Effects of Acute Gamma-irradiation on the Aquatic Microbial Microcosm in Comparison with Chemicals

Ecosystems consist of various organisms, and there are complex interactions among these organisms. Indirect effects through interspecies interactions should be considered as well as direct effects of ionising radiation when environmental effects are evaluated. We therefore began to investigate radiation effects on experimental model ecosystems, i.e., microcosms mimicking aquatic microbial communities. The microcosm used in this study consisted of green algae (Chlorella sp. and Scenedesmus sp.) and blue-green alga (Tolypothrix sp.) as producers; oligochaete (Aeolosoma hemprichi), rotifers (Lecane sp. and Philodina sp.) and ciliate protozoa (Cyclidium glaucoma) as consumers; and four or more species of bacteria as decomposers. The steady-state microcosm was acutely irradiated with 60Co gamma-rays at 100, 500, 1000 and 5000 Gy, and population changes were observed for 25 days. In the control microcosm, population densities of the constituent organisms were almost constant during the experiment. At 100 Gy, significant effects were not observed for any organisms. At 500 Gy, A. hemprichi died out. Green algae and bacteria were not affected until day 15, but were decreased on day 25. C. glaucoma was temporarily decreased just after irradiation, but the cell density was recovered to a control level on day 7. Tolypothrix sp. was not affected until day 15, but was increased on day 25. Rotifers were not affected. At 1000 Gy, A. hemprichi died out. Bacteria were decreased just after irradiation, and the cell density remained slightly lower than controls during the experiment. Chlorella sp. and Scenedesmus sp. were not affected until day 15 and 10, respectively, but were decreased thereafter. C. glaucoma was temporarily decreased just after irradiation, but the cell density was recovered to a control level on day 10. Tolypothrix sp. was not affected until day 15, but was increased on day 25. Rotifers were not affected. At 5000 Gy, A. hemprichi, Lecane sp., and C. glaucoma died out. Bacteria were decreased just after irradiation, and the cell density remained significantly lower than controls during the experiment. Green algae were not affected until day 10, but were decreased thereafter. Tolypothrix sp. was not affected until day 15, but was increased on day 25. Philodina sp. was not affected. Most of observed effects were adverse, and were generally depended on radiation doses. But Tolypothrix sp. was increased at 500 Gy or higher doses. It is thought that this unexpected result arose from indirect effects. In the irradiated microcosm, decrease in green algae might provide Tolypothrix sp. with an advantage in the competition for light and inorganic nutrients. Breakdown products such as CO2 and inorganic nutrients originated from dead organisms might also contribute to increase in Tolypothrix sp. Equi-dosimetric analysis will be carried out between gamma-rays and some chemicals. That is, effective radiation doses evaluated in this study will be quantitatively compared with effective concentrations of herbicides (simazine and thiobencarb), surfactants (LAS and soap) and copper, which had been evaluated in other studies.International Conference on Radioecology and Enviornmental Radioactivit