Cytological evidence for cytoplasmic volume control inPlatymonas subcordiformisafter osmotic stress

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Abstract
Platymonas subcordiformis, a marine phytoplankton flagellate, shows partial control of cell volume in response to osmotic stress. Electron micrographs of samples taken at short intervals after application of a hypo‐osmotic stress revealed that a large vacuolar system is formed by fusion of vesicles originating from the Golgi apparatus. The chloroplast is swollen and the thylakoid stacking is transiently disturbed for 1 to 5 min after the osmotic shock. It then regains the original size and stacked organization of the thylakoids, while the vacuoles remain permanently. It is assumed that mainly the cytoplasmic compartment is under volume control rather than the whole cell, as has been found in other wall‐less micro‐algae.