On the basis of a comparative study of 178 strains of cyanobacteria, representative of this group of prokaryotes, revised definitions of many genera are proposed. Revisions are designed to permit the generic identification of cultures, often difficult through use of the field-based system of phycological classification. The differential characters proposed are both constant and readily determinable in cultured material. The 22 genera recognized are placed in five sections, each distinguished by a particular pattern of structure and development. Generic descriptions are accompanied by strain histories, brief accounts of strain properties, and illustrations; one or more reference strains are proposed for each genus. The collection on which this analysis was based has been deposited in the American Type Culture Collection, where strains will be listed under the generic designations proposed here. I N T R O D U C T I O NThe cyanobacteria constitute one of the largest sub-groups of Gram-negative prokaryotes. As a result of their traditional assignment to the algae, the classification of these organisms was developed by phycologists, working under the provisions of the Botanical Code (Stafleu et al., 1972). Almost entirely on the basis of observations on field materials, about 150 genera and well over lo00 species have been described. The discriminatory properties, both generic and specific, are either structural or ecological, these being virtually the only characters determinable in the field. Types are represented by herbarium specimens or, failing these, by descriptions and illustrations; cultures are not recognized as valid type materials under the Botanical Code.The attempt to identify cyanobacteria in culture through this field-based system of classification leads to many difficulties and ambiguities. The limited and necessarily provisional taxonomic goal of the present article is to redefine certain cyanobacterial genera in such a way that simple and clear-cut generic assignments can be made for cultures. It is based on our experience over the past decade with pure strains representative of all major sub-groups of cyanobacteria. As far as possible, we have attempted to maintain the system of generic nomenclature and the generic definitions now used by phycologists (Bourrelly, 1970;Geitler, 1932;Desikachary, 1959). However, when the discriminatory characters that nominally distinguish two genera are either not determinable on cultures or within the range of variation of a single strain, the existing genera have been combined. Some of the proposed generic definitions include discriminatory characters that have not hitherto received taxo-

The cyanobacterial phylum encompasses oxygenic photosynthetic prokaryotes of a great breadth of morphologies and ecologies; they play key roles in global carbon and nitrogen cycles. The chloroplasts of all photosynthetic eukaryotes can trace their ancestry to cyanobacteria. Cyanobacteria also attract considerable interest as platforms for "green" biotechnology and biofuels. To explore the molecular basis of their different phenotypes and biochemical capabilities, we sequenced the genomes of 54 phylogenetically and phenotypically diverse cyanobacterial strains. Comparison of cyanobacterial genomes reveals the molecular basis for many aspects of cyanobacterial ecophysiological diversity, as well as the convergence of complex morphologies without the acquisition of novel proteins. This phylum-wide study highlights the benefits of diversity-driven genome sequencing, identifying more than 21,000 cyanobacterial proteins with no detectable similarity to known proteins, and foregrounds the diversity of lightharvesting proteins and gene clusters for secondary metabolite biosynthesis. Additionally, our results provide insight into the distribution of genes of cyanobacterial origin in eukaryotic nuclear genomes. Moreover, this study doubles both the amount and the phylogenetic diversity of cyanobacterial genome sequence data. Given the exponentially growing number of sequenced genomes, this diversity-driven study demonstrates the perspective gained by comparing disparate yet related genomes in a phylum-wide context and the insights that are gained from it.

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Cyanobacteria forged two major evolutionary transitions with the invention of oxygenic photosynthesis and the bestowal of photosynthetic lifestyle upon eukaryotes through endosymbiosis. Information germane to understanding those transitions is imprinted in cyanobacterial genomes, but deciphering it is complicated by lateral gene transfer (LGT). Here, we report genome sequences for the morphologically most complex true-branching cyanobacteria, and for Scytonema hofmanni PCC 7110, which with 12,356 proteins is the most gene-rich prokaryote currently known. We investigated components of cyanobacterial evolution that have been vertically inherited, horizontally transferred, and donated to eukaryotes at plastid origin. The vertical component indicates a freshwater origin for water-splitting photosynthesis. Networks of the horizontal component reveal that 60% of cyanobacterial gene families have been affected by LGT. Plant nuclear genes acquired from cyanobacteria define a lower bound frequency of 611 multigene families that, in turn, specify diazotrophic cyanobacterial lineages as having a gene collection most similar to that possessed by the plastid ancestor.

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Prochlorococcus marinus. The purification and properties of the axenic strain PCC 9511, derived from the same primary culture (SARG) as the type species, are reported here. Prochlorococcus PCC 9511 differs from the latter in possessing horseshoeshaped thylakoids, exhibiting a low chlorophyll b 2 content and lacking phycoerythrin, but shares these phenotypic properties with Prochlorococcus strain CCMP 1378. This relationship was confirmed by 16S rRNA sequence analyses, which clearly demonstrated that the axenic isolate is not co-identic with the nomenclatural type. Strain PCC 9511 has a low mean DNA base composition (32 mol % GMC) and harbours the smallest genome of all known oxyphotobacteria (genome complexity 13 GDa l 2 Mbp). Urea and ammonia are the preferred sources of nitrogen for growth, whereas nitrate is not utilized. Several different organic phosphorus compounds efficiently replace phosphate in the culture medium, indicative of ecto-phosphohydrolase activity. In order to distinguish strain PCC 9511 from the nomenclatural type, a new subspecies is proposed, Prochlorococcus marinus Chisholm et al. 1992 subsp. pastoris subsp. nov. This paper is dedicated in gratitude to Professor Germaine Cohen-Bazire on the occasion of her 80th birthday. Together with her late husband, Professor R. Y. Stanier, Germaine gave the members of the Physiologie Microbienne (Institute Pasteur, Paris) generous scientific guidance and spiritual support over many years (1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988) Abbreviations : chl, chlorophyll ; HL, high light ; LL, low light ; PPFD, photosynthetic photon flux density ; PE, phycoerythrin ; T m , temperature mid-point of denaturation. Keywords :The GenBank accession numbers for the 16S rRNA sequences of PCC 9511, CCMP 1426 and NATL1 are AF180967, AF133833 and AF133834, respectively. INTRODUCTIONIn the last edition of Bergey's Manual of Systematic Bacteriology, the class Oxyphotobacteria was subdivided into the Cyanobacteria, whose ordinal recognition still awaits validation, and the order Prochlorales Lewin 1977(Castenholz & Waterbury, 1989. In the light, like algae and plants, these photosynthetic prokaryotes use H # O for the generation of chemical energy and reducing power and liberate molecular O # as a by-product. The demand for cellular carbon is met by CO # fixation. Cyanobacteria synthesize monovinyl chlorophyll a (chl a " ) and harvest light by aid of watersoluble multimeric complexes, the phycobilisomes, composed of biliproteins and linker polypeptides R. Rippka and others (Glazer, 1987(Glazer, , 1989Sidler, 1994). In contrast, oxyphotobacteria of the order Prochlorales Lewin 1977 either lack phycobiliproteins entirely, or synthesize only trace amounts (Lewin, 1977(Lewin, , 1989 BurgerWiersma et al., 1986 ;Chisholm et al., 1992 ;Hess et al., 1996). Their light-harvesting complexes are membrane-associated proteins containing chlorophyll a (a " or a # ) and chlorophyll b (b " or b # ) as the major photosyntheti...

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An in situ hybridization method was applied to the identification of marine cyanobacteria assignable to the genus Prochlorococcus using horseradishperoxidase-labelled 16S rRNA-targeted oligonucleotide probes in combination with tyramide signal amplification (TSA). With this method very bright signals were obtained, in contrast to hybridizations with oligonucleotides monolabelled with fluorochromes, which failed to give positive signals. Genotype-specific oligonucleotides for high light (HL)-and low light (LL)-adapted members of this genus were identified by 16S rRNA sequence analyses and their specificities confirmed in whole-cell hybridizations with cultured strains of Prochlorococcus marinus Chisholm et al., 1992, Prochlorococcus sp. and Synechococcus sp. In situ hybridization of these genotype-specific probes to field samples from stratified water bodies collected in the North Atlantic Ocean and the Red Sea allowed a rapid assessment of the abundance and spatial distribution of HL-and LL-adapted Prochlorococcus. In both oceanic regions the LL-adapted Prochlorococcus populations were localized in deeper water whereas the HL-adapted Prochlorococcus populations were not only distinct in each region but also exhibited strikingly different depth distributions, HLI being confined to shallow water in the North Atlantic, in contrast to HLII, which was present throughout the water column in the Red Sea.