Kiriko Miyamoto

Ecological effects of various toxic agents on the aquatic microcosm in comparison with acute ionizing radiation

The purpose of this study was an evaluation of the effect levels of various toxic agents compared with acute doses of ionizing radiation for the experimental model ecosystem, i.e., microcosm mimicking aquatic microbial communities. For this purpose, the authors used the microcosm consisting of populations of the flagellate alga Euglena gracilis as a producer, the ciliate protozoan Tetrahymena thermophila as a consumer and the bacterium Escherichia coli as a decomposer. Effects of aluminum and copper on the microcosm were investigated in this study, while effects of gamma-rays, ultraviolet radiation, acidification, manganese, nickel and gadolinium were reported in previous studies. The microcosm could detect not only the direct effects of these agents but also the community-level effects due to the interspecies interactions or the interactions between organisms and toxic agents. The authors evaluated doses or concentrations of each toxic agent which had the following effects on the microcosm: (1) no effects; (2) recognizable effects, i.e., decrease or increase in the cell densities of at least one species; (3) severe effects, i.e., extinction of one or two species; and (4) destructive effects, i.e., extinction of all species. The resulting effects data will contribute to an ecological risk assessment of the toxic agents compared with acute doses of ionizing radiation.

Effects of gamma-rays on the populations of the steady-state ecological microcosm.

PURPOSE To investigate the dose-response of an aquatic microcosm exposed to gamma-rays and to test the suitability of microcosms for elucidation of the mechanisms that account for such ecological effects. MATERIALS AND METHODS The microcosm used in this study consisted of algae Euglena gracilis Z as a producer, protozoa Tetrahymena thermophila B as a consumer and bacteria Escherichia coli DH5alpha as a decomposer. After the steady-state microcosm was irradiated with 60Co gamma-rays at various dose levels, the population densities of each species were measured. RESULTS Irradiation at up to 100 Gy did not affect the population of the microcosm except for a temporary decrease of E. coli just after irradiation. At 500 or 1000Gy, E. coli died out just after irradiation. Only Eu. gracilis and T. thermophila could survive. Their populations, however, decreased compared with that of each control, except for a temporary increase of T. thermophila after 1000 Gy irradiation. These population changes were attributable to the extinction of E. coli in addition to the direct effects of radiation. Irradiation at 5000 Gy extinguished all species in the microcosm. CONCLUSIONS The response of the microcosm to radiation was dose-dependent over a range of high doses. The microcosm was also shown to be suitable for examining not only direct effects but also secondary effects.

Effects of acute γ-irradiation on the aquatic microbial microcosm in comparison with chemicals

Effects of acute gamma-irradiation were investigated in the aquatic microcosm consisting of green algae (Chlorella sp. and Scenedesmus sp.) and a blue-green alga (Tolypothrix sp.) as producers; an oligochaete (Aeolosoma hemprichi), rotifers (Lecane sp. and Philodina sp.) and a ciliate protozoan (Cyclidium glaucoma) as consumers; and more than four species of bacteria as decomposers. At 100 Gy, populations were not affected in any taxa. At 500-5000 Gy, one or three taxa died out and populations of two or three taxa decreased over time, while that of Tolypothrix sp. increased. This Tolypothrix sp. increase was likely an indirect effect due to interspecies interactions. The principal response curve analysis revealed that the main trend of the effects was a dose-dependent population decrease. For a better understanding of radiation risks in aquatic microbial communities, effect doses of gamma-rays compared with copper, herbicides and detergents were evaluated using the radiochemoecological conceptual model and the effect index for microcosm.

Transfer of technetium from paddy soil to rice seedling

Tracer experiments on the chemical transformation of technetium in paddy soil and the transfer to rice seedlings have been carried out using 95mTc as a tracer. Two common Japanese soils, Andosol and Gray lowland soil were used in the soil incubation experiments. The chemical form of soluble Tc in soil water was a mixture of Tc-organic matter complex, Tc-iron complex and pertechnetate. An uptake experiment with rice seedlings using nutrient solution showed that the Tc-organic matter complex was less available than pertechnetate or the Tc-iron complex. These chemical forms of Tc were also observed in the root bleeding sap of rice seedlings when introduced to the nutrient solution containing soluble Tc. These results suggested that the transfer of technetium from soil to rice would depend on the chemical form of Tc and they would transport from the root to the leaf without chemical transformation.

Measured and modelled tritium concentrations in freshwater Barnes mussels (Elliptio complanata) exposed to an abrupt increase in ambient tritium levels.

To improve understanding of environmental tritium behaviour, the International Atomic Energy Agency (IAEA) included a Tritium and C-14 Working Group (WG) in its EMRAS (Environmental Modelling for Radiation Safety) program. One scenario considered by the WG involved the prediction of time-dependent tritium concentrations in freshwater mussels that were subjected to an abrupt increase in ambient tritium levels. The experimental data used in the scenario were obtained from a study in which freshwater Barnes mussels (Elliptio complanata) were transplanted from an area with background tritium concentrations to a small Canadian Shield lake that contains elevated tritium. The mussels were then sampled over 88 days, and concentrations of free-water tritium (HTO) and organically-bound tritium (OBT) were measured in the soft tissues to follow the build-up of tritium in the mussels over time. The HTO concentration in the mussels reached steady state with the concentration in lake water within one or two hours. Most models predicted a longer time (up to a few days) to equilibrium. All models under-predicted the OBT concentration in the mussels one hour after transplantation, but over-predicted the rate of OBT formation over the next 24h. Subsequent dynamics were not well modelled, although all participants predicted OBT concentrations that were within a factor of three of the observation at the end of the study period. The concentration at the final time point was over-predicted by all but one of the models. The relatively low observed concentration at this time was likely due to the loss of OBT by mussels during reproduction.

A new method using enhanced green fluorescent protein (EGFP) to determine grazing rate on live bacterial cells by protists

Abstract A new method was developed for estimating the grazing rate of live bacteria by protists. Bacterial cells (Escherichia coli bearing plasmid pEGFP) expressing enhanced green fluorescent protein (EGFP) were used as a live bacterial tracer. Ciliates (Tetrahymena thermophila) were fed with EGFP-tagged bacterial cells, and the individual cells taken up by the ciliates were detected by epifluorescence microscopy. The EGFP fluorescence was stable during the storage of samples fixed with glutaraldehyde. Comparison of clearance rates based on the uptake of EGFP-tagged live cells and fluorescently-labeled heat-killed cells suggested that the use of heat-killed cells underestimates the clearance rates. We suggest that EGFP-tagged bacteria are a useful tracer for determining protist bacterivory in culture and aquatic environments.

Reconstruction of two victims' posturing based on the induced radioactivities in their bones in the criticality accident in Tokai-Mura, Japan.

On 30 September 1999, three workers were severely exposed to neutrons and gamma rays in a criticality accident that occurred at a uranium conversion facility in Tokai-mura, Ibaraki Prefecture, Japan. Radiochemical analyses of 32P and 45Ca induced by neutrons in bone matrix were carried out after the deaths of two of the victims. It was found that more than several million becquerels of both nuclides had been produced in their body skeletons. Results showed non-homogeneous distributions of neutron fluence in the bodies, from which it could be deduced how both workers were positioned relative to the fission source during exposure, i.e., at the moment of the first nuclear excursion. For the victim who died first, the activities in the central part of his body were more than those of his extremities. Also, in the central part of his body, the right side showed more activities than the left side. As for the second man, the activities indicated rather uniform exposure to neutrons to the whole body although the geometrical distribution of the activity varied enough to assume his orientation. Such information on the geometrical distribution of neutron-induced radioactivities in the skeleton can be used to reconstruct the posturing of the victims, which is necessary to estimate their apparent absorbed doses.

Comparative biokinetics of tritium in rats during continuous ingestion of tritiated water and tritium-labeled food.

Purpose : The biokinetics of tritium during continuous ingestion of tritiated water and tritiated wheat were investigated to estimate the radiation dose rates at the end of two modes of chronic exposure. Materials and methods : Wistar strain male rats continuously ingested tritiated water as drinking water and tritiated wheat as food for 14 weeks. Urine and tissue samples were obtained and total tritium in the fresh wet samples and organically bound tritium (OBT) in the freeze-dried samples were determined. Results : The biokinetics of tritium was different between the two modes of exposure. The concentration of total tritium in the tissues exposed to tritiated water attained a steady-state condition by 2-3 weeks. The steady-state condition in the case of exposure to tritiated wheat was not observed for 10 weeks after the start of exposure in the majority of tissues. The relatively efficient and prolonged OBT formation during chronic exposure to tritiated wheat resulted in relatively high incorporation and retention of tritium in the tissues compared with those for exposure to the same activity of tritiated water. Conclusion : Radiation dose rates estimated at the end of continuous ingestion showed that tritiated wheat gave higher dose rates than tritiated water by a factor of 1.3 to 4.5, but the factors were within 2.0 in the majority of tissues except for small intestine and adipose tissue.

Biokinetics of radiocarbon ingested as a food in rats.

Biokinetics of radiocarbon (14C) in rats during 14 wk of continuous ingestion of 14C-wheat was investigated to obtain the dosimetric data on organic 14C ingested as a food. After the start of continuous ingestion of 14C-wheat, the concentrations of 14C in both wet and dry tissues gradually increased and showed a tendency towards a steady-state condition for most of the tissues, except adipose tissue. The steady-state condition was observed at 10 wk in the majority of tissues, but was obtained at 3 wk in liver. The steady-state concentrations in wet tissues ranged from about 10% to 40%, and those in dry tissues ranged from about 80% to 100% of the concentration of 14C in the ingested food. The highest steady-state concentrations in both wet and dry tissues were observed in liver. When compared the concentrations among tissues at the end of 14 wk of continuous ingestion, adipose tissue was the highest in wet tissue and the lowest in dry tissue. From the concentrations of 14C in wet tissues at the 14th week, the radiation dose rates to individual tissues were estimated. The dose rates were different among the tissues. The highest dose rate was observed in adipose tissue. The next highest dose rates were found in liver and small intestine, while lower doses were measured in testis and lung. The differences in the dose rate among the tissues seemed to be mainly dependent on the variation in the organic content of each tissue.

Determination of 32P in urine for early estimation of the neutron exposure level for three victims of the JCO criticality accident

In the criticality accident which occurred on 30 September 1999 at a uranium conversion facility in Tokai-mura, Japan, three workers were severely exposed to neutron and gamma-ray irradiation. Preliminary estimations of doses from blood properties and 24Na concentration in blood were 16-20, 6-10 and 1-4 gamma-ray gray-equivalent (gammaGyEq) respectively for the three workers. For apparent dose estimation, neutron-induced radionuclides in biological materials such as blood, hair and urine were measured. Accordingly, we detected 32p in urine samples. The concentration ratios of 32P in the urine for the three workers showed a similar tendency to those of 24Na in blood. This result indicated that the radioactivity of 32P in urine could be used to estimate the neutron exposure level.