Small, coccoid and rod-shaped Synechococcus-type cyanobacteria with either phycoerythrin or phycocyanin as major accessory pigments were isolated from several large, temperate-zone lakes and the brackish Baltic Sea. The picocyanobacteria had two ribosomal operons with a long internal transcribed spacer (ITS-1) separating the 16S rDNA and 23S rDNA. A 16S rRNA-based phylogenetic analysis assigned all isolates to the picophytoplankton clade [sensu Urbach, E., 46,[188][189][190][191][192][193][194][195][196][197][198][199][200][201], which also comprises marine Synechococcus spp. and Prochlorococcus spp. The strains assorted to five paraphyletic clusters each containing two or more strains with 99?4-100 % 16S rRNA sequence identity. Five corresponding strain clusters were deduced from analysis of ITS-1 sequences. Sequence divergence in ITS-1 varied between 23 % in the most divergent and 1 % in the phylogenetically most conserved cluster. Clustered strains with low sequence divergence in ITS-1 were frequently isolated from a single ecosystem or hydrographically comparable lakes in the same region. They represent physiologically distinct ecotypes of species which, among other phenotypic variations, frequently differed in their major accessory pigments, the phycobiliproteins. The reproduction of the various pigment traits in different lineages was not correlated with the phylogenetic divergence deduced from 16S rRNA or ITS-1 sequences but rather seemed to be related to characteristics of the ecosystem and habitat from which the strains were isolated. The occurrence of a comparable spectrum of phenotypes in different lineages and ecosystems indicates that different strain clusters developed similar ecotypes during independent adaptive radiations.

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In many filamentous cyanobacteria nitrogen fixation occurs in differentiated cells called heterocysts. Filamentous strains that do not form heterocysts may ri nitrogen in vegetative cells, primarily under anaerobic conditions. We describe here two functional Mo-dependent nitrogenases in a single organism, the cyanobacterium Anabaena variabilis. Filamentous cyanobacteria of the genus Anabaena serve as a simple prokaryotic model for developmental control of gene expression. When deprived of a source of fixed nitrogen, about every 10th photosynthetic vegetative cell in the cyanobacterial filament differentiates into a morphologically and physiologically distinct cell called a heterocyst (1, 2). The primary function of heterocysts is nitrogen fixation, the reduction of atmospheric dinitrogen to ammonia mediated by the enzyme nitrogenase. Nitrogenase is very oxygen labile; hence, nitrogen fixation is restricted to anaerobic environments. Heterocysts provide the requisite anaerobic environment because their cell envelope limits oxygen entry and they lack oxygen-evolving photosystem II, which is characteristic of vegetative cells (2). Within a filament heterocysts differentiate in a semiregular pattern, thus providing spatial separation of nitrogen fixation from oxygenic photosynthesis in what is functionally a onedimensional multicellular organism (3).Among nitrogen-fixing cyanobacteria that do not differentiate heterocysts there does not appear to be a single mechanism for protection of nitrogenase from oxygen and different strains show a range in oxygen tolerance (4,5). In many nonheterocystous cyanobacteria, photosynthesis is temporally separated from nitrogen fixation, which occurs only at night (6-8). For other nonheterocystous cyanobacteria that fix nitrogen aerobically in the light without apparently differentiated cells, little is known of the mechanisms for protecting nitrogenase from oxygen (9, 10); however, nitrogenase activity in laboratory-grown cultures is significantly enhanced by lower oxygen tensions (4). Thus, low oxygen tensions are probably necessary for optimal nitrogenase activity.The heterocystous cyanobacterium, Anabaena sp. strain PCC 7120 (hereafter, Anabaena PCC 7120), has a large cluster of nif genes (including nifBSUHDKEN) that encode a Modependent nitrogenase system (11). The nifB-fdxN-nifS-nifU operon is interrupted by a 55-kb insertion infdxN and the nifD gene has an 11-kb insertion, both of which are excised during heterocyst differentiation (12-14). The 11-kb element is prevalent in heterocystous cyanobacteria (15) but is missing in all nonheterocystous cyanobacteria examined to date (4). The nif genes of Anabaena variabilis ATCC 29413 homologous to those of Anabaena PCC 7120 have been cloned and partially mapped (16); they contain the 11-kb excision element, but not the 55-kb excision element (17). In addition to that nifHDK cluster, a different putative nifHD segment, transcribed within hours after the onset of nitrogen starvation under anaerobic conditions, was cloned from ...

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