Effects of indoleamines and short photoperiods on the encystment of Gonyaulax polyedra.

At a temperature of 15 degrees C, Gonyaulax polyedra responds to short days (light less than or equal to 10 h) by transition to the stage of a resting cyst. At 20 degrees C, even an light:dark (LD) cycle of 6:18 is incapable of inducing this process. In otherwise cyst-inducing conditions (15 degrees C; 10 h of light per day), an interruption of the scotophase by 2 h of light (LDLD 8:2:2:12 or 2:2:8:12) prevented encystment. Cyst induction is, therefore, initiated by a photoperiodic mechanism rather than by light deficiency. In Gonyaulax, photoperiodism may be mediated by the action of indoleamines. Melatonin, which exhibits a circadian rhythmicity in this organism, leads to encystment when given 1 h before lights-off in LD 11:13 at 15 degrees C, i.e., under otherwise noninducing conditions. Again, at 20 degrees C, melatonin is inefficient. Some analogues of melatonin, in particular, 5-methoxytryptamine and N,N-dimethyl-5-methoxytryptamine, and, at high concentrations, their respective precursors, serotonin and bufotenin, are capable of inducing cyst formation at 20 degrees C and in LD 12:12, whereas N-acetyl-serotonin does not show this effect.