Circadian chronotypic death in heat-synchronized infradian mode cultures ofTetrahymena pyriformis W.

Continuous cultures ofTetrahymena pyriformis in the infradian ("slow-exponential") growth mode can be entrained to give circadian rhythms of cell division by the application of thermal cycles with periods of 24 hr. The degree of synchronization (phasing index) is dependent on the cycling of other environmental agents, particularly food in the form of the defined medium used. The phase angles of these multiple zeitgebers to one another and to the circadian cell cycle markedly influence the quality of synchronization achieved. In wellsynchronized oxygen-limited cultures, a circadian cytotoxia (circadian chronotypic death) is observed when the culture's oxygen requirements periodically exceed the oxygen support limit of the environment.