Characterization of biocenoses in the storage reservoirs of liquid radioactive wastes of Mayak PA. Initial descriptive report

Internal radiation dose to the herring gull embryo due to 90Sr in the egg

Birds are bioindicators of anthropogenic environmental stress, including the changes caused by radioactive contamination of ecosystems. Any radiation-induced biological effects can be the consequence of exposure both after hatching and during the embryonic period. Therefore, it is necessary to quantify radiation doses to the embryo when interpreting observed radiobiological effects in birds. This is especially true for areas contaminated with Ca-like 90Sr. The levels of radionuclide accumulation in the eggshell can be extremely high, which leads to chronic embryo exposure. Consequently, the objective of the present study was to develop a method to calculate the dose to a herring gull embryo exposed to 90Sr distributed in egg compartments (shell, embryo body, albumen and yolk). To achieve this, the time-dependent Sr distribution in the egg compartments was modeled. Additionally, dosimetric modeling was carried out to obtain dose factors that convert the radionuclide activity in different compartments of an egg to embryo dose at various stages of embryogenesis. It has been shown that the accumulated dose to the herring gull embryo can be calculated based on 90Sr total activity in the egg using a dose conversion factor of 0.44 μGy Bq-1. Since the eggshell contains more than 90% of total 90Sr activity, the conversion from eggshell activity to embryo dose would be practically the same as that from the total egg activity - 0.46 μGy Bq-1. The main dose fraction (~ 99%) accumulates at the last stage of embryogenesis (from 13 to 26 days). The proposed method allows for an estimation of individual radiation doses to embryos based on eggshell radiometry. This creates a new opportunity to study how dangerous any radiation exposure of birds could be during the embryonic period.

Bone and head kidney radiation dosimetry for roach from radioactively contaminated reservoirs: Method elaboration and application

To establish causal relationships between radiation exposure and tissue effects, it is necessary to calculate the doses to critical organs and tissues and to evaluate the dose-effect relationship. The tools of non-human biota dosimetry that exist nowadays do not take into account the heterogeneous radionuclide distribution between organs and tissues. In the current study, a computational phantom of the roach body segment was developed based on the assessment of the morphometric parameters of target organs. The spine, ribs and head kidney were considered as target organs for bone and hematological effects of radiation exposure. The dose factors to convert specific activity of incorporated 90Sr and 137Cs to dose rates in target organs were calculated based on Monte Carlo simulation of electron and photon transport. One Bq/g of 90Sr in bone tissue lead to dose rates equal to 1.98, 3.38 and 7.49 μGy/day in the head kidney, ribs and spine, respectively. The accumulation of 137Cs in the bones results in bone-specific dose rates that are 3-4 times lower than those at the same concentration of 90Sr. The obtained results were used to calculate doses to the roach from two radioactively contaminated waterbodies of Ural region. Maximum doses were typical of vertebra (1.4 ± 0.2 and 27.0 ± 8.3 mGy/day). They were 1.4-3 times higher than doses head kidney and ribs. One can expect more pronounced bone effects of radiation exposure in the vertebral bodies than in the ribs of fish, and they should be considered as separate target organs in the presence of Strontium isotopes in the environment.

Dynamic model of changes in the trophic structure of an ecosystem affected by chronic radiation exposure

The conceptual dynamic ecosystem model was developed to evaluate the self-organization of trophic structure in ecosystems during the course of biogenic succession. This model was applied to analyze the possible changes in the ecosystem under impact of the anthropogenic physical stressor - chronic exposure to ionizing irradiation. The model predicts that amount of the limiting biogenic nutrient in the environment can modify the ecological effects of ionizing radiation. Negative effects of the chronic exposure are less significant in ecosystems with high food supply. The model does not show presence of any ecological effect of radiation at the exposure rates less than the derived consideration reference levels, obtained by International Commission on Radiological Protection for individual nature organisms. If the dose rates are higher than those levels, radiation exposure can affect ecological interactions between species. The model shows that environmental hormesis can exist in the ecosystems, impacted by the chronic radiation exposure. The reason of this effect is change of the ecological coefficients (for example, decrease of the predation rate), which in the certain range of parameters leads to the increase of biomasses of all species at the same amount of the limiting biogenic nutrient in ecosystem. Trigger regimes exist in the model ecosystem with mixed-feeding consumers. Within the trigger area, the realization of a particular trophic structure depends on initial species biomasses. A hysteresis phenomenon exists in such ecosystems, which means that the successive changes in the trophic structures realized following the increase of the influencing factor are not reproduced in the same order if the influencing factor was gradually decreased back to its previous values. The model predicts for this case, that the radioactively contaminated ecosystem does not necessarily return to its initial trophic structure, despite the dose rate decreases to the initial levels.

Plant adaptation to ionizing radiation: Mechanisms and patterns

Adaptation to environmental stressors is an essential property of plants that allows them, despite an immobile lifestyle, to survive in a changeable environment. The chain of successive events culminating in the final radiobiological reaction begins with the absorption of energy of ionizing radiation in the cell. Starting from stochastic acts of molecular injury formation, radiation damage gradually acquires deterministic features, which are expressed in a limited number of phenomena that complete plant radiation damage. As plants undergo specialization, the differences between plants and animals become more pronounced, leading to distinct responses to radiation. Chronic radiation exposure may activate biological mechanisms resulting in increased radioresistance of the population. The higher the level of radiation exposure and the sensitivity of plants to radiation, the more intensive the selection. Depending on the circumstances, enhanced radioresistance of a population can be achieved in different ways or has not evolved at all. High dose rates of chronic irradiation leаd to selection for the efficiency of repair systems, while low dose rates activate epigenetic mechanisms that lead to the maintenance of oxidative balance, additional synthesis of chaperones, and control of TEs transposition. Due to huge differences in the radiosensitivity of organisms that make up the ecosystem, irradiation can result in disruption of connections between components of ecosystems which may lead to consequences that can differ drastically from those expected at the organismal and population levels. Therefore, the use of ecological knowledge is essential for understanding the responses of populations and ecosystems to radiation exposure.

Fish otoliths as biological dosimeter: internal dose calculation

Otoliths are organs used by fish for hearing and keeping balance. They consist of biogenic crystals of hydroxyapatite and do not contain any living cells. Upon exposure to ionizing radiation, otolith hydroxyapatite accumulates radiation-induced stable CO₂^- radicals whose amount is proportional to absorbed dose. In electron paramagnetic resonance (EPR) dosimetry, carbonate ions are registered and, hence, the total accumulated dose in the fish otolith can be quantified. Therefore, otoliths can be used as individual fish dosimeters to support radiobiological and radioecological studies. An important aspect of otolith-based EPR dosimetry on fish from contaminated water bodies is the potential presence of bone-seeking ⁹⁰Sr. Consequently, cumulative absorbed doses measured with EPR in otoliths may reflect the superposition of internal exposure to ⁹⁰Sr/⁹⁰Y and external exposure due to radionuclides circulating in soft tissue of the fish as well as due to environmental contamination. The objective of the present study was to develop a method that allows for an assessment of the contribution of ⁹⁰Sr to the total dose in otolith. The method has been tested using otoliths from seven fish taken from reservoirs located in the Southern Urals contaminated with radionuclides including ⁹⁰Sr. It has been shown that dose to otoliths is largely determined by ⁹⁰Sr in the hydroxyapatite. The internal dose component can be calculated using activity concentration-to-dose conversion factors, which vary slightly in the range of 2.0-2.8 × 10^-3 Gy year^-1 per Bq g^-1 depending on fish species and age. Internal doses to fish from water bodies with different levels of ⁹⁰Sr contamination were calculated in the range from 2 mGy to ~ 200 Gy. External dose contribution was derived for two fish only to be about 100 and 40 Gy. It is concluded that EPR dosimetry on fish otoliths is a promising tool when external exposure prevails or is comparable to internal exposure due to ⁹⁰Sr.

Biogeochemical Factors of Cs, Sr, U, Pu Immobilization in Bottom Sediments of the Upa River, Located in the Zone of Chernobyl Accident

Laboratory modeling of Cs, Sr, U, Pu immobilization by phytoplankton of the river Upa, affected after the Chernobyl accident, has been carried out. Certain conditions are selected for strong fixation of radionuclides in bottom sediments due to biogeochemical processes. The process of radionuclide removal from the water phase via precipitation was based on their accumulation by phytoplankton, stimulated by nitrogen and phosphorus sources. After eight days of stimulation, planktonic phototrophic biomass, dominated by cyanobacteria of the genus Planktothrix, appears in the water sample. The effectiveness of U, Pu and Sr purification via their transfer to bottom sediment was observed within one month. The addition of ammonium sulfate and phosphate (Ammophos) led to the activation of sulfate- and iron-reducing bacteria of the genera Desulfobacterota, Desulfotomaculum, Desulfosporomusa, Desulfosporosinus, Thermodesulfobium, Thiomonas, Thiobacillus, Sulfuritallea, Pseudomonas, which form sulphide ferrous precipitates such as pyrite, wurtzite, hydrotroillite, etc., in anaerobic bottom sediments. The biogenic mineral composition of the sediments obtained under laboratory conditions was verified via thermodynamic modeling.

Artificial radionuclides association with bottom sediment components from Mayak Production Association industrial reservoirs

About ten years have passed since the last published report in Russian on the speciation and distribution features of radionuclides in the bottom sediment of Production Association (PA) "Mayak" (further mentioned as Mayak) artificial reservoirs. Herein, the desorption parameters of ¹³⁷Cs, ⁹⁰Sr, ²⁴¹Am, and ^238,239+240Pu and their association with bottom sediment components were investigated in two Mayak reservoirs (R-17 (decommissioned) and R-4 (still in use) with big differences in size, activity and water regime). It was established that ¹³⁷Cs and ⁹⁰Sr desorption from the R-17 bottom sediment reach constant values after 24 h, and the desorption degree is not dependant on pH but on ionic strength. Sequential extraction indicated that the main accumulation mechanism of ¹³⁷Cs is incorporation into the clay minerals. The maximum plutonium content was detected in the residual fraction of the R-17 bottom sediment, which could be associated with the effects of precipitation aging. In R-4, plutonium is equally distributed between residual and bound to organic matter fractions. The ²⁴¹Am is associated with carbonates in the R-17 bottom sediment and with organic matter in the R-4 bottom sediment and to lesser extent with iron-manganese oxides. The radionuclides are becoming less environmentally available with time since deposition.

Realism and usefulness of multispecies experiment designs with regard to application in radioecology: A review

Multispecies experiments like microcosms and mesocosms are widely used in many fields of research but not in radioecology. In radioecology, size limitations are important as large experimental volumes involve problems with waste (radionuclides), or shielding, absorption and available space in gamma fields (often within a climate chamber). We have therefore performed a literature review (ISI Web of Science, n = 406) of the design and properties of multispecies effect studies <100 L in size and with three or more mentioned taxa in other research fields to assess their suitability to radioecology. Studies with more mentioned taxa assess structural ecosystem parameters more often than studies with fewer mentioned taxa, while the opposite trend is seen for indirect effects/interactions. Studies of indirect effects benefit from more replicates and longer experiments. Almost all studies assess some ecosystem level parameter but only a quarter take a holistic approach assessing both structural and functional as well as indirect effects. We find that most cosms are custom-made systems, rather than standardised designs. Unmanipulated cosms consist of excised portions of the natural environment with a higher number of mentioned taxa, high ecological complexity and high realism, but have a relatively low replicability. In contrast, standardised cosms with fewer mentioned taxa have less ecological complexity but much higher replicability. This literature review shows that smaller cosm sizes have similar ecological complexity (e.g. number of taxa and trophic levels) and experimental duration as larger sized cosms, allowing for ecologically-relevant investigations, despite their small size. We encourage multispecies radioecology studies, preferably with environmental relevant doses and sufficient detail on dosimetry.

Review of resistance to chronic ionizing radiation exposure under environmental conditions in multicellular organisms

Ionizing radiation resistance occurs among many phylogenetic groups and its mechanisms remain incompletely understood. Tolerances to acute and chronic irradiation do not always correlate because different mechanisms may be involved. The radioresistance phenomenon becomes even more complex in the field than in the laboratory because the effects of radioactive contamination on natural populations are intertwined with those of other factors, such as bioaccumulation of radionuclides, interspecific competition, seasonal variations in environmental conditions, and land use changes due to evacuation of humans from contaminated areas. Previous reviews of studies performed in radioactive sites like the Kyshtym, Chernobyl, and Fukushima accident regions, and of protracted irradiation experiments, often focused on detecting radiation effects at low doses in radiosensitive organisms. Here we review the literature with a different purpose: to identify organisms with high tolerance to chronic irradiation under environmental conditions, which maintained abundant populations and/or outcompeted more radiosensitive species at high dose rates. Taxa for which consistent evidence for radioresistance came from multiple studies conducted in different locations and at different times were found among plants (e.g. willow and birch trees, sedges), invertebrate and vertebrate animals (e.g. rotifers, some insects, crustaceans and freshwater fish). These organisms are not specialized "extremophiles", but tend to tolerate broad ranges of environmental conditions and stresses, have small genomes, reproduce quickly and/or disperse effectively over long distances. Based on these findings, resistance to radioactive contamination can be examined in a more broad context of chronic stress responses.

Radiation effects and ecological processes in a freshwater microcosm

Ecosystem response to gamma radiation exposure depends on the different species sensitivities and the multitude of direct and indirect pathways by which individual organisms can be affected, including the potential for complex interactions across multiple trophic levels. In this study, multi-species microcosms were used to investigate effects of ionizing radiation in a model freshwater ecosystem, including endpoints at both structural and functional levels and ecological interactions. Microcosms were exposed for 22 days to a gradient of gamma radiation with four dose rates from 0.72 to 19 mGy h^-1, which are within the range of those seen at contaminated sites. Results showed significant dose related effects on photosynthetic parameters for all macrophyte species. No significant effects of radiation were observed for the consumers in the microcosms, however trends indicate the potential for longer-term effects. We also witnessed a different response of Daphnia magna and Lemna minor compared to previous single-species studies, illustrating the importance of multispecies studies, which aim to encompass systems more realistic to natural ecosystems. Microcosms allowed us to isolate specific relationships between interacting species in an ecosystem and test the effects, both direct and indirect, of radiation on them. In addition, the ecological pathways and processes, and the experimental design itself, was central to understanding the results we witnessed. This type of study is important for radioecology research that has been very much limited to high dose rates and single species studies. This approach to radioecology has been strongly promoted in recent decades and, to our knowledge, this is the first microcosm study performed at dose rates similar to those at contaminated field sites.

The radiation exposure of fish in the period of the Techa river peak contamination

Waterborne radioactive releases into the Techa River from the Mayak Production Association in Russia during 1949-1956 resulted in downstream contamination of the river ecosystem. The discharged liquid waste contained both short-lived isotopes (⁹⁵Zr, ⁹⁵Nb, ^103,106Ru, ^141,144Ce, ⁹¹Y, ⁸⁹Sr and ¹⁴⁰Ba with half-life from 3 days to 1.02 years) and the long-lived ⁹⁰Sr and ¹³⁷Cs (half-life - 28.79 y and 30.07 y, respectively). Even now, when two half-lives of ⁹⁰Sr and ¹³⁷Cs have passed, the contamination in the upper river region (about 70 km from the source of releases) is still relatively high. Current anthropogenic dose rates calculated for the fish of the Techa River depend on the distance along the stream and decrease from 150 to 3 μGy day^-1. Radiation exposure of fish is expected to have been much higher at the time of the releases. The aim of the study was to evaluate the dose rates for the most common fish species of the river, viz., roach (Rutilus rutilus), perch (Perca fluviatilis) and pike (Esox lucius), in the period of peak contamination of the upper reaches of the Techa River from 1950 to 1951. To achieve this objective, calculation of both internal and external dose rates was performed. For dose rate calculation, the contamination of the river compartments was modeled, body-size dependent dose coefficients were evaluated, morphometric data were analyzed. Maximum dose rates were obtained for roach; minimum - for pike, it depends on fish lifestyle (time spent at the bottom). In the period before September 1950, fish of the upper reaches are assessed to have been exposed to dose rates exceeding the screening level equal to 240 μGy day^-1. From September 1950 up to the end of 1952 the fish dose rates along the Techa River were found to be close to the UNSCEAR threshold equal to 9.6 × 10³ μGy day^-1 or even much more higher (up to 1.9 × 10⁵ μGy day^-1). Extremely high historical dose rates did not lead to the difference in fish size and fish growth rate currently observed in the Techa River and in the comparison waterbody (the Miass River). Discussion includes the description of radiation effects observed currently in the river fish. Today the effects observed in hematopoietic system may be the consequence of radiation exposure of fish over several generations. For example, long term dwelling of fish in the radioactively contaminated environment leads to their adaptation to chronic radiation exposure. At the same time, an increase their sensitivity and decrease their adaptive capacity to respond to other stress factors can be observed.

Review of microbial resistance to chronic ionizing radiation exposure under environmental conditions

Ionizing radiation (IR) produces multiple types of damage to nucleic acids, proteins and other crucial cellular components. Nevertheless, various microorganisms from phylogenetically distant taxa (bacteria, archaea, fungi) can resist IR levels many orders of magnitude above natural background. This intriguing phenomenon of radioresistance probably arose independently many times throughout evolution as a byproduct of selective pressures from other stresses (e.g. desiccation, UV radiation, chemical oxidants). Most of the literature on microbial radioresistance is based on acute γ-irradiation experiments performed in the laboratory, typically involving pure cultures grown under near-optimal conditions. There is much less information about the upper limits of radioresistance in the field, such as in radioactively-contaminated areas, where several radiation types (e.g. α and β, as well as γ) and other stressors (e.g. non-optimal temperature and nutrient levels, toxic chemicals, interspecific competition) act over multiple generations. Here we discuss several examples of radioresistant microbes isolated from extremely radioactive locations (e.g. Chernobyl and Mayak nuclear plant sites) and estimate the radiation dose rates they were able to tolerate. Some of these organisms (e.g. the fungus Cladosporium cladosporioides, the cyanobacterium Geitlerinema amphibium) are widely-distributed and colonize a variety of habitats. These examples suggest that resistance to chronic IR and chemical contamination is not limited to rare specialized strains from extreme environments, but can occur among common microbial taxa, perhaps due to overlap between mechanisms of resistance to IR and other stressors.

Modeling species richness and abundance of phytoplankton and zooplankton in radioactively contaminated water bodies

Water bodies polluted by the Mayak nuclear plant in Russia provide valuable information on multi-generation effects of radioactive contamination on freshwater organisms. For example, lake Karachay was probably the most radioactive lake in the world: its water contained ∼2 × 10⁷ Bq/L of radionuclides and estimated dose rates to plankton exceeded 5 Gy/h. We performed quantitative modeling of radiation effects on phytoplankton and zooplankton species richness and abundance in Mayak-contaminated water bodies. Due to collinearity between radioactive contamination, water body size and salinity, we combined these variables into one (called HabitatFactors). We employed a customized machine learning approach, where synthetic noise variables acted as benchmarks of predictor performance. HabitatFactors was the only predictor that outperformed noise variables and, therefore, we used it for parametric modeling of plankton responses. Best-fit model predictions suggested 50% species richness reduction at HabitatFactors values corresponding to dose rates of 10⁴-10⁵ μGy/h for phytoplankton, and 10³-10⁴ μGy/h for zooplankton. Under conditions similar to those in lake Karachay, best-fit models predicted 81-98% species richness reductions for various taxa (Cyanobacteria, Bacillariophyta, Chlorophyta, Rotifera, Cladocera and Copepoda), ∼20-300-fold abundance reduction for total zooplankton, but no abundance reduction for phytoplankton. Rotifera was the only taxon whose fractional abundance increased with contamination level, reaching 100% in lake Karachay, but Rotifera species richness declined with contamination level, as in other taxa. Under severe radioactive and chemical contamination, one species of Cyanobacteria (Geitlerinema amphibium) dominated phytoplankton, and rotifers from the genus Brachionus dominated zooplankton. The modeling approaches proposed here are applicable to other radioecological data sets. The results provide quantitative information and easily interpretable model parameter estimates for the shapes and magnitudes of freshwater plankton responses to a wide range of radioactive contamination levels.

Otoliths as object of EPR dosimetric research

Otoliths are the organs which fish use for hearing and keeping balance. Otoliths are the most calcified tissues in the fish body. In contrast to bones, otoliths are not affected by remodeling and, therefore, they are expected to accumulate any dose from ionizing radiation during lifetime. Therefore, EPR dosimetry with fish otoliths could be an important tool for dose reconstruction in radiobiology and radioecology. It could also provide useful information remediation actions to de-contaminate waterbodies. Consequently, in the present study, otoliths of three contaminated fish species (roach (Rutilus rutilus), pike (Esox lucius) and perch (Perca Fluviatilis)) were examined with Electron Paramagnetic Resonance (EPR) spectroscopy. The fish were caught at storage reservoirs of liquid radioactive waste from Mayak PA and from the upper reach of the Techa River, which have been contaminated with different levels of radionuclide activity concentrations. It is shown that the radiation-induced EPR signal of otolith is stable and characterized by a linear dose response. However, the slope of the calibration curve (corresponding to the radiation sensitivity of the material) is not the same for different species; this may be caused by differences in mineralization. The reconstructed doses were found to be in the range from undetectable (in fish from the upper stream of the Techa River) up to 265 Gy (in roach from the most contaminated waterbody). In parallel, otoliths were measured with β-counter to detect ⁹⁰Sr/⁹⁰Y. Samples were also tested on the presence of alpha-emitters, but no alpha activity above background could be detected. However, a significant activity concentration of ⁹⁰Sr was detected (from 1 × 10¹ to 2 × 10⁴ Bq/g). The EPR doses measured correlated with the ⁹⁰Sr activity concentration measured in the otolith samples.

Ecological effects of exposure to enhanced levels of ionizing radiation

Irradiation of plants and animals can result in disruption of ecological relationships between the components of ecosystems. Such effects may act as triggers of perturbation and lead to consequences that may differ essentially from expected ones based on effects observed at the organismal level. Considerable differences in ecology and niches occupied by different species lead to substantial differences in doses of ionizing radiation absorbed by species, even when they all are present in the same environment at the same time. This is especially evident for contamination with α-emitting radionuclides. Radioactive contamination can be considered an ecological factor that is able to modify the resistance in natural populations. However, there are radioecological situations when elevated radioresistance does not evolve or persist. The complexity and non-linearity of the structure and functioning of ecosystems can lead to unexpected consequences of stress effects, which would appear harmless if they were assessed within the narrower context of organism-based traditional radioecology. Therefore, the use of ecological knowledge is essential for understanding responses of populations and ecosystems to radiation exposure. Integration of basic ecological principles in the design and implementation of radioecological research is essential for predicting radiation effects under rapidly changing environmental conditions.

Addressing ecological effects of radiation on populations and ecosystems to improve protection of the environment against radiation: Agreed statements from a Consensus Symposium

This paper reports the output of a consensus symposium organized by the International Union of Radioecology in November 2015. The symposium gathered an academically diverse group of 30 scientists to consider the still debated ecological impact of radiation on populations and ecosystems. Stimulated by the Chernobyl and Fukushima disasters' accidental contamination of the environment, there is increasing interest in developing environmental radiation protection frameworks. Scientific research conducted in a variety of laboratory and field settings has improved our knowledge of the effects of ionizing radiation on the environment. However, the results from such studies sometimes appear contradictory and there is disagreement about the implications for risk assessment. The Symposium discussions therefore focused on issues that might lead to different interpretations of the results, such as laboratory versus field approaches, organism versus population and ecosystemic inference strategies, dose estimation approaches and their significance under chronic exposure conditions. The participating scientists, from across the spectrum of disciplines and research areas, extending also beyond the traditional radioecology community, successfully developed a constructive spirit directed at understanding discrepancies. From the discussions, the group has derived seven consensus statements related to environmental protection against radiation, which are supplemented with some recommendations. Each of these statements is contextualized and discussed in view of contributing to the orientation and integration of future research, the results of which should yield better consensus on the ecological impact of radiation and consolidate suitable approaches for efficient radiological protection of the environment.