CIRCADIAN REGULATION OF BIOLUMINESCENCE IN THE DINOFLAGELLATE PYROCYSTIS LUNULA 1

In the unicellular algae Pyrocystis lunula Schütt and Gonyaulax polyedra Stein, bioluminescence and its circadian regulation are similar in several respects, but there are also several important differences. As in G. polyedra, P. lunula emits light both as bright flashes and as a low intensity glow. At 20° C, the individual flashes are considerably brighter than in G. polyedra , and their durations are typically less than 500 ms. Both species show a circadian rhythm in the frequency of spontaneous flashes, which peaks in the night‐phase under light–dark cycles and continues in both continuous light and dark. However, compared to G. polyedra , the circadian system in P. lunula is more sensitive to light: 10 min exposures (500 μmol · m –2 · s –1 white light) can shift the phase of the rhythm by more than 8 h, and rhythmicity is completely suppressed at an irradiance above 20 μmol · m –2 · s –1 , where the G. polyedra rhythym persists for weeks. Like G. polyedra , period length increases with increasing irradiance of continuous red light but decreases with increasing intensity of continuous blue light. The glow in P. lunula differs markedly from that in G. polyedra in that it occurs at about the same intensity at all times during the circadian cycle; thus, it is not under circadian control but may fluctuate 5–10‐fold in intensity within a time frame of seconds. This suggests that the glow may differ in its physiological basis in the two organisms. The results also indicate that the circadian regulation of luciferase activity differs in the two species. In G. polyedra , the organelle responsible for bioluminescence and luciferase is lost and then reformed on a daily basis; in P. lunula , the luciferase is conserved and localized elsewhere during the nonbioluminescent phase of the cycle.