Summary: An investigation has been carried out into the association between ciliate Protozoa and bacteria in a chalk stream. The ciliates are discussed in the present paper. In general, the largest number of ciliate species and individual species were associated with a growth of green or blue-green filamentous algae. A zonation in the distribution of species along the length of the stream was observed, and particular species were found to undergo cyclical variations in the numbers of individuals. Though there was no correlation between total bacterial numbers and total numbers of ciliates present at any time, the highest bacterial numbers occurred when diatom-eating ciliates were dominant. Statistical analysis showed that the presence of Gram-negative rods in the bacterial flora was correlated with the presence of bacteria-eating ciliates. Gram-positive rods, on the other hand, were correlated with the presence of diatom-eating ciliates. The bacterial flora of the stream was in part determined from that of the surrounding soil. The periods of abundance of ciliates seemed to be related to weather conditions, since ciliates were found to increase in numbers after heavy rain or during prolonged droughts when the banks tended to crumble. Various observations suggest that the ciliate faunas of soil and brook are largely identical. The addition of a small quantity of a broth culture of a micrococcus isolated from the brook to cultures of soil ciliates in soil-extract hay-infusion medium led to a rapid multiplication of ciliates.


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