Intracellular Structure of the Unicellular Red Alga Cyanidioschyzon merolae in Response to Phosphate Depletion and Resupplementation

In algae, vacuolar polyphosphate stores phosphate, regulates osmotic pressure and cytosolic pH, and buffers metallic cations. However, its metabolism is not well understood. Cyanidioschyzon merolae cells contain four to five vacuoles storing polyphosphate whose degradation and synthesis are induced by phosphate deprivation and resupplementation, respectively. To understand polyphosphate metabolism, we observed C. merolae cells cultured in different phosphate conditions by electron microscopy. In the phosphate-containing medium, the vacuoles contained electron-dense substances presumably consisting of polyphosphate and metallic cations. When phosphate was depleted, the vacuoles lost the electron-dense substances, and lipid bodies in close contact with vacuoles and starch grains appeared in the cytoplasm. When phosphate was added again, small, electron-dense granules or vesicles appeared in the cytoplasm. Some of them were in contact with the vacuoles and had been taken into the vacuolar lumen, suggesting the possibility that polyphosphate is synthesized and transported to the vacuoles in these structures.