Compartmentalization of specialized functions to discrete locales is a fundamental theme of eucaryotic organization in cells. We report here that bioluminescence of the dinoflagellate alga Gonyaulax originates in vivo from discrete subcellular loci that are intrinsically fluorescent. We demonstrate this localization by comparing the loci of fluorescence and bioluminescence as visualized by image-intensified video microscopy. These fluorescent particles appeared to be the same as the previously described in vitro "scintillons." We attribute the endogenous fluorescence to that of the bioluminescence substrate, luciferin, because (a) the fluorescence excitation and emission characteristics are comparable, (b) the autofluorescence is lost after exhaustive stimulation of bioluminescence, and (c) the fluorescence of discharged particles in vitro can be restored by adding luciferin. The fluorescence in vivo exhibits a standard property of circadian (daily) rhythmicity: under constant environmental conditions, the intensity of the particle fluorescence fluctuates cyclically (it is maximal during the night phase and is low during the day). Thus, luciferin is localized within the cell at discrete loci from which the bioluminescence emanates; the cellular quantity of luciferin is rhythmically modulated by the circadian clock.

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