Phasing of cell division by temperature cycles in Euglena cultured autotrophically under continuous illumination

Autotrophic cultures of Euglena in continuous light (LL) were exposed to temperature cycles spanning different temperature intervals in the physiological range. Each cycle consisted of a regular alternation between equal phases of two temperatures differing by 7°. Different temperature combinations varied in the degree to which they were capable of phasing cell division. The most effective combinations tested, 18/25° and 28/35°, produced almost as good a synchrony as has been observed from the use of light/dark cycles (at constant temperature) in this system. Both batch and chemostat cultures with a wide range of generation times were phased, and cycles with period lengths ranging from 8 to 24 hr appeared to be equally effective.Dry weight of cells per milliliter of sample was found to increase uniformly in phased cultures, indicating that most biosynthetic processes continue in such cultures even while all division is inhibited. Phasing cannot, therefore, be explained as a simple growth inhibition by the less favorable temperature of the cycle.The average generation times of cultures phased by 12,12 hr cycles were shorter than the "expected" times calculated from the results of the corresponding pairs of constant temperature experiments, indicating that temperature cycles have an overall accelerating effect on the growth of Euglena in continuous light. This and other evidence suggests that temperature cycles may be acting as Zeitgeber for Euglena in LL even though many trials revealed no persistence of a cell division rhythm in conditions of constant temperature and LL following temperature-cycle entrainment.