ULTRASTRUCTURE OF THE THERMOPHILIC BLUE‐GREEN ALGA, SYNECHOCOCCUS LIVIDUS COPELAND

The ultrastructure of Synechococcus lividus Copeland, a thermophilic blue‐green alga, was studied in thin sections. The cell envelope reveals striking similarities with that of some gram‐negative bacteria. In contrast to bacteria and to many other species of blue‐green algae, ribosomes are predominantly found in the central nuclear region and appear to be associated with the DNA fibrils. Thylakoids (photo‐synthetic lamellae) are arranged as concentric shells, around the nuclear equivalent, lying nearly parallel to one another and to the plasma membrane. Both plasma and thylakoidal membranes, as described by other authors for different Cyanophyceae, are of the unit membrane dimension and morphology. Various types of intracellular inclusions are found: (1) Lipid inclusions, located in the cytoplasm are similar to the osmiophilic globules of higher plant chloroplasts. (2) Polyphosphate inclusions (or volutin) resembling those of other species are generally found at the cell poles but within the nuclear region. (3) Polyhedral inclusions also located in the nuclear region are clearly recognized to be different from the polyphosphate bodies, but their function remains unknown.