A Study of Feeding Responses to Bacterial Prey by Estuarine Ciliates

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BERK, S. G., COLWELL, R. R. & SMALL, E. B. 1976. A study of feeding responses to bacterial prey by estuarine ciliates. Trans. Amer. Micros. Soc., 95: 514-520. Growth of two estuarine ciliates, Uronema nigricans and Potomacus pottsi, was inhibited when the bacterial food source was at a population level of < 106-107 cells/ml. Feeding rates of Uronema varied with the type and population density of bacteria. The feeding rate of Uronema was greater when a Vibrio sp. served as food source compared with a Bacillus sp. Significant differences noted in the ciliate population levels were related to the strains used as food source. As predators of bacteria, ciliated protozoa can be significant in the ecology of aquatic environments. Marine protozoa grazing on bacteria are important agents in the regeneration of nutrients (Johannes, 1965). Protozoa, plentiful in activated sludge systems, are responsible for the quality of the sewage effluent since they graze on coliform bacteria associated with the effluent, thereby removing a high percentage of these bacteria (Curds & Cockburn, 1968). In turn, protozoa serve as prey for organisms of higher trophic levels, thus constituting a link in aquatic food chains. It has been noted by Hamilton & Preslan (1969) that marine copepods feed on certain of the marine ciliates. In the present study, it has been observed that in estuarine water samples brought into the laboratory, copepod larvae attack and ingest Pleuronema. The feeding responses of ciliates to their estuarine bacterial prey, therefore, may have important consequences in the food web. Few investigations have been conducted on the feeding habits of estuarine bactivorous protozoa (Fenchel, 1968a,b; Hamilton & Preslan, 1969), although more observations have been made on fresh-water organisms (Barna & Weis, 1973; Curds & Cockburn, 1968; Hairston et al., 1968; Laybourn & Stewart, 1975; Proper & Carver, 1966; Seto & Tazaki, 1971). The present study examines some aspects of feeding responses of estuarine ciliates to their bacterial prey.