Unusual ultrastructural features in three strains of Cyanothece (cyanobacteria)

Three unicellular cyanobacterial strains (PCC 7425, PCC 8303, PCC 9308) ass igned to the genus Cyanothece Komarek 1976, which showed an unusually high content of light refractile inclusions when viewed by phase-contrast micros copy, were characterized by confocal laser scanning microscopy and transmis sion electron microscopy. All strains had concentric cortical thylakoids an d a compact central nucleoid. Frequently. the two innermost thylakoid membr anes protruded to form circular enclosures containing cytoplasm or electron -transparent granules, or both. The largest granules were partially immerse d in the nucleoid region, but they remained attached to the inner cortical thylakoids by a single narrow connection. The pattern of binary cell divisi on in strain PCC 7425 was different than that in strains PCC 8303 and PCC 9 308. Tn the former, all cell wall layers invaginated simultaneously, wherea s in the latter the invagination of the outer membrane was delayed compared to that of the cytoplasmic membrane and the peptidoglycan layer. Thus, pri or to completion of cell division, the new daughter cells of strains PCC 83 03 and PCC 9308 were transiently connected by a thick septum, which was not observed in strain PCC 7425. Nucleoid partitioning coincided with initiati on of cell division in all three strains and was unlike that reported in ot her bacteria and in archaea, in which separation of the nucleoids precedes cell division. Based on the common morphological and ultrastructural featur es, the three strains of Cyanothece examined constitute a distinct cluster, which might deserve independent generic status.