Abstract
Several factors were examined with respect to their influence on the attraction of protozoa towards yeast extract and on the inhibitory effect of cadmium on chemoattraction. These included: density of protozoa (numbers per volume), age of cultures, axenic and bacterized cultures, and starvation period before testing. A marine species, Miamiensis avidus, and a strain of Tetrahymena isolated from a local river, were tested separately. Two-fold increases in density of Miamiensis resulted in a three-fold increase in numbers attracted, whereas Tetrahymena resulted in a two-fold decrease in numbers attracted. However, inhibition by cadmium was not altered by using different densities. At 250 and 500 protozoa/25 μL, Tetrahymena was inhibited 87.8 ± 5.2 (average of four replicates ± 1 SD) and 89.0 ± 2.6%, respectively; at 117 and < 1 protozoa/25 μL, Miamiensis was inhibited 76.0 ± 4.4 and 67.5 ± 10.0%, respectively. Age of culture had an effect on cadmium sensitivity of Miamiensis but not of Tetrahymena. Older cultures of Miamiensis were more inhibited by cadmium (77.0 ± 2.0%) than younger cultures (57.3 ± 8.4%). Protozoa from axenic and bacterized cultures of Tetrahymena were equally affected by cadmium. This aspect was not tested for Miamiensis, since we did not have axenic cultures of this species. For both species, the starvation period clearly affected the numbers of protozoa attracted to yeast extract. Longer starvation periods resulted in greater attraction, yet their sensitivity to the metal did not change. After 1 and 72 h of starvation Tetrahymena was inhibited 79.0 ± 7.4 and 80.7 ± 10.8%, respectively, and Miamiensis was inhibited 78.5 ± 10.0 and 78.5 ± 5.0%, respectively. These results should provide suggestions for better standardization of behavioral tests using protozoa.
