ABSTRACT
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 deg. C, the individual flashes are considerably brighter than in G. polyedra, and their durations are typically less than 500 msec. Both species show a circadian rhythm in the frequency of spontaneous flashes, which peaks in the night-phase under light-dark cycles and continues both in 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 in dim light at an irradiance (20 µmol.m-2.s-1) where the G. polyedra rhythm 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 are lost and then reformed on a daily basis; in P. lunula the luciferase is conserved and localized elsewhere during the non-bioluminescent phase of the cycle.