Annual Report

52. Ecologically Responsive Phenomena of the Aquatic Microcosm System to Ultra Violet C

Kiriko Tanaka-Miyamoto, Hiroshi Takeda, Shoichi Fuma, Nobuyoshi Ishii, Kei Yanagisawa, Yoshikazu Inoue, Chitose Ishii, Kazunori Sugai and Zen-ichiro Kawabata* (*Kyoto Univ.)

Keywords: aquatic microcosm, ecological assessment, Escherichia coli, Euglena gracilis, model ecosystem, Tetrahymena thermophila, ultra violet C

It is necessary to establish a reasonable method to evaluate the ecological effects of any ecological toxicants on microbial communities in the environment, since various microorganisms play a big role in the life support system of human beings. In this study an aquatic microcosm system was adopted as a model ecosystem for accumulating basic data on ecologically responsive phenomena of the microbial communities, rather than on biologically responsive phenomena of single species. This microcosm is one of the simplest biological communities, which can demonstrate indirect effects on species caused by an ecological stress. It consists of three species of microorganisms in a small container like a test tube or a small plastic bottle, and the interactions among the three species have been well investigated. The three are flagellate algae Euglena gracilis as a producer which has chloroplast for photosynthesis, ciliated protozoa Tetrahymena thermophila B as a consumer which grazes bacteria, and bacteria Escherichia coli DH5

as a decomposer which decomposes metabolites and dead bodies of the other two species. The three species can survive by exchanging materials with each other in the closed container with limited nutrients when the microcosm is first started, and their population densities can be kept in a steady state for a long time, usually for more than a year.

During the past few years the authors have reported data on the impact of acidification, manganese and gadolinium on the population of the aquatic microcosm in its growth stage and the steady state. In the present paper, effects of ultra violet C on the population of the aquatic microcosm in the early steady state were investigated.

An experiment was carried out as follows: Each microorganism was preincubated following the literature method of Kawabata et al. Then the three species were inoculated into a culture medium (0.05 % proteose peptone in a half strength of modified Taub and Dollar's solution) in plastic bottles and incubated under 2500 lx and 12-12 h LD light regime at 25Åé. On the 56th day after composition, the microcosm was exposed to 1 kerg/mm2, 10 kerg/mm2, 50 kerg/mm2 and 100 kerg/mm2 of ultra violet C (wave length: 254nm). Population densities of each organism were determined at various time intervals after exposure. The population density of T. thermophila was counted microscopically, that of E. coli was measured by counting colonies formed in the broth-agar medium, and that of Eu. gracilis was measured by the plate culture method.

Fig.17 shows variation of the population densities of the three species in the microcosm. When the microcosm was exposed to 1 kerg/mm2 of ultra violet C, only the population density of E. coli soon decreased and then recovered to the same level as that of control within 3 days, but all three species were affected when the microcosm was exposed to 10 kerg/mm2. In the case of the exposure to 10 kerg/mm2, population density of T. thermophila decreased soon after the exposure and recovered within 4 days. This is an indirect effect of ultra violet C on T. thermophila since T. thermophila followed the time course variation of the population density of E. coli as the food for T. thermophila. Just after the exposure, population density of E. coli decreased soon and recovered within 2 days, while that of T. thermophila did not decrease on the day of exposure, but then decreased on the next day and recovered on the fourth day. In both cases of 50 kerg/mm2 and 100 kerg/mm2, Eu. gracilis and T. thermophila probably experienced a direct effect of ultra violet C, since they showed the same sudden disappearance when they were exposed to ultra violet C in a single species medium not in the microcosm system. However the effect of ultra violet C seems to be moderated by the microcosm system itself, as population density of Eu. gracilis recovered from 7 to 13 days after the exposure. In the case of exposure to a single species not in the microcosm system, Eu. gracilis did not recover at all after exposures of high doses (50 kerg/mm2 and 100 kerg/mm2). This moderation effect of the microcosm system was considered to be caused by the effective absorption activity of ultra violet C with the microcosm medium solution. A variety of chemical compounds derived from metabolism of the three species were capable of effectively absorbing ultra violet C.

The present study demonstrated indirect effects of ultra violet C on one species through other species. It also showed that the microcosm system itself worked to protect the microorganisms in the microcosm system from the impact of ultra violet C.

Kawabata, Z., et al.: J. Protozool. Res., 5, 23-26, 1995.


Fig.17 Population densities of three microorganisms in the microcosm exposed to ultra violet C on the 56 th day after inoculation.

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