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Abstract. Mathematical computer model is developed to simulate the population dynamics and dynamic mass budgets of the microbial community realized as a self sustainable aquatic ecological system in the tube. Autotroph algae, heterotroph protozoa and saprotroph bacteria live symbiotically with interspecies' interactions as predator-prey relationship, competition for the common resource, autolysis of detritus and detritus-grazing food chain, etc. The simulation model is the individual-based parallel model, built in the demographic stochasticity, environmental stochasticity by dividing the aquatic environment into patches.Validity of the model is checked by the multifaceted data of the microcosm experiments. In the analysis, intrinsic parameters of umbrella endpoint regarding to lethality are manipulated at the individual level, and tried to find the population level, community level especially focused on predator-prey relationship, and revealed the indirect effect of chronic exposure of radiation on the probability of Tetrahymena's extinction.

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.

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 heatkilled cells suggested that the use of heat-killed cells underestimates the clearance rates. We suggest that EGFPtagged bacteria are a useful tracer for determining protist bacterivory in culture and aquatic environments.

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.