Temporal variation of cesium isotope concentrations and atom ratios in zooplankton in the Pacific off the east coast of Japan

Status of the transfer state of 137Cs in zooplankton and surface water fish off Fukushima during 2018-2021

Radiocaesium introduced to coastal waters by the accident at the Fukushima Dai-ichi Nuclear Power Station (F1NPS) elevated the radioactivity level in biota. The radionuclide level in zooplankton, concentration of 137Cs radioactivity in surface water fish decreased rapidly, which was the primary food source for planktivorous fish, was recognized as not to be depurated at the same level prior to the accident. To evaluate the possible cause of this phenomenon, zooplankton and surface water fish were collected off Fukushima during 2018-2021, and the presence of radioactive particles was also examined. The concentrations of stable Cs and 137Cs radioactivity were analysed along with aluminium which was an indicator element of Cs that originated from suspended particles which were attached to or ingested by zooplankton. As a result, radioactive particles were often found in coastal zooplankton samples, and stable Cs and 137Cs of this inorganic fraction were identified. After removal of this excess radioactivity, the 137Cs radioactivity in whole-body tissue of zooplankton was derived. However, the level in the soft part of zooplankton during 2018-2020 was still greater than the levels which existed before 2010. Since habitat seawater was understood to not be a substantial source of 137Cs, then 137Cs transfer along the food chain, possibly from phytoplankton or detritus of enhanced radioactivity were suggested as important sources of 137Cs. In addition, enhanced 137Cs radioactivity in zooplankton was considered consequently elevate radioactivity levels in surface-dwelling water fish off Fukushima. Although the radioactivity level was not radiologically significant in relation to seafood safety limit, enhanced 137Cs radioactivity levels in biota was demonstrated in southern waters off Fukushima. In contrast, derived 137Cs/133Cs atom ratios of fish and seawater south of 37°E and west from 142°E indicated that radiocaesium transfer between fish and the environment was in an equilibrium state, showing the environment beyond these geographical coordinates had returned to the pre-accident state.

Transport and fate of Fukushima-derived 137Cs and 134Cs in the seawater of the Northwest Pacific in 2015

To understand the influence of the Fukushima accident on the Northwest Pacific, the distributions and transportations of 134Cs and 137Cs in the seawater in the Northwest Pacific in May and September 2015 were studied. The data showed that the Fukushima-derived 134Cs and 137Cs at some stations can still be distinguished from background level ~ 4 years later. On the whole, the activities of 137Cs and 134Cs in seawater were decreasing from May to Sep 2015. But the increased inventories and the surface activities of 137Cs imply that there has ever been an extra 137Cs from offshore water transported to this study area (from 31° N to 27° N, 145° E to 152.5° E) in May 2015. The average activities of 137Cs in subtropical gyre area in south of KE were the highest and the least were to the east of Luzon Strait in 2015. In vertical direction, 137Cs in subtropical gyre area were mainly distributed at 100 ~ 500 m layer and 137Cs only at 500 m layer in this area showed an increasing trend from May to Sep 2015 which reflects more 137Cs were still penetrating to deeper layer of 500 m from upper water. But they were almost not found below 1000 m layer. It was associated with the subsurface transport of radiocesiums by Northwest Pacific Mode Water (NPMW) and the diffusion of mesoscale eddy. Different distribution characteristics of 137Cs existed between north of KE and south of KE. The low-temperature-low-salinity water mass likely to be the first Oyashio Intrusion was the main factor that resulted in higher 137Cs appearing at the upper 100 m layers in north of KE.

Cesium-137 and 137Cs/133Cs atom ratios in marine zooplankton off the east coast of Japan during 2012-2020 following the Fukushima Dai-ichi nuclear power plant accident

We measured the concentrations of cesium isotopes (¹³³Cs, ¹³⁴Cs, and ¹³⁷Cs) in zooplankton samples collected in waters off the east coast of Japan from May 2015 to June 2020. By combining these data with those obtained previously from May 2012 to February 2015, we evaluated the long-term impacts of the Fukushima Dai-ichi Nuclear Power Plant accident on marine zooplankton. Relatively high ¹³⁷Cs concentrations in zooplankton, exceeding 10 Bq/kg-dry weight, were sporadically observed until June 2016, regardless of year or station. After May-June 2017, ¹³⁷Cs concentrations decreased to below 1 Bq/kg-dry at most stations, and by May 2020, concentrations were below 0.5 Bq/kg-dry except those off Fukushima Prefecture. Since the accident, the ¹³⁷Cs/¹³³Cs atom ratios of zooplankton samples were higher than those of ambient seawater until 2019, but in May-June 2020 the ratios matched those of seawater except off Fukushima Prefecture. Highly radioactive particles were not detected in zooplankton samples by autoradiography using imaging plates after May-June 2017, although they were before. Therefore, the persistence of elevated ¹³⁷Cs/¹³³Cs ratios in zooplankton relative to seawater for nine years after the accident was probably due to the incorporation of highly radioactive particles (cesium-bearing particles or clay-mineral aggregates with highly adsorbed radiocesium) onto/into zooplankton for several years after the accident. However, since at least May-June 2017, these elevated ratios have likely been caused by small highly radioactive particles (or larger particles disaggregated into small pieces) entering the ocean from land via rivers or directly discharged from the Fukushima Nuclear Power Plant. Microplastics enriched with radiocesium with higher ¹³⁷Cs/¹³³Cs ratios than seawater may have also contributed ¹³⁷Cs to the zooplankton.

Occurrence of highly radioactive microparticles in the seafloor sediment from the pacific coast 35 km northeast of the Fukushima Daiichi nuclear power plant

To understand the properties and significance of highly radioactive particles in the marine environment, we have examined seafloor sediment with a radioactivity of ∼1200 Bq/kg (dry weight; after decay correction to March 2011) collected 35 km northeast of the Fukushima Daiichi Nuclear Power Plant (FDNPP). Among the 697 highly radioactive particles separated from the sediment, two particles, D1-MAX and D1-MID, had a total Cs radioactivity of ∼56 and 0.67 Bq (after decay correction to March 2011), respectively. These particles were characterized with a variety of electron microscopic techniques, including transmission electron microscopy. The ¹³⁴Cs/¹³⁷Cs radioactivity ratio of D1-MAX, 1.04, was comparable to that calculated for Unit 2 or 3. D1-MAX consisted mainly of a Cs-rich microparticle (CsMP) with a silica glass matrix. The data clearly suggested that D1-MAX resulted from a molten core-concrete interaction during meltdowns. In contrast, D1-MID was an aggregate of plagioclase, quartz, anatase, and Fe-oxide nanoparticles as well as clay minerals, which had adsorbed soluble Cs. D1-MID was likely a terrestrial particle that had been transported by wind and/or ocean currents to a site 35 km from the FDNPP. The radioactive fractions of D1-MAX and D1-MID were 15% and 0.36%, respectively, of the total radioactivity in the bulk sediment. These highly radioactive particles have a great impact on the movement of radioactive Cs in the marine environment by carrying condensed Cs radioactivity with various colloidal and desorption properties depending on the host phase.

Changes in radioactive cesium concentrations from 2011 to 2017 in Fukushima coastal sediments and relative contributions of radioactive cesium-bearing microparticles

Sedimentary cesium-137 concentrations around the Fukushima Daiichi Nuclear Power Plant (FDNPP) were measured from 2011 to 2017 at eight stations. Although high values were observed until 2013, decreasing trends were observed at the surface sediments of seven stations. We isolated 25 radioactive Cs-bearing microparticles (CsMPs; 1.0-5385 Bq per particle). The contribution ratio of CsMPs to each sample ranged from 4.1% to 99.5% (median 58.8%), with the contribution ratio of the CsMPs in the southern part of the FDNPP was low compared to that from the northern part. In the southern part of the FDNPP, small CsMPs that could not be isolated in this study were present in large quantities immediately after the accident, and gradually diffused away and/or were dissolved over time. In contrast, the CsMPs in the northern part of the FDNPP have most likely accumulated over time, as suggested by the silty nature of the sediments there.

Celastrol Alleviates Gamma Irradiation-Induced Damage by Modulating Diverse Inflammatory Mediators

The present study aimed to explore the possible radioprotective effects of celastrol and relevant molecular mechanisms in an in vitro cell and in vivo mouse models exposed to gamma radiation. Human keratinocytes (HaCaT) and foreskin fibroblast (BJ) cells were exposed to gamma radiation of 20 Gy, followed by treatment with celastrol for 24 h. Cell viability, reactive oxygen species (ROS), nitric oxide (NO) and glutathione (GSH) production, lipid peroxidation, DNA damage, inflammatory cytokine levels, and NF-κB pathway activation were examined. The survival rate, levels of interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) in blood, and p65 and phospho-p65 expression were also evaluated in mice after exposure to gamma radiation and celastrol treatment. The gamma irradiation of HaCaT cells induced decreased cell viability, but treatment with celastrol significantly blocked this cytotoxicity. Gamma irradiation also increased free radical production (e.g., ROS and NO), decreased the level of GSH, and enhanced oxidative DNA damage and lipid peroxidation in cells, which were effectively reversed by celastrol treatment. Moreover, inflammatory responses induced by gamma irradiation, as demonstrated by increased levels of IL-6, TNF-α, and IL-1β, were also blocked by celastrol. The increased activity of NF-κB DNA binding following gamma radiation was significantly attenuated after celastrol treatment. In the irradiated mice, treatment with celastrol significantly improved overall survival rate, reduced the excessive inflammatory responses, and decreased NF-κB activity. As a NF-κB pathway blocker and antioxidant, celastrol may represent a promising pharmacological agent with protective effects against gamma irradiation-induced injury.

Depthprofile distribution of Cs and its toxicity for canola plants grown on arid rainfed soils as affected by increasing K-inputs

Radioactive cesium (Cs) is more likely to be trans-located via rainfall into surrounding environments. Upon Cs-contaminated water reaching soil, Cs is retained on soil components, mainly organic matter and clay fraction. This study aims are i) comparing the relative ability of five arid soils, differing in their textural and chemical properties, to accumulate Cs when subjected to Cs-artificially contaminated rain droplets and ii) testing whether K fertilizer can decrease the uptake of Cs and its translocation within plants or not. A lab experiment was then conducted to simulate artificial rain droplets contaminated with 1000 becquerel (Bq) of ¹³⁴Cs L^-1 precipitated on soil columns each of 10.5 cm inner diameter at a rate of 1.15 mL cm^-2 over a period of 2-months. At least 89% of ¹³⁴Cs accumulated within the uppermost 5-cm layer of these soils. Another greenhouse experiment was set to test the hypothesis which indicates that Cs uptake increases unexpectedly by supplying plants with K-fertilizers. In this experiment, canola (Brassica napus L.) seeds were cultivated into three K-deficient soils (Typic Haplotorrent, Typic Haplocalcid, and Typic Torripsamment) which were contaminated with 100 mg Cs kg^-1 soil (stable-Cs was used instead of radioactive-Cs to designate its behavior on the long run). Canola plants were fertilized with 0, 80 and 120 mg K₂SO₄ kg^-1 soil. Results carried on Typic Haplotorrent soil confirmed the aforementioned assumption as K-addition increased Cd-uptake up to 40.1%. Contradictory results were achieved in the other two soils where Cs-uptake decreased by 21.5 and 15.3% in Typic Haplocalcid and Typic Torripsamment soils, respectively due to the application of the aforementioned dose of K. In the K non-amended soils, Cs shoot-root translocation factor was >1; yet, it was <1 in response to K addition, regardless of its application rate.

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.

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.

Contribution of 137Cs-enriched particles to radiocesium concentrations in seafloor sediment: Reconnaissance experiment

Autoradiography was used to detect ¹³⁷Cs-enriched particles in sediment samples. The contributions of ¹³⁷Cs-enriched particles to ¹³⁷Cs concentrations in sediment samples ranged from 9% to 64%. These experiments revealed that the variability of ¹³⁷Cs concentrations was due mainly to the heterogeneous distribution of ¹³⁷Cs-enriched particles in the samples. Therefore, the heterogeneous distribution of ¹³⁷Cs-enriched particles is probably one of the main factors responsible for the temporal and spatial variations of ¹³⁷Cs concentrations in sediment samples.

Potential Protective Effects of Ursolic Acid against Gamma Irradiation-Induced Damage Are Mediated through the Modulation of Diverse Inflammatory Mediators

This study was aimed to evaluate the possible protective effects of ursolic acid (UA) against gamma radiation induced damage both in vitro as well as in vivo. It was observed that the exposure to gamma radiation dose- and time-dependently caused a significant decrease in the cell viability, while the treatment of UA attenuated this cytotoxicity. The production of free radicals including reactive oxygen species (ROS) and NO increased significantly post-irradiation and further induced lipid peroxidation and oxidative DNA damage in cells. These deleterious effects could also be effectively blocked by UA treatment. In addition, UA also reversed gamma irradiation induced inflammatory responses, as indicated by the decreased production of TNF-α, IL-6, and IL-1β. NF-κB signaling pathway has been reported to be a key mediator involved in gamma radiation-induced cellular damage. Our results further demonstrated that gamma radiation dose- and time-dependently enhanced NF-κB DNA binding activity, which was significantly attenuated upon UA treatment. The post-irradiation increase in the expression of both phospho-p65, and phospho-IκBα was also blocked by UA. Moreover, the treatment of UA was found to significantly prolong overall survival in mice exposed to whole body gamma irradiation, and reduce the excessive inflammatory responses. Given its radioprotective efficacy as described here, UA as an antioxidant and NF-κB pathway blocker, may function as an important pharmacological agent in protecting against gamma irradiation-induced injury.

Temporal changes in radiocesium contamination derived from the Fukushima Dai-ichi Nuclear Power Plant accident in oceanic zooplankton in the western North Pacific

We investigated temporal changes of the contamination of oceanic zooplankton with radiocesium (¹³⁴Cs and ¹³⁷Cs) derived from the Fukushima Dai-ichi Nuclear Power Plant accident one month to three years after the accident at subarctic and subtropical stations (1900 and 900-1000 km from the plant, respectively) in the western North Pacific. The maximum activity concentrations of ¹³⁷Cs in zooplankton were two orders of magnitude higher than the pre-accident level. In the first four months after the accident, the activity concentrations of radiocesium in subtropical zooplankton decreased rapidly, but no similar change was observed at the subarctic station. The radiocesium derived from atmospheric deposition rapidly decreased as a result of seawater mixing. Thus, most of the subtropical zooplankton (with short lifespans) that had taken up radiocesium just after the accident were probably replaced by newly hatched zooplankton within four months of the accident, whereas subarctic zooplankton (with long lifespans) that were highly contaminated with radiocesium were still alive four months after the accident. By the end of the study, ¹³⁷Cs activity concentrations in subtropical zooplankton were still high, whereas the activity concentrations in subarctic zooplankton had decreased to nearly the pre-accident level. The former concentrations were probably influenced by a secondary supply of radiocesium via advection of subtropical mode water that was highly contaminated with Fukushima-derived radiocesium. Unexpectedly, at the subarctic station, the radiocesium activity concentrations in surface zooplankton were lower than those in subsurface zooplankton, whereas the opposite relationship was observed in surface and subsurface seawater. Because carnivores predominated in the subsurface zooplankton community, we hypothesize that the higher radiocesium activity concentrations in subsurface zooplankton were influenced by bioaccumulation. We conclude that radiocesium activity concentrations in zooplankton are influenced not only by the supply of radiocesium to the environment but also by the characteristics of the zooplankton community.