THE PHYLOGENY OF UNICELLULAR, EXTREMELY HALOTOLERANT CYANOBACTERIA

We examined the morphology, physiology, and 16S rRNA gene sequences of three culture collection strains and of ten novel isolates of unicell ular cyanobacteria from hypersaline environments. The strains were mor phologically diverse, with average cell widths ranging from 2.8 to 10. 3 mu m. There were single-celled, colonial, and baeocyte-forming strai ns. However, morphological traits were markedly variable with culture conditions, in contrast, all strains displayed extreme halotolerance ( growing close to optimally at above 12% salinity); all were obligately marine, euryhaline, and moderately thermophilic; and all shared a sui te of chemotaxonomic markers including phycobilins, carotenoids, and m ycosporine-like amino acids. 16S rRNA gene sequence analysis indicated that the strains were related to each other. Sequence similarity anal ysis placed the strains in a monophyletic cluster (which we named the Halothece cluster) apart from all cultured or uncultured, not extremel y halotolerant cyanobacteria whose 16S rRNA gene sequences are availab le in public nucleotide sequence databases. This represents the first case in which a phylogenetically coherent group of cyanobacteria can b e defined on the basis of physiology. The Halothece cluster contained two subclusters that may be divergent at the generic level, one encomp assing 12 strains (spanning 5% 16S rRNA gene sequence divergence and n amed the Euhalothece subcluster), and a single deep-branching isolate. Phenotypic characterization of the isolates, including morphological, physiological, and chemotaxonomic traits, did not distinguish these s ubclusters and only weakly suggested the existence of two separate cla des, one encompassing strains of small cell size (cell width < 5 m) an d another one encompassing strains of larger cell size.