Revisiting the life cycle of Margalefidinium polykrikoides Group III

Dinoflagellates produce cysts as a strategy to withstand environmental stressors, with nutrient depletion generally considered a key trigger for cyst production. Resting cysts are thick-walled, typically composed of one to several layers, and characterized by a prolonged dormancy period. In contrast, pellicle cysts possess a thin, single wall and exhibit no dormancy or a markedly shorter dormancy than resting cysts of the same species. Margalefidinium polykrikoides produces pellicle and resting cysts, whereas its congener, M. fulvescens, has been shown to produce pumpkin-like structures. Using phase-contrast microscopy, time-lapse microscopy, FlowCam, and attenuated total reflection Fourier transform infrared microspectroscopy (ATR μ-FTIR), we investigated the environmental triggers of sac production, the role of the sacs in the life cycle of the M. polykrikoides American/Malaysian ribotype (Group III) strain, and the chemical composition of the sacs. The sacs observed here differed from pellicle cysts described in M. polykrikoides Group III NY and Group I Korea strains but were indistinguishable from the pumpkin-like structures of M. fulvescens. Phosphate depletion of the culture medium was determined as the main trigger for sac production. Sacs were produced through both sexual and asexual processes. We report the presence of sac transporters-diploid cells that facilitate sac movement. The ATR μ-FTIR data indicated that the sacs were primarily composed of sulfated polysaccharides, closely resembling the extracellular polymeric substances produced by M. polykrikoides (Japan strain) and by Gymnodinium sp. Based on their single thin wall and absence of dormancy, these sacs are considered pellicle cysts.