Electron microscope studies on the morphogenesis of plastids. VIII. Further studies on the fine structure of Cyanidium caldarium with special regard to the photosynthetic apparatus.

Electron microscopic observations were made on Cyanidium caldarium which is a thermophilic, acidophilic and unicellular alga. The organism, whose taxonomic position has been undetermined, contained a well defined nucleus and a large photosynthetic apparatus composed of a single envelope, single thylakoids closely resembling those of red algae, phycobilisomes and matrix, etc. The algal cell is enveloped by a thick cell wall made from at least three dense sublayers. Only one dumbbell shaped mitochondrion was located between nucleus and chloroplast. Other organelles, golgi bodies and microbodies etc. seen in eucaryotic cells were not observed in the algal cell in this study. However, the cytoplasm was occupied by a large number of dense ribosomes. From the standpoint of cell ultrastructure, C. caldarium may be a primitive rhodophyte, although from the characteristics of chloroplast fine structure, the alga shows a transitional form from blue green algae to red algae. From these observations, the taxonomic position of the present alga and the evolution of photosynthetic apparatus are discussed.