The influence of growth phase and culture conditions of Tetrahymena on effects of cadmium.

The batch culture system is used in the majority of toxicity studies employing protozoa and other cell cultures, although it is known that metal toxicity may vary depending on the age and therefore on the physiological state of the culture. The ciliate Tetrahymena was exposed to cadmium during short-term incubations at different stages of the growth cycle. In cultures with cells proliferating at a high rate (log phase cells) the toxicity of cadmium was found to increase with increasing initial cell density. However, in the subsequent stationary growth phase with decreased rate of cell proliferation, the sensitivity against cadmium again decreases. Thus the effect of a given concentration of cadmium depends on the initial cell density of the culture. This effect of cadmium may be explained either by changes in the chemical composition of the culture medium during the growth cycle or by a changed cellular metabolism. Furthermore, it is shown that the cadmium concentration of the medium decreases during prolonged exposure when the cells are grown in batch cultures. Some of the problems associated with the toxicity bioassay performed in a static system may be overcome by using a continuous flow system. In such a system, Tetrahymena can be kept under optimal growth conditions with a generation time of less than 3 h, and any cell density may be maintained for extended periods of time. Furthermore, the cadmium concentration of the medium remained constant during prolonged exposure when maintaining a cell density of about 50 x 10(3) cells/ml. This illustrates the great advantage of using a continuous flow system in such investigations, where for example, long-term adaptations of the organism to a metal may be studied under constant conditions.