Effects of γ-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 micro-cosms 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 DH5 alpha as a decomposer. After the steady-state microcosm was irradiated with Co-60 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 1000 Gy, 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.