Phasing effects of light on cell division in exponentially increasing cultures of Tetrahymena grown at low temperatures

Cultures of Tetrahymena pyriformis (W) grown at 10 ° ± 0.05 °C on proteose peptone-liver extract either in darkness or in relatively low light intensities exhibit marked differences in generation times during the exponential growth phase: about 20 h and 30 h, respectively. If a diurnal LD: 12, 12 or LD: 6, 18 light cycle was imposed on cultures that had been growing in the dark, cell division was phased so that the division bursts (each resulting in an approximate doubling of cell number) occurred once every 24 h and were confined primarily to the dark periods. Microscopic determination of the division index demonstrated that divisions virtually ceased during at least a portion of the light periods; indeed, a decrease in the index often anticipated the actual onset of light. Long trains of 24 h oscillations in apparent cell number could also be obtained in semicontinuous culture in LD: 6, 18. Furthermore, these entrained rhythmic division bursts persisted for at least 6 days with a circadian period if the culture was placed in constant darkness. These results are consistent with the hypothesis that an endogenous, self-sustaining circadian clock mechanism may underlie the observed persisting division rhythmicity as it does in many other microorganisms.