Experimental studies on the biokinetics of 134Cs and 24Am in mussels (Mytilus galloprovincialis)

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.

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.

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.

Allometric relationship in the bioaccumulation of radionuclides (134Cs &241Am) and delineation of contamination pathways (food and seawater) in bloody cockle Anadara senilis using radiotracer techniques

The uptake and depuration kinetics of ¹³⁴Cs and ²⁴¹Am were investigated in the bloody cockle Anadara senilis exposed via seawater and food in controlled conditions, using animals of different weight groups in order to assess how their bioaccumulation is affected by allometry and, hence, the individual's age. This study is one of the few experiments investigating bioaccumulation capacities of radionuclides in a West-African bivalve. Results showed that allometric relationships were mainly dependent on the exposure pathway considered. Significant relationships with body weight of bloody cockles were found during the uptake from dissolved phase for both radionuclides; they followed inverse power functions: smaller cockles concentrated both radionuclides more than larger ones. In contrast, radionuclide absorption and assimilation efficiencies from water and food, respectively, did not show any significant relationship with weight: only slight variation was observed between small and large organisms for the retention of ²⁴¹Am accumulated from food. A bioaccumulation model was used to assess the contribution of each pathway of exposure (food vs. water) in organisms grouped in small and large individuals. We found that, regardless of the size, ¹³⁴Cs was mainly bioaccumulated through the dietary pathway. In the case of ²⁴¹Am, the relative contribution of each pathway is weight-dependent: major contribution of dissolved pathway in smaller organisms and the major dietary contribution in larger organisms.

Acidified seawater increases accumulation of cobalt but not cesium in manila clam Ruditapes philippinarum

The pH of seawater around the world is expected to continue its decline in the near future in response to ocean acidification that is driven by heightened atmospheric CO₂ emissions. Concomitantly, economically-important molluscs that live in coastal waters including estuaries and embayments, may be exposed to a wide assortment of contaminants, including trace metals and radionuclides. Seawater acidification may alter both the chemical speciation of select elements as well as the physiology of organisms, and may thus pose at risk to many shellfish species, including the manila clam Ruditapes philippinarum. The bioconcentration efficiency of two common radionuclides associated with the nuclear fuel cycle, ¹³⁴Cs and ⁵⁷Co, were investigated by exposing live clams to dissolved ¹³⁴Cs and ⁵⁷Co at control (pH = 8.1) and two lowered pH (pH = 7.8 and 7.5) levels using controlled aquaria. The uptake and depuration kinetics of the two radionuclides in the whole-body clam were followed for 21 and 35 days, respectively. At steady-state equilibrium, the concentration factor (CF_ss) for ⁵⁷Co increased as the pH decreased (i.e. 130 ± 5, 194 ± 6, and 258 ± 10 at pH levels 8.1, 7.8 and 7.5, respectively), whereas the ¹³⁴Cs uptake was not influenced by a change in pH conditions. During depuration, the lowest depuration rate constant of ⁵⁷Co by the manila clam was observed at the intermediate pH of 7.8. An increase in the accumulation of ⁵⁷Co at the intermediate pH value was thought to be caused mainly by the aragonitic shell of the clam, as well as the low salinity and alkalinity of seawater used in the experiment. Considering that accumulation consists of uptake and depuration, among the three pH conditions moderately acidified seawater enhanced most the accumulation of ⁵⁷Co. Accumulation of ¹³⁴Cs was not strongly influenced by a reduced pH condition, as represented by an analogous uptake constant rate and CF_ss in each treatment. Such results suggest that future seawater pH values that are projected to be lower in the next decades, may pose a risk for calcium-bearing organisms such as shellfish.

Differential bioaccumulation of (134)Cs in tropical marine organisms and the relative importance of exposure pathways

Bioaccumulation of (134)Cs was determined in 5 tropical marine species: three bivalves (the oysters Isognomon isognomum and Malleus regula, and the clam Gafrarium pectinatum), one decapod (shrimp Penaeus stylirostris) and one alga (Lobophora variegata). Marine organisms were exposed to the radionuclides via different pathways: seawater (all of them), food (shrimp and bivalves) and sediment (bivalves). Our results indicate that the studied tropical species accumulate Cs similarly than species from temperate regions whereas retention capacities seems to be greater in the tropical species. Bioaccumulation capacities of the two oysters were similar for all the exposure pathways. The alga, and to a lesser extent the shrimp, concentrated dissolved Cs more efficiently than the bivalves (approx. 14 and 7 times higher, respectively). Assimilation efficiencies of Cs in bivalves and shrimp after a single feeding with radiolabelled food were comprised between 7.0 ± 0.4 and 40.7 ± 4.3%, with a variable retention time (half-life -Tb1/2- ranging from 16 ± 3 to 89 ± 55 d). Although the clam lives buried in the sediment, this exposure pathway resulted in low bioaccumulation efficiency for sediment-bound Cs (mean transfer factor: 0.020 ± 0.001) that was lower than the two oyster species, which are not used to live in this media (0.084 ± 0.003 and 0.080 ± 0.005). Nonetheless, Cs accumulated from sediment was similarly absorbed (61.6 ± 9.7 to 79.2 ± 2.3%) and retained (Tb1/2: 37 ± 2 to 58 ± 25 d) for the three bivalves species. Despite the poor transfer efficiency of Cs from food, the use of a global bioaccumulation model indicated that the trophic pathways was the main uptake route of Cs in the bivalves and shrimp. In shelled organisms, shells played a non-negligible role in Cs uptake, and their composition and structure might play a major role in this process. Indeed, most of the Cs taken up from seawater and sediment was principally located on the hard parts of the bivalves and shrimp, with the exception of G. pectinatum, where Cs was mainly distributed in the soft-parts.

Delineation of ¹³⁴Cs uptake pathways (seawater and food) in the variegated scallop Mimachlamys varia

Among bivalves, scallops have been shown to be good bioindicator species for radionuclide monitoring. The present paper looked at the Cs bioaccumulation capacities of the variegated scallop Mimachlamys varia exposed separately via seawater and food under laboratory conditions. Results were compared with data previously obtained for the king scallop Pecten maximus, the only Pectinid species for which Cs accumulation has been studied in laboratory. Results indicated that M. varia has higher uptake capacity (CF: 1.86 ± 0.08) but lower absorption efficiency (A0l: 33 ± 5%) than P. maximus when exposed to waterborne Cs (CF of P. maximus: 0.94 ± 0.05 and A0l: 45 ± 3%). When scallops were fed radiolabeled phytoplankton, the assimilation efficiency of Cs was similar for the two species (AE: 24 ± 3% for M. varia and 28 ± 4% for P. maximus). Interspecific differences in terms of accumulation and retention, can be explained by physiological factors (including size of individuals) and/or difference in storage mechanisms. Indeed, organotropism differed between the two scallop species, suggesting the occurrence of specific redistribution mechanisms towards the tissues involved in Cs storage, excretion and detoxification. Finally, the present study examined the relative contribution of the different exposure pathways (seawater and food) to global (134)Cs bioaccumulation for M. varia. Results showed that food constitutes the main accumulation pathway, contributing for 77% of the global (134)Cs bioaccumulation.

Assessment of growth, genotoxic responses and expression of stress related genes in the Pacific oyster Crassostrea gigas following chronic exposure to ionizing radiation

Marine organisms are exposed to low doses of anthropogenic contaminants during their entire life. Authorized amounts of radionuclides are discharged in the Channel by nuclear facilities. The Pacific oyster was used to investigate the potential impact of chronic exposure to ionizing radiation. Though we exposed larvae and spat for two weeks to much higher concentrations than those encountered near nuclear facilities, oyster growth and expression of 9 selected stress genes were not significantly changed. To determine potential DNA damage, 2year old oysters were exposed for two weeks to tritiated water. The comet assay was used to evaluate the level of DNA strand breaks in haemocytes, whilst the 'clearance rate' was used as a measure of physiological effects. Whilst other parameters did not alter, DNA damage significantly increased. Our results highlight the significance of the observed DNA damage and their potential consequences at higher levels of biological organization.

Biokinetics of radiocesium in shrimp (Palaemon adspersus): seawater and food exposures

The bioaccumulation of (134)Cs was studied in the shrimp Palaemon adspersus (Rathke, 1837) using dissolved or food pathways. The uptake and loss kinetics (following seawater and food uptake) were followed for 27 and 38 days, respectively. The steady state concentration factor (CFss) value of (134)Cs in the whole body of the shrimp was found to be 15 ± 0.08. The loss kinetics of radiocesium was described by a two-component exponential model, with a biological half-life of 85.5 days for the whole body. The depuration kinetics of (134)Cs was best fitted to a single-component exponential model for both edible and inedible parts. The depuration kinetics of (134)Cs following exposure via pulse-chase feeding was also described by a two-component exponential model, with a biological half-life of 84.2 days. Assimilation efficiency (AE) was found to be 38.5%. Most of the radioactivity was accumulated in muscular tissues (the edible part) of the shrimp compared to the remaining soft parts. The average of the total body burden of (134)Cs eliminated with molting was %15.3 ± 8.1.

Contamination of terrestrial gastropods, Helix aspersa maxima, with 137Cs, 85Sr, 133Ba and 123mTe by direct, trophic and combined pathways

(137)Cs, (85)Sr, (133)Ba and (123m)Te contaminations of terrestrial gastropods, Helix aspersa maxima, by direct deposition, labelled food ingestion or combined (trophic and direct pathways) exposure were carried out under laboratory conditions. The aim of this study was to compare the three contamination pathways: direct, trophic and combined, in terms of individual mortality, radionuclide uptake, depuration and distribution in the tissues. An initial group of 30 snails (2 years old) was exposed to radioactive aerosols during a 20-h period. These aerosols were assumed to be representative of those that would be released during a nuclear accident occurring in a PWR. A second group of 50 snails (same age) was submitted to an ingestion of commercial food contaminated by the same aerosols, twice a week for 21 days (flour at a feeding rate of about 0.2g). A third group of 40 snails was submitted to a combined exposure: exposure to radioactive aerosols (20h), followed by ingestion of flour contaminated by the same aerosols, twice a week for 21 days. No significant difference between the three groups and a reference group of 10 snails was observed, neither in growth nor in mortality. Concerning the direct pathway, at the end of direct deposition (about 1 day after the beginning), cesium was the most bioavailable element, distributed rather homogeneously throughout the whole body (13% of the total Cs in all organs excepting the digestive system and 28% in the muscle). Strontium was measured in the shell (about 70%). Barium was found in the muscle (20%) and in the shell (65%). Tellurium was mainly present in the shell (70%) and in the digestive system (20%). After 21 days of depuration, the faeces eliminated 42% of the Te. As for contamination by ingestion, Te mainly accumulated in the digestive system (72% of Te present in the total body), Ba accumulated in the muscle (75%) and Sr in the shell (70%). Concerning contamination by combined pathways, at the end of the 21-day exposure, the 4 radionuclides had the same tendency as direct deposition. However, the effect of the trophic pathway was significant: it causes an 18% increase of Sr in the shell and an 7% increase of Cs in the digestive system in comparison to direct deposition, resulting in a final 86% in the shell and 27% in the digestive system.