Derivation of hazardous doses for amphibians acutely exposed to ionising radiation.

Derivation of effect benchmark values for each taxonomic group, which has been difficult due to lack of experimental effects data, is required for more adequate protection of the environment from ionising radiation. Estimation of effects doses from nuclear DNA mass and subsequent species sensitivity distribution (SSD) analysis were proposed as a method for such a derivation in acute irradiation situations for assumed nuclear accident scenarios. As a case study, 5% hazardous doses (HD5s), at which only 5% of species are acutely affected at 50% or higher lethality, were estimated on a global scale. After nuclear DNA mass data were obtained from a database, 50% lethal doses (LD50s) for 4.8 and 36% of the global Anura and Caudata species, respectively, were estimated by correlative equations between nuclear DNA mass and LD50s. Differences between estimated and experimental LD50s were within a factor of three. The HD5s obtained by the SSD analysis of these estimated LD50s data were 5.0 and 3.1 Gy for Anura and Caudata, respectively. This approach was also applied to the derivation of regional HD5s. The respective HD5s were 6.5 and 3.2 Gy for Anura and Caudata inhabiting Japan. This HD5 value for the Japanese Anura was significantly higher than the global value, while Caudata had no significant difference in global and Japanese HD5s. These results suggest that this approach is also useful for derivation of regional benchmark values, some of which are likely different from the global values.

Environmental protection: researches in National Institute of Radiological Sciences.

Some studies for radiological protection of the environment have been made at the National Institute of Radiological Sciences (NIRS). Transfer of radionuclides and related elements has been investigated for dose estimation of non-human biota. A parameter database and radionuclide transfer models have been also developed for the Japanese environments. Dose (rate)-effect relationships for survival, growth and reproduction have been investigated in conifers, Arabidopsis, fungi, earthworms, springtails, algae, duckweeds, daphnia and medaka. Also genome-wide gene expression analysis has been carried out by high coverage expression profiling (HiCEP). Effects on aquatic microbial communities have been studied in experimental ecosystem models, i.e., microcosms. Some effects were detected at a dose rate of 1 Gy day(-1) and were likely to arise from interspecies interactions. The results obtained at NIRS have been used in development of frameworks for environmental protection by some international bodies, and will contribute to environmental protection in Japan and other Asian countries.

mRNA expression of a cadmium-responsive gene is a sensitive biomarker of cadmium exposure in the soil collembolan Folsomia candida.

The gene expression of environmental organisms is useful as a biomarker of environmental pollution. One of its advantages is high sensitivity. We identified the cDNA of a novel cadmium-responsive gene in the soil collembolan Folsomia candida. The deduced protein, designated "metallothionein-like motif containing protein" (MTC), was cysteine-rich and contained a metallothionein-like motif with similarity to metallothionein, but had a much longer sequence than metallothionein and contained repeated sequences of amino acids. Expression of MTC mRNA was sensitively induced by cadmium exposure at 0.3 mg/kg of dry food, a concentration at which toxic effects are not observed, but expression was not affected by gamma-ray exposure (an inducer of oxidative stress). These findings suggest that MTC is involved in cadmium-binding processes rather than in oxidative-stress responses. In conclusion, we suggest that gene expression of MTC may be a candidate biomarker for detecting low levels of cadmium contamination in soil.

Application of HiCEP to screening of radiation stress-responsive genes in the soil microarthropod Folsomia candida (Collembola).

The field of ecotoxicogenomics has received increasing attention for its potential to provide insight into pressing ecological issues. However, its applications are limited due to a lack of genetic sequence information for organisms used in ecotoxicological studies. We used high-coverage expression profiling (HiCEP), a method that requires no prior sequence knowledge, to examine stress-responsive genes and their dose dependence in the springtail Folsomia candida using gamma radiation as the stressor. Radiation-responsive genes and their dose dependency were detected at effective doses for reproduction, and 16 up-regulated transcript-derived fragments (TDFs) were sequenced. Quantitative PCR analysis also found that most of the TDFs were up-regulated. The sequences of the TDFs showed resemblance to known genes, such as glutathione S-transferase and poly(ADP-ribose) polymerase, but most showed no similarity to any genes in the gene databases. These results suggest that HiCEP is effective for discovering differently expressed genes and their dose dependence, even in organisms for which few sequence data are available. The limited length of the TDFs, however, may impede functional annotation of the genes. In conclusion, HiCEP is useful for ecotoxicogenomic studies in which various organisms with few available genomic resources are involved.

Effects of acute gamma irradiation on Folsomia candida (Collembola) in a standard test.

An understanding of the effects of ionizing radiation on non-human biota is required by the International Commission on Radiological Protection for the radiological protection of the environment. We examined dose-effect relationships for gamma radiation on survival, growth, and reproduction in the soil invertebrate Folsomia candida (Collembola) in a standard laboratory test. F. candida were acutely irradiated at increasing doses of gamma radiation, and subsequent survival, growth in body length, and number of neonates produced by irradiated specimens were examined. The 50% lethal dose was at 1356 Gy, and the 10% and 50% effective doses (ED10 and ED50) for growth were at 32 and 144 Gy, respectively. The ED10 and ED50 values for reproduction were at 7.1 and 21.9 Gy, respectively. These data establish important baselines for the radiological protection of terrestrial ecosystems based on scientific principles.

Rate of iodine volatilization and accumulation by filamentous fungi through laboratory cultures.

Five strains of basidiomycetes (Lentinula edodes, Coprinus phlyctidosporus, Hebeloma vinosophyllum, Pleurotus ostreatus and Agaricus bisporus), one strain of ascomycete (Hormoconis resinae) and six strains of imperfect fungi (Penicillium chrysogenum, Penicillium roquefortii, Cladosporium cladosporioides, Alternaria alternata, Aspergillus niger and Aspergillus oryzae) were cultured in a liquid medium containing a radioactive iodine tracer ((125)I), and were tested for their abilities to volatilize or accumulate iodine. Of the fungal strains tested, 11 strains volatilized a considerable amount of iodine, with L. edodes showing the highest volatilization rate of 3.4%. The volatile organic iodine species emitted from imperfect fungi cultures was identified as methyl iodide (CH(3)I). In contrast, six fungal strains in 12 strains accumulated a considerable amount of iodine from the medium with concentration factors of more than 1.0. Among these, Alt. alternata and Cl. cladosporioides accumulated more than 40% of the iodine in their hyphae, and showed high concentration factors of 22 and 18, respectively. These results suggest that filamentous fungi have a potential to influence the mobility and speciation of iodine by volatilization and accumulation. Considering their great biomass in soils, filamentous fungi may contribute to the global circulation of stable iodine and also the long-lived radioiodine, (129)I (half-life: 1.6 x 10(7)years), released from nuclear facilities into the environment.

Isolation of iodide-oxidizing bacteria from iodide-rich natural gas brines and seawaters.

Iodide-oxidizing bacteria (IOB), which oxidize iodide (I-) to molecular iodine (I2), were isolated from iodide-rich (63 microM to 1.2 mM) natural gas brine waters collected from several locations. Agar media containing iodide and starch were prepared, and brine waters were spread directly on the media. The IOB, which appeared as purple colonies, were obtained from 28 of the 44 brine waters. The population sizes of IOB in the brines were 10(2) to 10(5) colony-forming units (CFU) mL(-1). However, IOB were not detected in natural seawaters and terrestrial soils (fewer than 10 CFU mL(-1) and 10(2) CFU g wet weight of soils(-1), respectively). Interestingly, after the enrichment with 1 mM iodide, IOB were found in 6 of the 8 seawaters with population sizes of 10(3) to 10(5) CFU mL(-1). 16S rDNA sequencing and phylogenetic analyses showed that the IOB strains are divided into two groups within the alpha-subclass of the Proteobacteria. One of the groups was phylogenetically most closely related to Roseovarius tolerans with sequence similarities between 94% and 98%. The other group was most closely related to Rhodothalassium salexigens, although the sequence similarities were relatively low (89% to 91%). The iodide-oxidizing reaction by IOB was mediated by an extracellular enzyme protein that requires oxygen. Radiotracer experiments showed that IOB produce not only I2 but also volatile organic iodine, which were identified as diiodomethane (CH2I2) and chloroiodomethane (CH2ClI). These results indicate that at least two types of IOB are distributed in the environment, and that they are preferentially isolated in environments in which iodide levels are very high. It is possible that IOB oxidize iodide in the natural environment, and they could significantly contribute to the biogeochemical cycling of iodine.

Concentrations of 137Cs and 40K in mushrooms consumed in Japan and radiation dose as a result of their dietary intake.

Mushrooms are known to accumulate radiocesium. To estimate the intake of radiocesium through the eating of mushrooms, about 30 samples belonging to 4 commonly consumed species (Lentinula edodes, Hypsizigus marmoreus, Grifola frondosa, and Tricholoma matsutake), were analyzed for (137)Cs and (40)K. The concentration ranges were 0.060-29 Bq kg(-1) (wet wt) for (137)Cs and 38-300 Bq kg(-1) (wet wt) for (40)K. The geometric mean concentration for (137)Cs was 0.56 Bq kg(-1) (wet wt), and the mean concentration for (40)K was 92 Bq kg(-1) (wet wt). The (137)Cs concentrations in L. edodes cultivated in mushroom beds (sawdust-rice bran media) were lower than those cultivated on bed logs (natural wood with bark). The annual intake of (137)Cs per person through mushrooms was calculated, by using the current analytical results and food consumption data in Japan, to be 3.1 Bq for (137)Cs, which is about 28% of the total dietary intake of this nuclide. The effective dose equivalent of (137)Cs through mushrooms was estimated to be 4.0 currency 10(-8) Sv, which is about the half the value obtained in our previous study. The decrease of the (137)Cs intake through mushrooms is probably related to changes in cultivation methods in recent years, from the use of bed logs to mushroom beds.

Transfer factors of radioiodine from volcanic-ash soil (Andosol) to crops.

In order to obtain soil-to-plant transfer factors (TFs) of radioiodine from volcanic-ash soil to agricultural crops, we carried out radiotracer experiments. The mean values of TFs (on a wet weight basis) of radioiodine from Andosol to edible parts of crops were as follows: water dropwort, 0.24; lettuce, 0.00098; onion, 0.0011; radish, 0.0044; turnip, 0.0013 and eggplant, 0.00010. The mean value of the TFs of radioiodine for edible parts of wheat (on a dry weight basis) was 0.00015. We also studied the distributions of iodine in crops. There was a tendency for the TFs of leaves to be higher than those of tubers, fruits and grains. A very high TF was found for water dropwort, because this plant was cultivated under a waterlogged condition, in which iodine desorbed from soil into soil solution with a drop in the Eh value. The data obtained in this study should be helpful to assess the long-lived 129I (half life: 1.57 x 10(7) yr) pathway related to the fuel cycle.