XVI International Botanical Congess

The diurnal movements of chloroplast and scintillons (the source of bioluminescence) have been observed by numerous authors to occur in the non-motile vegetative cells of Pyrocystis. Here we report evidence that diurnal ultrastructural changes accompany the organelle movements in Pyrocystis, yet with each organelle moving in opposite directions. Daytime chloroplasts extend radically from the cell center with triple layers of thylakoid membranes. Scintillons are found in the middle of the cell. During the night, chloroplasts are closely associated and their thylakoids are stacked in a grana-like arrangement. The scintillons are scattered to the cell periphery. The opposing movements of those two organelles appear to be under control of the cellular clock and may be regulated by cytoskeletal mechanisms.