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76. Bio-toxicity of GdCl3 on the Movement Activity of Euglena Gracilis
Tetsuya Sakashita, Harald Tahedl*, Donat-P. Hader*, Masahiro Doi and Yuji Nakamura
(*Friedrich-Alexander Universitat, Germany)
Keywords: Euglena gracilis, GdCl3, motility, ECOTOX, toxicant
Gadolinium is one of the rare earth elements, which is utilized in several industrial technologies, semi-conductor manufacturing being a typical example. In the nuclear fuel cycle, Gd is also used as a neutron absorber and is encountered in spent fuel waste disposal. The element may be regarded as a potential environmental toxicant, though its bio-toxicity is still open to study. For the purpose of environmental protection for human beings, not only the effects on the human body itself, but also those on the surrounding natural environment, that is the natural ecosystem, due to possible toxicants should be taken into account.
In this paper, a preliminary result on bioassay of microorganisms (Euglena glacilis) exposed to Gd is presented. The toxicity of Gd to microorganisms has already been reported, from the viewpoint of the long-term fatality measured by the temporal change in their population density. Fatality is not, however, a unique indicator to measure toxicity of materials. There may be other indicators which represent non-fatal, but serious effects on the healthy survival of living things. Then, this study discusses three parameters related to movement activity of bioassay organisms.
After logarithmically growing during one week, Euglena gracilis cells were cultivated for 5 days in a batch medium containing no organic carbon source to prevent heterotrophic growth, under conditions of continuous white light from fluorescent lamps (18 Wm-2) at a temperature of 19°C. The cells were automatically mixed with toxic water containing GdCl3 in the ECOTOX bio-monitoring system developed by Tahedl and Hader (Water Res., 33, 426-432, 1999) to observe the change in movement activity of the organisms, including motility (motile activity), r-value (gravitational orientation) and cell shape parameter (compactness). The motility represents the proportion of active cells among total cells. Euglena gracilis normally moves upward in the ECOTOX cell against the orientation of gravity. Randomness in moving orientation of microorganisms will increase with increasing toxicity in the environment. The degree of randomness in moving cells is expressed by the gravitational orientation parameter. The shape parameter represents the defensive structure of cells against toxicants.
Results of the experiment are illustrated in Fig. 26. These three parameters change with increasing concentration of GdCl3. At addition of 50 
M GdCl3, inhibitions from the normal state of both motility and r-value exceed 50% for each. This means that GdCl3 may affect the activity of Euglena gracilis, though not fatally. The bio-toxic effect on the shape of the cells seems to be relatively small compared to that on motile activity. At lower concentration of GdCl3, it seems that the gravitational orientation (r-value) is the most sensitive parameter indicating bio-toxicity of the element.
