Growth responses of cohabiting ciliate protozoa to various prey bacteria

Four species of ciliate protozoans isolated from a small pond were grown monoxenically on eight species of bacteria. Four of the bacterial species were common "laboratory" species, the others were isolated from the same pond as the ciliates. The ciliates showed different growth responses to the wild bacteria, confirming the hypothesis that they were specialized with respect to the bacterial species on which they could grow. The laboratory bacteria all permitted good growth of all ciliates, suggesting that the wild bacteria possessed mechanisms to discourage predation. The results are interpreted as supporting the concept that food specialization may be part of a niche partitioning that accounts for the high species richness observed in protozoan communities.