THE PRESENCE AND FUNCTION OF MELATONIN AND STRUCTURALLY RELATED INDOLEAMINES IN A DINOFLAGELLATE, AND A HYPOTHESIS ON THE EVOLUTIONARY SIGNIFICANCE OF THESE TRYPTOPHAN-METABOLITES IN UNICELLULARS

The bioluminescent dinoflagellate Gonyaulax polyedra contains various indoleamines, in particular, melatonin and 5-methoxytryptamine, as well as enzymes of their biosynthetic pathway. Melatonin exhibits a high-amplitude circadian rhythm characterized by a dramatic increase shortly after the onset of darkness. The maximum of melatonin is followed by a peak of 5-methoxytryptamine. These 5-methoxylated indoleamines seem to be involved in the mediation of the information 'darkness'. G. polyedra shows a short-day response, which consists in the formation of asexual cysts. Light break experiments demonstrate the photoperiodic nature of this reaction. Cells become sensitive to short days only upon exposure to a lowered temperature ( < 16-degrees-C). Melatonin mimics the short-day effect, but only at decreased temperature. 5-Methoxytryptamine is even a better inducer of cyst formation, acting also at 20-degrees-C and in any lighting schedule, including LL. Cyst induction is associated with stimulation of bioluminescence and cytoplasmic acidification. A model on the intracellular pathway of photoperiodic information transduction assumes increased deacetylation of melatonin under cyst-inducing conditions, binding of 5-methoxytryptamine to the membrane of an acidic vacuole, proton transfer to the cytoplasm, and decreased intracellular pH as the stimulus for encystment. Melatonin shows the property of a scavenger of superoxide anions. This reaction, which is efficiently catalyzed by hemin, leads to the formation of a substituted kynuramine (AFMK). Destruction of melatonin by light-induced superoxide anions in the presence of cellular hemin may represent a property which, during evolution, has made this molecule suitable as an indicator of darkness. On the other hand, AFMK, which is formed under illumination, might have become a mediator of the information 'light'. Photoperiodism in Gonyaulax shows surprising parallels to that in mammals, but allows the analysis of this phenomenon at an entirely cellular level.