine biota is being revealed. In this work, the evolution of FDNPPA derived ¹³⁴Cs, ¹³⁷Cs and ^110mAg in dolphin fishes (Coryphaena hippurus) in the Northwest Pacific from Dec. 2011 to Sept. 2018 were studied. Bioconcentration factors (BCF) of radiocesium (29-69 with the average of 48) in dolphin fishes were calculated. The background level of ¹³⁷Cs in dolphin fishes (<0.14 Bq/kg_{fresh weight}) before FDNPPA was estimated. The radioactive levels of these three radionuclides in dolphin fishes decreased with time. Among them, ¹³⁴Cs and ^110mAg decreased at the half-lives of 158 days and 54 days at the population level, respectively. After May 2014, ¹³⁴Cs and ^110mAg cannot be detected and the activity of ¹³⁷Cs returned to the background level before FDNPPA. Radiation dose assessment demonstrated that it was far from causing radiation harm to dolphin fishes in the open ocean of Northwest Pacific and humans who ingested them.

Numerical study of transport pathways of 137Cs from forests to freshwater fish living in mountain streams in Fukushima, Japan

The accident at the Fukushima Dai-ichi Nuclear Power Plant in 2011 released a large quantity of radiocesium into the surrounding environment. Radiocesium concentrations in some freshwater fish caught in rivers in Fukushima Prefecture in October 2018 were still higher than the Japanese limit of 100 Bq kg^-1 for general foodstuffs. To assess the uptake of ¹³⁷Cs by freshwater fish living in mountain streams in Fukushima Prefecture, we developed a compartment model for the migration of ¹³⁷Cs on the catchment scale from forests to river water. We modelled a generic forest catchment with Fukushima-like parameters to ascertain the importance of three export pathways of ¹³⁷Cs from forests to river water for the uptake of ¹³⁷Cs by freshwater fish. The pathways were direct litter fall into rivers, lateral inflow from the forest litter layer, and lateral transfer from the underlying forest soil. Simulation cases modelling only a single export pathway did not reproduce the actual trend of ¹³⁷Cs concentrations in river water and freshwater fish in Fukushima Prefecture. Simulations allowing a combined effect of the three pathways reproduced the trends well. In the latter simulations, the decreasing trend of ¹³⁷Cs in river water and freshwater fish was due to a combination of the decreasing trend in the forest leaves/needles and litter compartments, and the increasing trend in soil. The modelled ¹³⁷Cs concentrations within the forest compartments were predicted to reach an equilibrium state at around ten years after the fallout due to the equilibration of ¹³⁷Cs cycling in forests. The model suggests that long term ¹³⁷Cs concentrations in freshwater fish in mountain streams will be controlled by the transfer of ¹³⁷Cs to river water from forest organic soils.

Natural and anthropogenic radionuclides in Norwegian farmed Atlantic salmon (Salmo salar)

Norway is one of the main producers of farmed fish and the world's second-largest exporter of seafood. Farmed Atlantic salmon (Salmo salar) represents the most exported species. This is the first comprehensive survey of anthropogenic (¹³⁷Cs, ⁹⁰Sr, ²³⁸Pu, ^239,240Pu and ²⁴¹Am) and natural (⁴⁰K, ²²⁶Ra, ²²⁸Ra, ²¹⁰Pb, ²¹⁰Po) radionuclides in farmed salmon and manufactured fish feed from Norway. The only anthropogenic radionuclide detected in salmon and fish feed was ¹³⁷Cs. The levels were low with arithmetic means in salmon and feed of 0.13 and 0.30 Bq/kg fresh weight (fw), respectively. The natural radionuclide ⁴⁰K exhibited the highest levels with arithmetic means in salmon and feed of 115 and 239 Bq/kg fw, respectively. The arithmetic means of ²¹⁰Po and ²¹⁰Pb in salmon were 0.013 and 0.044 Bq/kg fw, respectively, with a mean ²¹⁰Po:²¹⁰Pb activity ratio of 0.32. For fish feed, the situation was reversed: the arithmetic means of ²¹⁰Po and ²¹⁰Pb were 3.8 and 0.67 Bq/kg fw, respectively, with a mean ²¹⁰Po:²¹⁰Pb activity ratio of 5.7. The radionuclide levels found in farmed salmon in the present study are comparable to or lower than the levels found in other fish species in the North Atlantic Ocean. A highly conservative dose estimate for consumption showed that doses were no higher than 1.2 μSv/year for toddlers and 4.0 μSv/year for adults. This suggests that the risk associated with radioactivity in farmed salmon is very low even when considering individuals with high consumption and the highest radionuclide levels found in this study.

Effective UO22+ removal from aqueous solutions using lichen biomass as a natural and low-cost biosorbent

The UO₂²⁺ biosorption properties of a lichen, Evernia prunastri, from aqueous solutions were investigated. The widely occurring lichen samples were collected from the forest in Bilecik-Turkey. The UO₂²⁺ biosorption onto lichen was characterized by FT-IR and SEM-EDX analysis techniques before and after biosorption. The effects of the solution pH, biosorbent dosage, UO₂²⁺ concentration, contact time, and temperature on UO₂²⁺ biosorption on lichen sample were studied by using the batch method. The isotherm experimental data were described using isotherm models of Langmuir, Freundlich and Dubinin Radushkevich. The maximum UO₂²⁺ biosorption capacity of the lichen sample was estimated by the Langmuir equation to be 0.270 mol kg^-1. The adsorption energy from the Dubin Radushkevich model was found to be 8.24 kJ mol^-1. Kinetic data determined that the biosorption was best described by the pseudo-second-order kinetic model. Thermodynamic findings showed that the biosorption process was endothermic, entropy increased and spontaneous. In conclusion, the lichen appears to be a promising biosorbent for the removal of UO₂²⁺ ions from aqueous solutions because of high biosorption capacity, easy usability, low cost, and high reusability performance.

Strong contrast of cesium radioactivity between marine and freshwater fish in Fukushima

A proper understanding of radioactive contamination levels of food resources near the Fukushima Dai-ichi Nuclear Power Plant is necessary to estimate the potential effects of radionuclide contamination on human health. This study was conducted to present a direct comparison of radiocesium (¹³⁴Cs and ¹³⁷Cs) concentrations in marine and freshwater fish inhabiting different water bodies in Fukushima Prefecture (coastal waters, 6.3-54.5 km from the plant; forest rivers and irrigation ponds, 1.4-71.6 km), and to reveal plausible contamination mechanisms for each habitat. In contrast to marine demersal fish (7 species, n = 50), which showed lower and less variable radiocesium concentrations (0.234-3.41 Bq kg^-1-wet), freshwater fish (6 species, n = 463) showed higher and more site-specific variations for each species and habitat (4.09 Bq kg^-1-wet - 25.6 kBq kg^-1-wet) in 2015-2016. The apparent concentration ratio (aCR, L/kg) of ¹³⁷Cs in fish to water is higher for fish of freshwater habitats (mean 1240-12900 for each site) than in those of coastal waters (mean 200). Radiocesium contamination is more severe and persistent in freshwater fish, especially those distributed within the designated evacuation zone (salmon in rivers and bass in ponds). Continuous radiocesium uptake through the food web in relation to fish feeding habits and size (size effect), and biotic/abiotic characteristics in water and surrounding environments are main factors affecting site/habitat-specific bioaccumulation of radiocesium in freshwater fish. By contrast, uniformly lower radiocesium concentrations in marine demersal fish are mainly attributable to decreased radiocesium transfer intensity from the benthic food web because of lowered radiocesium contamination in sediments, and low physiological ability to retain radiocesium. Our results revealed a strong contrast of radiocesium contamination levels and mechanisms between marine and freshwater fish in natural habitats. Particularly, a close relation between ¹³⁷Cs accumulation in river salmon and contamination of prey items in forest ecosystems (mainly terrestrial and aquatic insects) is peculiar to the upstream areas affected by the Fukushima accident.

A 30-year record reveals re-equilibration rates of 137Cs in marine biota after the Fukushima Dai-ichi nuclear power plant accident: Concentration ratios in pre- and post-event conditions

Concentration ratios (CRs), expressed by dividing ¹³⁷Cs activity in seawater by that in marine biota (mainly fish), were obtained from the monitoring of ¹³⁷Cs in coastal areas around Japan between 1984 and 2016. Before the TEPCO Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident (1984-2010), mean CRs of ¹³⁷Cs, mainly from global fallout (i.e. CR_GF), were almost constant for each species throughout the monitoring period, but were different among species, while the values for several species were dependent on their length (i.e. CR_GF-SIZE). Thus, CR_GF and CR_GF-SIZE values for 29 of marketable species are given here as references for conditions where marine biota are in approximate equilibrium (or steady state) with their host water with respect to ¹³⁷Cs activities in the marine environment. After the FDNPP accident (2011-2016), the impact of the accident has been sustained in eastern Japan waters as indicated by apparent CRs (CR_as) which are being used here as indicators of disequilibrium between organisms and their host water. The recession rates of this disequilibrium (the effective CR_a half-lives) ranged from 100 to 1100 days. The identified distinct variation was due to the sample locations, even for the same species, because of the change in ¹³⁷Cs activity concentrations in their host water and diet preference differences. Variation among species, even those captured from the same area, was mainly due to diet differences as well as metabolic-physiological differences in ¹³⁷Cs retention. Thus, our results from >30 years of systematically monitoring have helped quantify the recession rates of post-FDNPP disequilibrium of ¹³⁷Cs in biota for assessment of how long term is required from contaminated condition by underlying spatial, inter- and intra-species factors.

Assessing tritium internalisation in zebrafish early life stages: Importance of rapid isotopic exchange

Tritium (³H) is mainly released into the environment in the form of tritiated water (HTO) by nuclear power plants and nuclear fuel reprocessing plants. To better understand how organisms may be affected by contamination to ³H it is essential to link observed effects to a correct estimation of absorbed dose rates. Due to quick isotopic exchanges between ³H and hydrogen, ³H measurement is difficult in small organisms such as zebrafish embryo, a model in ecotoxicological assay. This work aimed to optimise tritium measurement protocol to better characterise internalisation by early life stages of zebrafish. Zebrafish eggs were exposed at one HTO activity concentration of 1.22 × 10⁵ Bq/mL. This activity was calculated to correspond to theoretical dose rates of 0.4 mGy/h, where some deleterious effects are expected on young fish. A protocol for the preparation of biological samples was adapted from the method classically used to segregate the different forms of tritium in organisms. To deal with very quick isotopic exchanges of ³H with hydrogen, the impacts of washing by non-tritiated water as well as the bias induced by absorbed tritium around organisms on the measured activity concentration were studied. We managed to develop protocols to perform total tritium and total organically bound tritium (OBT) activity concentrations measurements in zebrafish eggs and larvae. The measurement of these both forms allowed the calculation of tissue-free-water-tritium (TFWT). To better understand total tritium internalisation, a study of total tritium kinetics from 4 hpf (hour post-fertilization) to 168 hpf was performed. OBT and TFWT were also assessed to complete the total tritium internalisation kinetics. The internalisation is a rapid phenomenon reaching a steady-state within 24 h after the beginning of contamination for total tritium and TFWT, with concentration factors and TFWT/HTO close to unity. OBT formation seemed to be slower. It appeared that OBT content in organisms was low with an OBT/TFWT ratio of about 8% for both stages (24 hpf and 96 hpf). To verify absorbed dose rates at key developmental stages (24 hpf eggs and 96 hpf larvae), they were calculated from total tritium activity concentrations after exposure at 1.22 × 10⁵ and 1.22 × 10⁶ Bq/mL, as these two activity concentrations were used to assess effects of tritium in another part of the study. Dose rates calculated from total tritium activity concentrations measured in 24 hpf eggs and 96 hpf larvae were consistent with the nominal ones, which validates the robustness of the protocol developed in the present study.

Bioaccumulation of 137Cs in anuran larvae utilizing a contaminated effluent canal on the U.S. Department of Energy's Savannah River Site

As a result of activities such as nuclear weapons testing, nuclear power generation and waste disposal, and nuclear accidents, radiocesium (¹³⁷Cs) is a widely distributed radio-contaminant of concern that readily accumulates in exposed wildlife. Although bioaccumulation of ¹³⁷Cs is an important factor for understanding its fate within the environment, there are currently limited data available on bioaccumulation patterns of ¹³⁷Cs in amphibians, despite their widespread distribution and potential to transport contaminants between aquatic and terrestrial ecosystems. Therefore, the objective of this study was to determine the amount of time necessary for anuran larvae experimentally placed in a contaminated system to reach a steady-state whole-body ¹³⁷Cs concentration, and to determine the threshold at which that steady-state ¹³⁷Cs concentration occurred for tadpoles within our study system. By restricting uncontaminated bullfrog (Lithobates catesbeianus) larvae to three experimental enclosures located along a¹³⁷Cs contaminated effluent canal on the U.S. Department of Energy's Savannah River Site, we modeled ¹³⁷Cs uptake through time using the von Bertalanffy modification of the Richards Model. The results of our modified Richards Model indicate that bullfrog tadpoles achieved steady-state ¹³⁷Cs concentrations of 3.68-4.34 Bq/g¹³⁷Cs dry whole-body weight after 11.63-15.50 days of exposure among sampling sites, with an average of 3.94 Bq/g after 14.07 days exposure. Radiocesium accumulation in bullfrog tadpoles was more rapid than that reported for other biota studied from other contaminated systems, likely due to incidental ingestion of sediments and a diet consisting of periphyton and other items that accumulate high levels of ¹³⁷Cs. Given their rapid accumulation of ¹³⁷Cs and inability to leave aquatic environments prior to metamorphosis, our data suggest amphibian larvae may be useful indicators for monitoring ¹³⁷Cs distributions and bioavailability within aquatic systems.

Kinetic and equilibrium of U(VI) biosorption onto the resistant bacterium Bacillus amyloliquefaciens

This study evaluated U(VI) biosorption properties by the resistant bacterium, Bacillus amyloliquefaciens, which was isolated from the soils with residual radionuclides. The effect of biosorption factors (uptake time, pH, ionic concentration, biosorbent dosage and temperature) on U(VI) removal was determined by batch experiments. The uptake processes were characterized by using SEM, FTIR, and XPS. The experimental data of U(VI) biosorption were fitted by the pseudo-second-order. The maximum uptake capacity was 179.5 mg/g at pH 6.0 by Langmuir model. The thermodynamic results: ΔG^о, ΔH^о and ΔS^о for uptake processes were calculated as -6.359 kJ/mol, 14.20 kJ/mol and 67.19 J/mol/K, respectively. The results showed that the biosorption of Bacillus amyloliquefaciens will be an ideal method to remove radionuclides.

Uranium biosorption by Lemna sp. and Pistia stratiotes

Biosorption-based technologies have been proposed for the removal of radionuclides from radioactive liquid waste containing organic compounds. Nevertheless, pytoremediation potential of uranium (U) by nonliving aquatic macrophytes Lemna sp. and Pistia stratiotes has not been previously addressed. In this study, uranium biosorption capacity by Pistia stratiotes and Lemna sp. was evaluated by equilibrium and kinetics experiments. The biomasses were added to synthetic and real waste solutions. The assays were tested in polypropylene vials containing 10 mL of uranium nitrate solution and 0.20 g of biomass. Solutions ranging from 0.25 to 84.03 mmol l^-1 were employed for the assessment of uranium concentration in each macrophyte. The equilibrium time was 1 h for both macrophytes. Lemna sp. achieved the highest sorption capacity with the use of the synthetic solution, which was 0.68 mmol g^-1 for the macrophyte. Since Lemna sp. exhibit a much higher adsorption capacity, only this biomass was exposed to the actual waste solution, being able to adsorb 9.24 × 10^-3 mmol g^-1 U (total). The results show that these materials are potentially applicable to the treatment of liquid radioactive waste.

Mechanisms of radiocesium depuration in Sebastes cheni derived by simulation analysis of measured 137Cs concentrations off southern Fukushima 2014-2016

The cesium depuration mechanisms were studied in Japanese rockfish Sebastes cheni off Fukushima, in which the radiocesium level remains higher than in other teleost. Samples were collected approximately 5 km south from the nuclear power plant during 2014-2016, and the ¹³⁷Cs concentrations in fish, stomach content and prey species were measured. The stable cesium content in fish was also analyzed and compared with fish age which was determined by annual ring analysis in otoliths. The ¹³⁷Cs concentrations in the dominant prey species, mysids and brown shrimp, were several Bq kg-w.w.^-1; indicating that transfer via the food chain was substantial compared to that from seawater during the study period. The ¹³⁷Cs concentrations in S. cheni decreased from 2014 to 2016 due to the metabolic excretion and the rate of decrease in its diet. Biokinetic model analyses confirmed the slower turnover of stable cesium in S. cheni, represented as a biological half-life (Tb_1/2) of 140-215 d, and was associated with stable Cs levels in food of 5-7 ng g-w.w.^-1. The ¹³⁷Cs levels in S. cheni were also simulated, which showed that the ¹³⁷Cs depuration in fish exposed to the initial contaminated plume in 2011 resulted from slower metabolic excretion, while the ¹³⁷Cs levels in fish born after 2012 could be regarded as equilibrated with the environmental levels of ¹³⁷Cs. Furthermore, the simulation results suggest that ¹³⁷Cs depuration in S. cheni population was also caused by the alternation of generation, which can be substantial by the addition of new year class population hatched after 2012 that were not contaminated by the initial contaminated plume from the 2011 accident.

Evaluation of body size and temperature on 137Cs uptake in marine animals

¹³⁷Cs bioaccumulation and retention in seven different marine animal species, including crustaceans, mollusc larvae, and fish larvae were compared under different temperature conditions (10 °C, 18 °C and 25 °C). Replicate animals were experimentally exposed to 0.5 nM ¹³⁷Cs dissolved in filtered seawater for 3 days, and their ¹³⁷Cs contents were periodically measured using gamma spectrometry. Among the seven species, ¹³⁷Cs bioconcentration factors ranged from 14 to 239 at the end of the exposure periods. Following uptake, the¹³⁷Cs loss rate constants from the animals ranged from 5 to 50% d^-1 and were unaffected by temperature or animal size. The ¹³⁷Cs bioconcentration factors were directly related to animal size and hence their surface: volume ratios, consistent with the conclusion that Cs sorption from the aqueous phase is the principal uptake mechanism in these animals. With the exception of gastropods, temperature had no major influence on Cs uptake and efflux in the experimental species.

Aspergillus niger changes the chemical form of uranium to decrease its biotoxicity, restricts its movement in plant and increase the growth of Syngonium podophyllum

Aspergillus niger (A. niger) and Syngonium podophyllum (S. podophyllum) have been used for wastewater treatment, and have exhibited a promising application in recent years. To determine the effects of A. niger on uranium enrichment and uranium stress antagonism of S. podophyllum, the S. podophyllum-A. niger combined system was established, and hydroponic remediation experiments were carried out with uranium-containing wastewater. The results revealed that the bioaugmentation of A. niger could increase the biomass of S. podophyllum by 5-7%, reverse the process of U(VI) reduction induced by S. podophyllum, and increase the bioconcentration factor (BCF) and translocation factor (TF) of S. podophyllum to uranium by 35-41 and 0.01-0.06, respectively, thereby improving the reduction of uranium in wastewater. Moreover, A. niger could promote the cell wall immobilization and the subcellular compartmentalization of uranium in the root of S. podophyllum, reduce the phytotoxicity of uranium entering root cells, and inhibit the calcium efflux from root cells, thereby withdrawing the stress of uranium on S. podophyllum.

137Cs and 40K in gray seals Halichoerus grypus in the southern Baltic Sea

This study presents levels of ¹³⁷Cs and ⁴⁰K concentrations in the placentas of seals gathered in the period 2007-2015. The mean activity of ¹³⁷Cs and ⁴⁰K was 5.49 Bq kg^-1w.w. and 136.6 Bq kg^-1 ww respectively. Statistically significant correlation was observed between the ¹³⁷Cs activities in placenta and in herring-the staple food for seals. The concentrations of ¹³⁷Cs and ⁴⁰K were also determined in other tissues (muscle, liver, lung, and brain) of wild seals. The concentrations of ¹³⁷Cs were from 2.59 Bq^-1 ww (lungs) to 24.3 Bq kg^-1 ww (muscles). The transfer factor values for ¹³⁷Cs (seal tissue/fish) ranged from 0.89 to 2.42 in the case of the placentas and from 1.35 to 8.17 in the case of the muscle. For adults seal, the effective dose from ¹³⁷Cs was 2.98 nGy h^-1. The mean external radiation dose to pup was 0.77 nGy h^-1 from ¹³⁷Cs and 6.69 nGy h^-1 from ⁴⁰K.

Numerical modelling of 137Cs content in the pelagic species of the Japanese Pacific coast following the Fukushima Dai-ichi Nuclear Power Plant accident using a size-structured food-web model

As result of the great east Japan earthquake on March 2011 and the damages of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), huge amount of radionuclides, especially ¹³⁷Cs, were released to the Japanese Pacific coast. By consequence, several marine species have been contaminated by direct uptake of radionuclides from seawater or through feeding on contaminated preys. In the present study we propose a novel radioecological modelling approach aiming to simulate the radionuclides transfer to pelagic marine species by giving to the organism body-size a key role in the model. We applied the model to estimate the ¹³⁷Cs content in 14 commercially important species of the North-Western Pacific Ocean after the FDNPP accident. Firstly, we validated the model and evaluated its performance using various observed field data, and we demonstrated the importance of using such modelling approach in radioecological studies. Afterwards, we estimated some radioecological metrics, such as the maximum activity concentration, its corresponding time and the ecological half-life, which are important in assessment of the previous, current and future contamination levels of the studied species. Finally, we estimated the time duration required for each species to reach the pre-accident ¹³⁷Cs activity concentrations. The results showed that the contamination levels in the planktivorous species have generally reached the pre-accident levels since about 5 years after the accident (since 2016). While in the case of the higher trophic level species, although the activity concentrations are much lower than the regulatory limit for radiocesium in seafood in Japan (100 Bq kg^-1), these species still require another 6–14 years (2018–2026) to reach the pre-accident levels.

Radiocesium accumulation in aquatic organisms: A global synthesis from an experimentalist's perspective

A better understanding of the fate of radiocesium in aquatic organisms is essential for making accurate assessments of potential impacts of radiocesium contamination on ecosystems and human health. Studies of the accumulation of ¹³⁴Cs, ¹³⁶Cs and ¹³⁷Cs in diverse biota have been the subject of many field investigations; however, it may often be difficult to understand all the mechanisms underlying the observations reported. To complement field investigations, laboratory experiments allow better understanding the observations and predicting dynamics of Cs within aquatic ecosystems by accurately assessing bioaccumulation of Cs in living organisms. The present review summarizes selected relevant laboratory studies carried out on Cs bioaccumulation in aquatic organisms over a period of more than 60 years. To date, 125 experimental studies have been carried out on 227 species of aquatic organisms since 1957. The present review provides a synthesis of the existing literature by highlighting major findings and identifying gaps of key information that need to be further addressed in future works on this topic. Thus, influences of some environmental parameters such as water chemistry both for marine and freshwater ecosystems, and biotic factors such as the life-stages and size of the organisms on radiocesium bioaccumulation should be examined and become priority topics for future research on Cs accumulation in aquatic organisms.

Accumulation and effects of uranium on aquatic macrophyte Nymphaea tetragona Georgi: Potential application to phytoremediation and environmental monitoring

This study analyzed the ability of Nymphaea tetragona Georgi (N. tetragona) to accumulate water-borne uranium and any effects this could exert on this plant species. In accumulation experiments, N. tetragona was exposed (21 d) to different concentrations of uranium (0-55 mg L^-1) and the content of uranium was determined in water and plant tissues (leaves, submerged position and plant) to determine the translocation factor (TF) and bioconcentration factor (BCF). The content of uranium in the plant and plant tissues showed concentration-dependent uptake, leaves were the predominant tissue for uranium accumulation, and TF and BCF values were both affected by the concentration of uranium in the water. In this research, the uranium content and BCF value in the leaves of N. tetragona were upto 3446 ± 155 mg kg^-1 and 73 ± 3, respectively. In physiological experiments, uranium treatment boosted the activity of peroxidase (POD), superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) in the leaves, and increasing uranium concentrations aggravated damage to the cell membrane system. Uranium contamination significantly inhibited the content of soluble protein, as well as chlorophyll-a, chlorophyll-b and carotene in the leaves, indicating the structure and function of chloroplast were destroyed, reducing the photosynthetic performance of plants. These results indicate that the macrophyte N. tetragona can accumulate uranium while showing a stress response via metabolic mechanisms under uranium exposure, and it may be a suitable bioremediation candidate for aquatic marine contamination.

Analysis of uranium removal capacity of anaerobic granular sludge bacterial communities under different initial pH conditions

The bacterial community of an anaerobic granular sludge associated with uranium depletion was investigated following its exposure to uranium under different initial pH conditions (pH 4.5, 5.5, and 6.5). The highest uranium removal efficiency (98.1%) was obtained for the sample with an initial pH of 6.5, which also supported the highest bacterial community richness and diversity. Venn diagrams visualized the decrease in the number of genera present in both the inoculum and the uranium-exposed biomass as the initial pH decreased from 6.5 to 4.5. Compared with the inoculum, a significant increase in the abundances of the phyla Chloroflexi and Proteobacteria was observed following uranium exposure. At initial pH conditions of 6.5 to 4.5, the proportions of the taxa Anaerolineaceae, Chryseobacterium, Acinetobacter, Pseudomonas, and Sulfurovum increased significantly, likely contributing to the observed uranium removal. Uranium exposure induced a greater level of dynamic diversification of bacterial abundances than did the initial pH difference.

Exploring taxonomic and phylogenetic relationships to predict radiocaesium transfer to marine biota

One potentially useful approach to fill data gaps for concentration ratios, CRs, is based upon the hypothesis that an underlying taxonomic and/or phylogenetic relationship exists for radionuclide transfer. The objective of this study was to explore whether these relationships could be used to explain variation in the transfer of radiocaesium to a wide range of marine organisms. CR data for ¹³⁷Cs were classified in relation to taxonomic family, order, class and phylum. A Residual Maximum Likelihood (REML) mixed-model regression modelling approach was adopted. The existence of any patterns were then explored using phylogenetic trees constructed with mitochondrial COI gene sequences from various biota groups and mapping the REML residual means onto these trees. A comparison of the predictions made using REML with blind datasets allowed the efficacy of the procedure to be tested. The only significant correlation between predicted and measured activity concentrations was revealed at the taxonomic level of order when comparing REML analysis output with data from the Barents Sea Region. For this single case a correlation 0.80 (Spearman rank) was derived which was significant at the 0.01 level (1-tailed test) although this was not the case once a (Bonferroni) correction was applied. The application of the REML approach to marine datasets has met with limited success, and the phylogenetic trees illustrate complications of using predictions based on values from different levels of taxonomic organization, where predicted values for the order level can mask the values at lower taxonomic levels. Any influence of taxonomy and phylogeny on transfer is not immediately conspicuous and categorizing marine organisms in this way is limited in providing a potentially robust prognostic extrapolation tool. Other factors may plausibly affect transfer to a much greater degree in marine systems, such as quite diverse life histories and different diets, which may confound any phylogenetic pattern.

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.

The absence of the pCO2 effect on dissolved 134Cs uptake in select marine organisms

Ocean acidification have been shown to not affect the capacity of bivalves to bioaccumulation ¹³⁴Cs in their tissue; but as this was studied on only one species to date. There is therefore a need to verify if this holds true for other bivalve species or other marine invertebrates. The present short communication confirms that in the scallop Mimachlamys varia and the prawn Penaeus japonicus, two species that supposedly have a record to preferentially concentrates this radionuclide, that bioconcentration of ¹³⁴Cs was shown not to be influenced by a decreasing pH (and thereby increasing seawater pCO₂). Although the dissolved ¹³⁴Cs was taken up in a similar manner under different pH values (8.1, 7.8, and 7.5) in both species, being described by a saturation state equilibrium model, the species displayed different bioconcentration capacities of ¹³⁴Cs: CF_ss in the prawns was approximately 10-fold higher than in scallops. Such results suggest that the Cs bioconcentration capacity are mainly dependent of the taxa and that uptake processes are independent the physiological ones involved in the biological responses of prawns and scallops to ocean acidification.

Radioecological aftermath: Maternal transfer of anthropogenic radionuclides to shark progeny is sustained and enhanced well beyond maternal exposure

Cartilaginous dogfish Scyliorhinus canicula continued to transfer four anthropogenic radionuclides (⁶⁵Zn, ⁶⁰Co, ¹³⁴Cs and ²⁴¹Am) to their eggs for over six months, after two months of continued maternal exposure to radio-labelled food. Unexpectedly, rates of radionuclide transfers to eggs and their yolk & embryo during maternal depuration were equivalent for ⁶⁰Co and ²⁴¹Am, or even enhanced for ⁶⁵Zn and ¹³⁴Cs by factors of c.200-350%, over two-three months, compared to their maximal transfer rates at the end of the maternal uptake phase. These rates of maternal transfer of radionuclides to yolk & embryo were positively associated with their previously determined efficiencies of assimilation (AE) from ingested radio-labelled food. Thus progeny may be more exposed via maternal transfer to those radionuclides which have greater rates of maternal assimilation from food. As maternal depuration continued beyond 60-80 up to 180-200 days the transfers of all four radionuclides to eggs did diminish but were still substantial at mean values of 18% for ²⁴¹Am, 17% for ¹³⁴Cs and 9 and 8% for ⁶⁰Co and ⁶⁵Zn, respectively. In the yolk & embryo the mean rates of transfer over this period were further reduced for ²⁴¹Am (13.5%), ⁶⁰Co (2.5%) and ⁶⁵Zn (5.8%), but were still appreciable for ¹³⁴Cs at 56%. These results for S. canicula have demonstrated a potential enhanced radiological risk of extended duration due to the particular biokinetics of maternal transfer in this species. This study draws further attention to the current paucity of knowledge about the maternal: progeny transfer pathway, particularly in the context of the known heightened radio-sensitivity of early life stages in fish and other vertebrates, compared to later life stages.

Effects of ocean acidification on 109Cd, 57Co, and 134Cs bioconcentration by the European oyster (Ostrea edulis): Biokinetics and tissue-to-subcellular partitioning

The uptake and depuration kinetics of dissolved ¹⁰⁹Cd, ⁵⁷Co and ¹³⁴Cs were determined experimentally in the European flat oyster Ostrea edulis (Linnaeus, 1758) under different pH conditions (i.e., 8.1, 7.8 and 7.5) for 59 days. Uptake and depuration rates were variable within these elements; no effects were observed under different pH conditions for the uptake biokinetics of ¹⁰⁹Cd and ⁵⁷Co and depuration of ¹⁰⁹Cd and ¹³⁴Cs in oyster. The uptake and depuration rate constants of ¹³⁴Cs differed during the exposure phase between treatments, while the steady state concentration factors (CF_ss) were similar. The resulting Cs activity that was purged during short- and long-term depuration phases differed, while the remaining activities after thirty-nine days depuration phase (RA_39d) were similar. Co-57 depuration was affected by pCO₂ conditions: RA_39d were found to be significantly higher in oysters reared in normocapnia (pCO₂ = 350 μatm) compared to high pCO₂ conditions. Co-57 tissue distribution did not differ among the variable pCO₂ conditions, while ¹⁰⁹Cd and ¹³⁴Cs accumulated in soft tissue of oysters were found to be higher under the highest pCO₂. Additionally, Cd, Co and Cs were stored differently in various compartments of the oyster cells, i.e. cellular debris, metal-rich granules (MRG) and metallothionein-like proteins (MTLP), respectively. The subcellular sequestration of the elements at the end of the depuration phase did not differ among pH treatments. These results suggest that bioconcentration and tissue/subcellular distribution are element-specific in the oyster, and the effects of higher pCO₂ driven acidification and/or coastal acidification variably influence these processes.

Relative comparison of tissue specific bioaccumulation and radiation dose estimation in marine and freshwater bivalve molluscs following exposure to phosphorus-32

With respect to environmental protection, understanding radionuclide bioconcentration is necessary to relate exposure to radiation dose and hence to biological responses. Few studies are available on tissue specific accumulation of short-lived radionuclides in aquatic invertebrates. Short-lived radionuclides such as ³²Phosphorus (³²P), although occurring in small quantities in the environment, are capable of concentrating in the biota, especially if they are chronically exposed. In this study, we firstly compared tissue specific bioaccumulation and release (depuration) of ³²P in adult marine (Mytilus galloprovincialis, MG) and freshwater bivalve molluscs (Dreissena polymorpha, DP). Secondly, using the Environmental Risk from Ionising Contaminants Assessment and Management (ERICA) tool, we calculated tissue specific doses following determination of radionuclide concentration. Marine and freshwater bivalves were exposed for 10 days to varying ³²P concentrations to acquire desired whole body average dose rates of 0.10, 1.0 and 10 mGy d^-1. Dose rates encompass a screening dose rate value of 10 μGy h^-1 (0.24 mGy d^-1), in accordance with the ERICA tool. This study is the first to relate tissue specific uptake and release (via excretion) of ³²P from two anatomically similar bivalve species. Results showed highly tissue specific accumulation of this radionuclide and similarity of accumulation pattern between the two species. Our data, which highlights preferential ³²P accumulation in specific tissues such as digestive gland, demonstrates that in some cases, tissue-specific dose rates may be required to fully evaluate the potential effects of radiation exposure on non-human biota. Differential sensitivity between biological tissues could result in detrimental biological responses at levels presumed to be acceptable when adopting a 'whole-body' approach.

Tritium in fish from remote lakes in northwestern Ontario, Canada

Tritium is most commonly generated as a by-product of nuclear reactors. As such, environmental concentrations are typically only reported near regions of interest, and background concentrations in areas unaffected by anthropogenic disturbance are not well characterized. To provide information on background levels of tritium in the natural environment, tissue-free water tritium (TFWT) and organically-bound tritium (OBT) were measured in the flesh of 106 fish collected within three lakes located at the IISD-Experimental Lakes Area (ELA) in Ontario, Canada in 2014. For the three ELA lakes studied, water tritium (HTO) activity concentration was determined to be below reliably detectable levels (0.6 Bq/L). Fish TFWT was found to be below 0.7 Bq/L, similar to the surrounding water tritium activity concentration. Fish OBT activity concentrations, at below 5 Bq/L, were also very low. Fish size was significantly related to OBT activity in Lake Whitefish and White Sucker from Lake 302, but not in other lakes. Though we observed significant differences in potential tritium exposure to humans among lakes, the levels of tritium reported here are below the Canadian natural background radiation of 1.8 mSv/y. These results provide information on background levels of tritium in freshwater fishes in Canada.

Co-expression of YieF and PhoN in Deinococcus radiodurans R1 improves uranium bioprecipitation by reducing chromium interference

Overexpression of the enzyme phosphatase (PhoN/PhoK) in the radiation-resistant bacterium Deinococcus radiodurans could be an efficient strategy for uranium remediation. However, the presence of other metals in nuclear wastes often interferes with uranium bioprecipitation. In our study, the uranium-precipitating ability of the PhoN-expressing D. radiodurans strain (Deino-phoN) significantly decreased by 45.4% in 13 h in the presence of chromium (VI); however, it was partially recovered after supplementation with chromium (III). Therefore, the reduction of chromium (VI) to chromium (III) was obtained by the co-expression of the YieF protein and PhoN in D. radiodurans (Deino-phoN-yieF). As a result, an increase in the chromium (VI) reduction (25.1%) rate was observed in 24 h. Furthermore, uranium precipitation also increased by 28.0%. For the decontamination of groundwater, we immobilized Deino-phoN-yieF cells using Polyvinyl alcohol (PVA)-sodium alginate (SA) beads, followed by incubation in a bioreactor. Approximately 99% of chromium (VI) and uranium (VI) was removed after 4 continuous cycles operated for a period of over 20 days at room temperature (25 °C). Therefore, Deino-phoN-yieF could be used as a potential biological agent for mixed radioactive nuclear waste remediation.

Uptake, depuration, dose estimation and effects in zebrafish exposed to Am-241 via dietary route

Zebrafish were chronically exposed to Am-241, an alpha-emitting radionuclide via daily consumption of an enriched artificial diet. Am-241 uptake was quantified in Danio rerio after 5 and 21 days of exposure via daily Am-spiked food ingestion and after 21 days of exposure followed by 5 days of depuration. Americium accumulates mostly in digestive tract, muscle, rest of the body but the accumulation levels and trophic transfer rate (0.033-0.013%) were low. Corresponding cumulative doses were calculated for the whole body (9 mGy) and for the digestive tract (42 mGy) with internal alpha radiation contributing to more than 99% of the total dose. Genotoxic effects (gamma-H2AX assay) and differential gene expressions of main biological functions were examined. Although fish were exposed to a low dose rate of 13 μGy h^-1, DNA integrity and gene expression linked to oxidative stress, hormonal signaling and spermatogenesis were altered after 21 days of Am-241 exposure. These results underline the higher toxicity of alpha emitter Am-241, as compared to other studies on gamma radiation exposure.

A double-tracer radioisotope approach to assess simultaneous bioaccumulation of caesium in the olive flounder Paralichthys olivaceus

To better understand bioaccumulation of radiocaesium in the commercially important Japanese flatfish, Paralichthys olivaceus, the uptake and depuration kinetics of caesium via both seawater and food were assessed simultaneously using controlled aquaria. The pre-conditioned fish were exposed to radionuclides via the two different pathways (aqueous versus dietary) concurrently using two isotopes of caesium, ¹³⁷Cs and ¹³⁴Cs, respectively. Dissolved caesium uptake was linear and did not reach a steady state over the course of the 8-day exposure period. Consumption of ¹³⁴Cs-labelled food led to higher bioaccumulation rates of radioactive Cs than via seawater exposure of ¹³⁷Cs during uptake and following depuration, though the model-derived long-lived biological half-lives of both pathways was approximately 66 d. Further development of this method for assessing multiple radiocaesium bioaccumulation pathways simultaneously could lead to a promising new approach for studying Cs contamination in marine organisms.

Detoxification of U(VI) by Paecilomyces catenlannulatus investigated by batch, XANES and EXAFS techniques

Paecilomyces catenlannulatus (P. catenlannulatus) as a genus of entomogenous fungus presented a variety of surface reactive groups by batch characterizations. The detoxification of U(VI) by P. catenlannulatus was investigated under different water chemistry (pH, incubation time, foreign anions and U(VI) concentration) by batch techniques. Approximately 75% of U(VI) was reduced to U(IV) (i.e., U^(IV)O₂(s)) by P. catenlannulatus at pH 5.5 and 7 days under glovebox conditions, therefore the formation of precipitates decreased the toxicity of U(VI) for P. catenlannulatus. In addition, phosphate facilitate the U(VI) reduction, whereas carbonate and sulfate inhibited the U(VI) reduction. The activities of catalase (CAT), superoxide dismutase (SOD) and glutathione (GSH) level were stimulated exposure to 1-30 mg/L U(VI), indicating that CAT, SOD and GSH were antagonized for the oxidant stress derived from U(VI) at low concentrations. According to XPS and XANES analysis, the occurrence of U(IV) revealed the reduction of adsorbed U(VI) to U(IV) by P. catenlannulatus. The results of EXAFS analysis indicated that the fitting of U-O and U-U shell for U-loaded P. catenlannulatus was similar to that of U^(IV)O₂(s)). The formation of U-bearing precipitates decreased the toxicity of U(VI) for P. catenlannulatus. These findings indicated that P. catenlannulatus is capable to detoxify U(VI) by extracellar/intracellar defense systems. Therefore, P. catenlannulatus can be utilized as a promising bioadsorbents for remediation of uranium-contaminated wastewater in environmental cleanup.

Interspecific comparison of radiocesium trophic transfer in two tropical fish species

The trophic transfer of radiocesium (¹³⁴Cs) was investigated in two tropical fish, the silver moony Monodactylus argenteus and the spotted scat Scatophagus argus. Juveniles of both species were exposed to dietary ¹³⁴Cs using the pulse-chase feeding methodology. The food was brine shrimp (Artemia salina) previously exposed to the dissolved radiotracer. Depuration kinetics of ¹³⁴Cs were followed for 45 d. Results showed that Cs was similarly efficiently assimilated by both species (AE > 50%). The estimated trophic transfer factors in the two species ranked from 1 to 2, suggesting that ¹³⁴Cs could be biomagnified in both omnivorous species. In complement, dissections of 7 body compartments were carried out at three different times in order to highlight ¹³⁴Cs organotropism. ¹³⁴Cs organotropism was similar in both species: more than 50% of ¹³⁴Cs was quickly distributed in the muscles and skeleton (after 3 days of depuration), which is likely related to the analogous behavior between Cs and K, an essential element for muscle contractions and bone formation.

The role of salinity in the trophic transfer of 137Cs in euryhaline fish

In order to better understand the influence of changing salinity conditions on the trophic transfer of ¹³⁷Cs in marine fish that live in dynamic coastal environments, its depuration kinetics was investigated in controlled aquaria. The juvenile turbot Scophthalmus maximus was acclimated to three distinct salinity conditions (10, 25 and 38) and then single-fed with compounded pellets that were radiolabelled with ¹³⁷Cs. At the end of a 21-d depuration period, assimilation efficiencies (i.e. AEs = proportion of ¹³⁷Cs ingested that is actually assimilated by turbots) were determined from observational data acquired over the three weeks. Our results showed that AEs of ¹³⁷Cs in the turbots acclimated to the highest salinity condition were significantly lower than for the other conditions (p < 0.05). Osmoregulation likely explains the decreasing AE observed at the highest salinity condition. Indeed, observations indicate that fish depurate ingested ¹³⁷Cs at a higher rate when they increase ion excretion, needed to counterbalance the elevated salinity. Such data confirm that ambient salinity plays an important role in trophic transfer of ¹³⁷Cs in some fish species. Implications for such findings extend to seafood safety and climate change impact studies, where the salinity of coastal waters may shift in future years in response to changing weather patterns.

An investigation on the seasonal variations of the biomarkers of oxidative stress response and their correlations to Polonium-210 in mussel (Mytilus galloprovincialis) and common sole (Solea solea) from İzmir Bay, Turkey

It is well known that the marine organisms are used as biological indicators for environmental pollution studies. Among these studies, the research on oxidative stress has been increasing in recent years. In this study, mussels (Mytilus galloprovincialis) and fish (Solea solea) samples were collected seasonally from İnciraltı, İzmir, Turkey. This station was in an area where fishing is carried out for human consumption. The relationship between ²¹⁰Po and oxidative stress markers (lipid peroxidation (LPO), H₂O₂ and proline) was investigated in the mussel tissue (digestive gland, gills) and fish tissue (liver, gills) samples. The present study indicated that H₂O₂ accumulated with increasing ²¹⁰Po concentration in mussel samples. Statistically significant correlation were found between H₂O₂ and ²¹⁰Po and LPO and proline in mussel samples. This correlation between LPO and proline can be attributed to common environmental parameters (other than ²¹⁰Po) affecting expression of both LPO and proline levels. There was not a significant correlation between ²¹⁰Po and LPO levels. Similarly, a significant correlation was not found between ²¹⁰Po and proline.

Microbial communities associated with uranium in-situ recovery mining process are related to acid mine drainage assemblages

A large fraction (47%) of the world's uranium is mined by a technique called "In Situ Recovery" (ISR). This mining technique involves the injection of a leaching fluid (acidic or alkaline) into a uranium-bearing aquifer and the pumping of the resulting solution through cation exchange columns for the recovery of dissolved uranium. The present study reports the in-depth alterations brought to autochthonous microbial communities during acidic ISR activities. Water samples were collected from a uranium roll-front deposit that is part of an ISR mine in operation (Tortkuduk, Kazakhstan). Water samples were obtained at a depth of ca 500 m below ground level from several zones of the Uyuk aquifer following the natural redox zonation inherited from the roll front deposit, including the native mineralized orebody and both upstream and downstream adjacent locations. Samples were collected equally from both the entrance and the exit of the uranium concentration plant. Next-generation sequencing data showed that the redox gradient shaped the community structures, within the anaerobic, reduced, and oligotrophic habitats of the native aquifer zones. Acid injection induced drastic changes in the structures of these communities, with a large decrease in both cell numbers and diversity. Communities present in the acidified (pH values < 2) mining areas exhibited similarities to those present in acid mine drainage, with the dominance of Sulfobacillus sp., Leptospirillum sp. and Acidithiobacillus sp., as well as the archaean Ferroplasma sp. Communities located up- and downstream of the mineralized zone under ISR and affected by acidic fluids were blended with additional facultative anaerobic and acidophilic microorganisms. These mixed biomes may be suitable communities for the natural attenuation of ISR mining-affected subsurface through the reduction of metals and sulfate. Assessing the effect of acidification on the microbial community is critical to evaluating the potential for natural attenuation or active bioremediation strategies.

Uptake and distribution of organo-iodine in deep-sea corals

Understanding iodine concentration, transport, and bioavailability is essential in evaluating iodine's impact to the environment and its effectiveness as an environmental biogeotracer. While iodine and its radionuclides have proven to be important tracers in geologic and biologic studies, little is known about transport of this element to the deep sea and subsequent uptake in deep-sea coral habitats. Results presented here on deep-sea black coral iodine speciation and iodine isotope variability provides key information on iodine behavior in natural and anthropogenic environments, and its geochemical pathway in the Gulf of Mexico. Organo-iodine is the dominant iodine species in the black corals, demonstrating that binding of iodine to organic matter plays an important role in the transport and transfer of iodine to the deep-sea corals. The identification of growth bands captured in high-resolution scanning electron images (SEM) with synchronous peaks in iodine variability suggest that riverine delivery of terrestrial-derived organo-iodine is the most plausible explanation to account for annual periodicity in the deep-sea coral geochemistry. Whereas previous studies have suggested the presence of annual growth rings in deep-sea corals, this present study provides a mechanism to explain the formation of annual growth bands. Furthermore, deep-sea coral ages based on iodine peak counts agree well with those ages derived from radiocarbon (¹⁴C) measurements. These results hold promise for developing chronologies independent of ¹⁴C dating, which is an essential component in constraining reservoir ages and using radiocarbon as a tracer of ocean circulation. Furthermore, the presence of enriched ¹²⁹I/¹²⁷I ratios during the most recent period of skeleton growth is linked to nuclear weapons testing during the 1960s. The sensitivity of the coral skeleton to record changes in surface water ¹²⁹I composition provides further evidence that iodine composition and isotope variability captured in proteinaceous deep-sea corals is a promising geochronometer as well as an emerging tracer for continental material flux.

A Novel Adaptation Mechanism Underpinning Algal Colonization of a Nuclear Fuel Storage Pond

Geochemical analyses alongside molecular techniques were used to characterize the microbial ecology and biogeochemistry of an outdoor spent nuclear fuel storage pond at Sellafield, United Kingdom, that is susceptible to seasonal algal blooms that cause plant downtime. 18S rRNA gene profiling of the filtered biomass samples showed the increasing dominance of a species closely related to the alga Haematococcus pluvialis, alongside 16S rRNA genes affiliated with a diversity of freshwater bacteria, including Proteobacteria and Cyanobacteria High retention of 137Cs and 90Sr on pond water filters coincided with high levels of microbial biomass in the pond, suggesting that microbial colonization may have an important control on radionuclide fate in the pond. To interpret the unexpected dominance of Haematococcus species during bloom events in this extreme environment, the physiological response of H. pluvialis to environmentally relevant ionizing radiation doses was assessed. Irradiated laboratory cultures produced significant quantities of the antioxidant astaxanthin, consistent with pigmentation observed in pond samples. Fourier transform infrared (FT-IR) spectroscopy suggested that radiation did not have a widespread impact on the metabolic fingerprint of H. pluvialis in laboratory experiments, despite the 80-Gy dose. This study suggests that the production of astaxanthin-rich encysted cells may be related to the preservation of the Haematococcus phenotype, potentially allowing it to survive oxidative stress arising from radiation doses associated with the spent nuclear fuel. The oligotrophic and radiologically extreme conditions in this environment do not prevent extensive colonization by microbial communities, which play a defining role in controlling the biogeochemical fate of major radioactive species present.IMPORTANCE Spent nuclear fuel is stored underwater in large ponds prior to processing and disposal. Such environments are intensively radioactive but can be colonized by microorganisms. Colonization of such inhospitable radioactive ponds is surprising, and the survival mechanisms that microbes use is of fundamental interest. It is also important to study these unusual ecosystems, as microbes growing in the pond waters may accumulate radionuclides present in the waters (for bioremediation applications), while high cell loads can hamper management of the ponds due to poor visibility. In this study, an outdoor pond at the U.K. Sellafield facility was colonized by a seasonal bloom of microorganisms, able to accumulate high levels of 137Cs and 90Sr and dominated by the alga Haematococcus This organism is not normally associated with deep water bodies, but it can adapt to radioactive environments via the production of the pigment astaxanthin, which protects the cells from radiation damage.

Iodate and nitrate transformation by Agrobacterium/Rhizobium related strain DVZ35 isolated from contaminated Hanford groundwater

Nitrate and radioiodine (¹²⁹I) contamination is widespread in groundwater underneath the Central Plateau of the Hanford Site. ¹²⁹I, a byproduct of nuclear fission, is of concern due to a 15.7 million year half-life, and toxicity. The Hanford 200 West Area contains plumes covering 4.3 km² with average ¹²⁹I concentrations of 3.5 pCi/L. Iodate accounts for 70.6% of the iodine present and organo-iodine and iodide make up 25.8% and 3.6%, respectively. Nitrate plumes encompassing the ¹²⁹I plumes have a surface area of 16 km² averaging 130 mg/L. A nitrate and iodate reducing bacterium closely related to Agrobacterium, strain DVZ35, was isolated from sediment incubated in a ¹²⁹I plume. Iodate removal efficiency was 36.3% in transition cultures, and 47.8% in anaerobic cultures. Nitrate (10 mM) was also reduced in the microcosm. When nitrate was spiked into the microcosms, iodate removal efficiency was 84.0% and 69.2% in transition and anaerobic cultures, respectively. Iodate reduction was lacking when nitrate was absent from the growth medium. These data indicate there is simultaneous reduction of nitrate and iodate by DVZ35, and iodate is reduced to iodide. Results provide the scientific basis for combined nitrogen and iodine cycling throughout the Hanford Site.

Complexation of Eu(III), Pb(II), and U(VI) with a Paramecium glycoprotein: Microbial transformation of heavy elements in the aquatic environment

This study investigated the interaction of inorganic aqueous Eu(III), Pb(II), and U(VI) with Paramecium sp., a representative single-celled protozoan that lives in freshwater. Living and prekilled Paramecium cells were tested. The prekilled cells were killed with a fixative. After 24 h exposure of the cells to inorganic aqueous solutions containing Eu(III) or U(VI), analyses by microparticle-induced X-ray emission with a focused beam (<1 μm) did not detect Eu and U in the living cells, whereas Eu and U were detected in the prekilled cells. Size exclusion chromatography coupled with on-line ultraviolet-visible detection and elemental detection by inductively coupled plasma mass spectrometry of the aqueous phases collected after the living cell experiments revealed that a fraction of the Eu, Pb, and U in the aqueous phase bound to a large (ca. 250 kDa) Paramecium biomolecule and formed a metal-organic complex. The characteristics of the biomolecule were consistent with those of the soluble glycoproteins covering the surfaces of Paramecium cells. These results show that Paramecium cells transform inorganic aqueous Eu, Pb, and U to organic complexes. This paper discusses the relation between this novel complexation and the sorption of these heavy elements on Paramecium cells.

Humic acids facilitated microbial reduction of polymeric Pu(IV) under anaerobic conditions

Flavins and humic substances have been extensively studied with emphasis on their ability to transfer extracellular electrons to insoluble metal oxides. Nevertheless, whether the low-solubility Pu(IV) polymers are microbially reduced to aqueous Pu(III) remains uncertain. Experiments were conducted under anaerobic and slightly alkaline conditions to study the difference between humic acids and flavins to transport extracellular electrons to Pu(IV) polymers. Our study demonstrates that Shewanella putrefaciens was unable to directly reduce polymeric Pu(IV) with a notably low reduction rate (3.4×10^-12mol/L Pu(III)_aq within 144h). The relatively high redox potential of flavins reveals the thermodynamically unfavorable reduction: E_h(PuO₂(am)/Pu³⁺)<E_h^o'(FMN/FMNH₂)≈E_h^o'(RBF/RBFH₂)≈-220mV at pH7.2. The microbially reduced humic acids facilitated the extracellular electron transfer to the polymers and reduced polymeric Pu(IV) (2.1×10^-10mol/L Pu(III)_aq) 62 times more rapidly than the flavins. The driving force for electron transfer explains the observed reduction: E_h(HA_ox/HA_red)<E_h(PuO₂(am)/Pu³⁺) when S. putrefaciens oxidized lactate and respired on the humic acids. In contrast, flavins were able to substantially reduce aqueous Pu(IV)-EDTA (1.9×10^-9mol/L Pu(III)_aq) because of the available driving force for electron transfer: Δ_rG_m=-F[E_h(PuL₂^4-/PuL₂^5-)-E_h^o'(FMN/FMNH₂)]=-33.5kJ/mol is a result of E_h(PuL₂^4-/PuL₂^5-)≫E_h(PuO₂(am)/Pu³⁺), where L is the EDTA ligand. In the presence of humic acids, the reduction of Pu(IV)-EDTA exhibited the most rapid rate (2.2×10^-9mol/L Pu(III)_aq). This result further demonstrates that humic acids facilitated the extracellular electron transfer to polymeric and aqueous Pu(IV). Reductive solubilization of the polymers may enhance Pu mobility in the geosphere and hence increases risks to human health.

Nitrogen species coupled with transpiration enhance Fe plaque assisted aquatic uranium removal via rhizofiltration of Phragmites australis Trin ex Steud

The influences of N species and transpiration on the Fe plaque (IP) formation and related aquatic U rhizofiltration had not revealed yet, especially when these factors were co-existed. It was evaluated in a mesocosm experiment in the condition of respective ammonium (NH₄⁺)/nitrate (NO₃^-) cultivation of Phragmites australis Trin ex Steud. coupled with different transpiration rates (TRs). The results suggested that the enhanced transpiration of P. australis improved the aquatic U rhizofiltration in both NO₃^- and NH₄⁺ rich milieus. However, the NO₃^- dependent oxidizing milieu restricted aquatic U uptake by the root of P. australis (up to 47.6 ± 1.8 mg kg^-1 under high TR) via IP assisted rhizofiltration. The high aquatic U availability and limited IP formation in NO₃^- rich milieu benefited the U retention within root tissue. On the contrary, the aquatic U rhizofiltration (up to 62.1 ± 1.0 mg kg^-1 under high TR) was enhanced under NH₄⁺ dependent reductive milieu. It was mainly contributed by U retention within IP. The area related U accumulation in different N species cultured roots was enhanced but did not significantly different under higher TR condition. The result suggested that the supplied NH₄⁺ coupled with enhanced transpiration was supposed to be more optimized option for IP assisted aquatic U rhizofiltration via P. australis.

Nuclear reprocessing-related radiocarbon (14C) uptake into UK marine mammals

To evaluate the transfer of Sellafield-derived radiocarbon (¹⁴C) to top predators in the UK marine environment, ¹⁴C activities were examined in stranded marine mammals. All samples of harbour porpoise (Phocoena phocoena) obtained from the Irish Sea showed ¹⁴C enrichment above background. Mammal samples obtained from the West of Scotland, including harbour porpoise, grey seals (Halichoerus grypus) and harbour seals (Phoca vitulina) showed ¹⁴C enrichment but to a lesser extent. This study demonstrates, for the first time, enriched ¹⁴C is transferred through the marine food web to apex predators as a consequence of ongoing nuclear reprocessing activities at Sellafield. Total Sellafield ¹⁴C discharge activity 24months prior to stranding and, in particular, distance of animal stranding site from Sellafield are significant variables affecting individual ¹⁴C activity. ¹⁴C activities of West of Scotland harbour porpoises suggest they did not forage in the Irish Sea prior to stranding, indicating a high foraging fidelity.

Experimental comparison of the bioaccumulation of anthropogenic radionuclides by egg and juvenile life stages of a small shark

This study compared the bioaccumulation of anthropogenic nuclides (⁶⁵Zn, ¹³⁴Cs, ⁶⁰Co and ²⁴¹Am) between the egg and juvenile life stages of a small shark (Scyliorhinus canicula), based on previously published experimental data. Rates of accumulation over 15 days were derived and summed for the transfer pathways which were specific to these two life stages. Radionuclide transfers to the egg and its embryo & yolk were quantified for i) the maternal pathway following her uptake of radionuclides via food and seawater and ii) from seawater following its oviposition. For the juvenile, the transfer of radionuclides were measured for aqueous & dietary pathways. The results show that, compared to juveniles, eggs have equivalent rates of accumulation of ⁶⁵Zn and ¹³⁴Cs but enhanced accumulation of ²⁴¹Am by a factor of five and of ⁶⁰Co by two orders of magnitude. The radiological exposure of the embryo due to radionuclides maternally transferred to the embryo & yolk is also enhanced for the alpha-emitting ²⁴¹Am. This enhanced accumulation of ²⁴¹Am and ⁶⁰Co, as well as the equivalent accumulation of ⁶⁵Zn and ¹³⁴Cs, suggest greater likely vulnerability to radiation damage in eggs as compared to juveniles. Radiological dose assessment confirmed highest doses to the egg which is predominantly due to accumulated ²⁴¹Am.

Trophic transfer of 134Cs in the Manila clam Ruditapes philippinarum

Bioaccumulation of radiocaesium in many marine organisms occurs through complex trophic transfer mechanisms. The present study addresses the trophic transfer of ¹³⁴Cs in the widely distributed marine bivalve, the Manila clam Ruditapes philippinarum, by experimentally determining the assimilation efficiency (AE) and the specific role of food quality or diet on the AE in this marine invertebrate. Pulse-chase feeding experiments were carried out on this clam using the phytoplankton species Tetraselmis chuii, Phaeodactylum tricornutum and Isochrysis galbana. Depuration kinetics of ¹³⁴Cs over 21 days were analysed using a two-component exponential model. Observed assimilation efficiencies were consistently less than 10% but slightly varied among individuals fed on the three different phytoplankton species diets (T. chuii: AE = 8.4 ± 0.6%; P. tricornutum: AE = 9.8 ± 0.5%; I. galbana: AE = 5.3 ± 0.6%), although no statistical differences were observed. Comparing results from these experiments with existing data from the literature on the same species exposed to caesium through seawater, it appears that trophic transfer processes are the main accumulation pathway, contributing up to 96% of the global ¹³⁴Cs bioaccumulation in this bivalve species.

Radionuclide biokinetics in the Russian sturgeon and phylogenetic consistencies with cartilaginous and bony marine fishes

The biokinetics of eight radionuclides (²⁴¹Am, ¹⁰⁹Cd, ¹³⁴Cs, ⁷⁵Se, ⁵⁴Mn, ^110mAg, ⁶⁵Zn, ⁶⁰Co) absorbed from the aquatic medium by juvenile Russian sturgeon (Acipenser gueldenstaedtii) were experimentally determined in fresh (0.42‰) and brackish (9.0‰) waters, of a similar salinity range to the Caspian Sea, and in conjunction with chemical speciation modelling. Uptake and loss rate constants were determined for each radionuclide for a 14 day exposure at each salinity and during 28 days of exposure to radionuclide-free conditions. Whole body (wet): water concentration factors (CF) achieved over 14 days for these eight radionuclides were used in a comparison with the same radionuclide CFs previously determined experimentally for six species of marine teleosts and chondrichthyans, to further test a phylogeny-based model of multi-nuclide bioaccumulation based on marine chordates. Multivariate analyses (multidimensional scaling and hierarchical clustering) identified the relative affinities among these taxa and also those radionuclides which distinguished most between them, in their differing CFs. They consistently showed that sturgeon aggregated as a group, which was also slightly differentiated with salinity. Sturgeon were distinguished from all teleosts and chondrichthyans but were more dissimilar from chondrichthyans than teleosts, in accordance with sturgeon's different periods of divergence from them in evolutionary time. Variable salinity among experiments may also cause changes in radionuclide bioaccumulation due to variations in (i) bioavailability (ii) osmolarity, and (iii) competitive inhibition of a radionuclide's bioaccumulation by its stable analogue or metabolic model. Their potentially confounding effects on these patterns of radionuclide CFs among taxa were critically evaluated for those radionuclides which discriminated most between sturgeon and teleosts or chondrichthyans. Bioavailability, osmolarity and competitive inhibition effects were identified among salinity treatments, however they were not appreciable enough to override the phylogeny-based signal. The results of this study are thus consistent with a phylogeny-based model of radionuclide bioaccumulation by marine chordates being valid for a fish species living in lower salinity regimes.

Equilibrium, thermodynamic and kinetic investigations for biosorption of uranium with green algae (Cladophora hutchinsiae)

Removal of toxic chemicals from environmental samples with low-cost methods and materials are very useful approach for especially large-scale applications. Green algae are highly abundant biomaterials which are employed as useful biosorbents in many studies. In the present study, an interesting type of green algae, Cladophora hutchinsiae (C. hutchinsiae) was used for removal of highly toxic chemical such as uranium. The pH, biosorbent concentration, contact time and temperature were optimized as 5.0, 12 g/L, 60 min and 20 °C, respectively. For the equilibrium calculations, three well known isotherm models (Langmuir, Freundlich and Dubinin-Radushkevich) were employed. The maximum biosorption capacity of the biosorbent was calculated as about 152 mg/g under the optimum batch conditions. The mean energy of biosorption was calculated as 8.39 kJ/mol from the D-R biosorption isotherm. The thermodynamic and kinetic characteristics of biosorption were also investigated to explain the nature of the process. The kinetic data best fits the pseudo-second-order kinetic model with a regression coefficient of >0.99 for all studied temperatures. The calculated ΔH° and ΔG° values showed that the biosorption process is exothermic and spontaneous for temperatures between 293 and 333 K. Furthermore, after seven cycling process, the sorption and desorption efficiencies of the biosorbent were found to be 70, and 58%, respectively meaning that the biosorbent had sufficiently high reusability performance as a clean-up tool.

210Po bioaccumulation and trophic transfer in marine food chains in the northern Arabian Gulf

The tendency of ²¹⁰Po to concentrate in body tissue poses a serious concern of radiological safety. This study compiles available information and presents recent ²¹⁰Po data for the marine food web in the northern Gulf waters. Since ²¹⁰Po is concentrated in marine biota, a large number of samples of various marine organisms covering several trophic levels, from microalgae to sharks, were analyzed. ²¹⁰Po was found to be highly concentrated in several marine species with the highest ²¹⁰Po concentrations found in yellowfin tuna, i.e. 37.3-44.9, 451-548, and 1511-1693 Bq kg^-1 wwt in muscle, digestive system and liver, respectively. In most dissected fish samples, ²¹⁰Po showed increasing concentrations in the following order: edible tissue, gills, digestive system, liver and fecal matter. Fish feces had ²¹⁰Po concentrations several orders of magnitude higher than that in seawater, fish muscle, and the fishes' ingested food. The high ²¹⁰Po concentration in fish fecal matter suggests that the bulk of ²¹⁰Po content in fish is eventually excreted back into the environment as fecal pellets. In most fish high concentrations were noted in liver, with the highest ²¹⁰Po concentration recorded in yellowfin tuna liver. Moreover, ²¹⁰Po concentration in the soft tissue of tunicate and bryozoan samples were 872-1012 and 402-527 Bq kg^-1 wwt, respectively, far higher than that in fish muscle (0.04-44.9 Bq kg^-1 wwt). It was observed that the maximum ²¹⁰Po concentration in edible fish tissue among the fish in trophic level 2 was an order of magnitude lower than those in trophic level 3 and two orders of magnitude lower compared to fish in trophic level 4. The highest concentrations in the muscle tissue were observed in the following order: tunicate > bryozoan > mollusc > crustacean > algae > fish. Among all the biota analyzed, the highest overall concentration of ²¹⁰Po was noted in yellowfin tuna (Thunnus albacores) indicating a potential biomagnification of ²¹⁰Po in this particular top predator species. In general, ²¹⁰Po concentrations found in the commercially important fish from Kuwaiti waters were comparable to levels that have been reported for similar fish species from several other marine areas worldwide.

Water hardness determines 226Ra uptake in the tropical freshwater mussel

Chemical data for freshwater mussels (Velesunio spp.) and water from 15 sampling sites in the Alligator Rivers Region and Rum Jungle uranium provinces in tropical Northern Australia were analysed to develop a predictive model of radium-226 (²²⁶Ra) bioaccumulation for variable water calcium (Ca) and magnesium (Mg) concentrations. Application of the model as a ²²⁶Ra screening approach for freshwater mussels in tropical waterbodies potentially impacted by operational or remediated uranium mine sites is discussed in relation to Mudginberri Billabong, located approximately 12 km downstream of Ranger uranium mine in the Alligator Rivers Region.

Isotherms, thermodynamic and mechanism studies of removal of low concentration uranium (VI) by Aspergillus niger

In order to develop an effective and economical method for removing low concentration radioactive wastewater of uranium, the biomass of 'CMCC(F)-98003' Aspergillus niger was investigated in a batch system. The maximum uranium adsorption capacity of 12.5 mg g^-1 was obtained at the initial uranium concentration of 0.75 mg L^-1. The biosorption data on a biomass concentration of 0.029 g L^-1 fitted well to the Freundlich isotherm with a correlation coefficient (R²) of 0.987. The calculated thermodynamic parameters showed that the biosorption of uranium ions was endothermic (ΔH° < 0). The results of scanning electron microscope and Fourier transform infrared spectrometry analysis revealed that nano-particles of uranium precipitation were formed on the cell surfaces after biosorption, and the functional groups of -CH, N-H, -COOH, P = O and the carbohydrates and alcohols were involved in the biosorption process between A. niger and uranium ions.

External dose reconstruction for the former village of Metlino (Techa River, Russia) based on environmental surveys, luminescence measurements, and radiation transport modelling

In the first years of its operation, the Mayak Production Association, a facility part of the Soviet nuclear weapons program in the Southern Urals, Russia, discharged large amounts of radioactively contaminated effluent into the nearby Techa River, thus exposing the people living at this river to external and internal radiations. The Techa River Cohort is a cohort intensely studied in epidemiology to investigate the correlation between low-dose radiation and health effects on humans. For the individuals in the cohort, the Techa River Dosimetry System describes the accumulated dose in human organs and tissues. In particular, organ doses from external exposure are derived from estimates of dose rate in air on the Techa River banks which were estimated from measurements and Monte Carlo modelling. Individual doses are calculated in accordance with historical records of individuals' residence histories, observational data of typical lifestyles for different age groups, and age-dependent conversion factors from air kerma to organ dose. The work here describes an experimentally independent assessment of the key input parameter of the dosimetry system, the integral air kerma, for the former village of Metlino, upper Techa River region. The aim of this work was thus to validate the Techa River Dosimetry System for the location of Metlino in an independent approach. Dose reconstruction based on dose measurements in bricks from a church tower and Monte Carlo calculations was used to model the historic air kerma accumulated in the time from 1949 to 1956 at the shoreline of the Techa River in Metlino. Main issues are caused by a change in the landscape after the evacuation of the village in 1956. Based on measurements and published information and data, two separate models for the historic pre-evacuation geometry and for the current geometry of Metlino were created. Using both models, a value for the air kerma was reconstructed, which agrees with that obtained in the Techa River Dosimetry System within a factor of two.