Nucleoid morphology in the coccal cyanophyte Cyanothece halobia (Chroococcales, Cyanophyta) -: a DAPI fluorescence study

Morphology of the nucleoid in the halobiotic, planktic cyanophyte Cyanothece halobia Roussomoustakaki et Anagnostidis was investigated. DAPI fluorescence staining revealed that nucleoid morphology strongly resembles that of Cyanothece aeruginosa (Nageli) Komarek. The nucleoids consist of many globular, oval, and elongated skeins of DNA connected by thin filaments of DNA. Red fluorescence of chlorophyll is dispersed among the patches of DNA. In some cases DNA is concentrated into one large skein located in the cell centre. A large skein of DNA was also recorded in dividing cells at the place of cell partition. It was confirmed that nucleoid morphology is a reliable character distinguishing Cyanothece from other Synechococcus-like organisms. A similar nucleoid morphology was observed in other cyanophytes (coccal or filamentous) possessing keritomized cells (cells with irregularly arranged dark and light spaces observable in a light microscope, caused by grouped thylakoids), e.g. Stanieria and Tychonema. I propose that nucleoid morphology (number, size and compactness of the DNA skeins) correlates with the extent of keritomy. The relationship of nucleoid morphology and thylakoid arrangement in algae possessing keritomized thylakoids is discussed.