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.

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Abstract
EM observations were made on C. caldarium, 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 3 dense sublayers. Only one dumbbell-shaped mitochondrion was located betwen nucleus and chloroplast. Other organelles, Golgi bodies and microbodies, etc., seen in eukaryotic cells, were not observed. 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.

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