GROWTH AND CELL CYCLE OF TWO CLOSELY RELATED RED TIDE‐FORMING DINOFLAGELLATES:GYMNODINIUM NAGASAKIENSEAND G. CF.NAGASAKIENSE1

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Abstract
Small‐sized vegetative cells were found to co‐occur with normal‐sized cells in populations of the European bloom‐forming dinoflagellateGymnodiniumcf.nagasakienseTakayama et Adachi, currently known asGyrodinium aureolumHulburt, but not in populations of the closely related Japanese speciesGymnodiniumn agasakiense.We examined how cell size differentiation may influence growth and cell cycle progression under a 12:12‐h light: dark cycle in the European taxon, as compared to the Japanese one. Cell number and volume and chlorophyll red fluorescence in both species varied widely during the photocycle. These variations generally appeared to be related lo the division period, which occurred at night, as indicated by the variations of the fraction of binucleated cells (mitotic index) as well as the distribution of cellular DNA content. “Small” cells ofG.cf.nagasakiensedivided mainly during the first part of the dark period, although a second minor peak of dividing cells could occur shortly before light onset. In contrast, “large” cells displayed a sharp division peak that occurred 9 h after the beginning of the dark period. The lower degree of synchrony of “small” cells could be a consequence of their faster growth. Alternatively, these data may suggest that cell division is lightly controlled by an endogenous clock in “large” cells and much more loosely controlled in “small” cells. Cells of the Japanese species, which were morphologically similar to “large” cells of the European taxon, displayed an intermediate growth pattern between the two cell types ofG.cf.nagasakiense, with a division period that extended to most of the dark period.