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-

For years, microbiologists characterized the Archaea as obligate extremophiles that thrive in environments too harsh for other organisms. The limited physiological diversity among cultivated Archaea suggested that these organisms were metabolically constrained to a few environmental niches. For instance, all Crenarchaeota that are currently cultivated are sulphur-metabolizing thermophiles. However, landmark studies using cultivation-independent methods uncovered vast numbers of Crenarchaeota in cold oxic ocean waters. Subsequent molecular surveys demonstrated the ubiquity of these low-temperature Crenarchaeota in aquatic and terrestrial environments. The numerical dominance of marine Crenarchaeota--estimated at 10(28) cells in the world's oceans--suggests that they have a major role in global biogeochemical cycles. Indeed, isotopic analyses of marine crenarchaeal lipids suggest that these planktonic Archaea fix inorganic carbon. Here we report the isolation of a marine crenarchaeote that grows chemolithoautotrophically by aerobically oxidizing ammonia to nitrite--the first observation of nitrification in the Archaea. The autotrophic metabolism of this isolate, and its close phylogenetic relationship to environmental marine crenarchaeal sequences, suggests that nitrifying marine Crenarchaeota may be important to global carbon and nitrogen cycles.

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Cultured isolates of the marine cyanobacteria Prochlorococcus and Synechococcus vary widely in their pigment compositions and growth responses to light and nutrients, yet show greater than 96% identity in their 16S ribosomal DNA (rDNA) sequences. In order to better define the genetic variation that accompanies their physiological diversity, sequences for the 16S-23S rDNA internal transcribed spacer (ITS) region were determined in 32 Prochlorococcus isolates and 25 Synechococcus isolates from around the globe. Each strain examined yielded one ITS sequence that contained two tRNA genes. Dramatic variations in the length and G؉C content of the spacer were observed among the strains, particularly among Prochlorococcus strains. Secondary-structure models of the ITS were predicted in order to facilitate alignment of the sequences for phylogenetic analyses. The previously observed division of Prochlorococcus into two ecotypes (called high and low-B/A after their differences in chlorophyll content) were supported, as was the subdivision of the high-B/A ecotype into four genetically distinct clades. ITS-based phylogenies partitioned marine cluster A Synechococcus into six clades, three of which can be associated with a particular phenotype (motility, chromatic adaptation, and lack of phycourobilin). The pattern of sequence divergence within and between clades is suggestive of a mode of evolution driven by adaptive sweeps and implies that each clade represents an ecologically distinct population. Furthermore, many of the clades consist of strains isolated from disparate regions of the world's oceans, implying that they are geographically widely distributed. These results provide further evidence that natural populations of Prochlorococcus and Synechococcus consist of multiple coexisting ecotypes, genetically closely related but physiologically distinct, which may vary in relative abundance with changing environmental conditions.In open-ocean ecosystems, carbon fixation is dominated by the marine cyanobacteria Prochlorococcus and Synechococcus. Together they have been shown to contribute between 32 and 80% of the primary production in oligotrophic oceans (14,21,24,60). Prochlorococcus is closely related to the marine cluster A Synechococcus, based on analyses using gene sequences from 16S rRNA (16S rDNA) and rpoC1, a subunit of DNAdependent RNA polymerase (37, 59). However, the two genera have very different light-harvesting systems. Prochlorococcus contains divinyl chlorophyll a (chl a 2 ) and both monovinyl and divinyl chlorophyll b (chl b) as its major photosynthetic pigments, rather than chlorophyll a and phycobiliproteins that are typical of cyanobacteria (7,8,13).Cultured isolates of Prochlorococcus have been divided into two genetically and physiologically distinct groups, referred to as ecotypes because their differing physiologies have implications for their ecological distributions (28,31,44). High-B/A isolates have larger ratios of chl b/a 2 and are able to grow at extremely low irradiances (less than 10 mol of ...

Fixed nitrogen (N) often limits the growth of organisms in terrestrial and aquatic biomes, and N availability has been important in controlling the CO2 balance of modern and ancient oceans. The fixation of atmospheric dinitrogen gas (N2) to ammonia is catalysed by nitrogenase and provides a fixed N for N-limited environments. The filamentous cyanobacterium Trichodesmium has been assumed to be the predominant oceanic N2-fixing microorganism since the discovery of N2 fixation in Trichodesmium in 1961 (ref. 6). Attention has recently focused on oceanic N2 fixation because nitrogen availability is generally limiting in many oceans, and attempts to constrain the global atmosphere-ocean fluxes of CO2 are based on basin-scale N balances. Biogeochemical studies and models have suggested that total N2-fixation rates may be substantially greater than previously believed but cannot be reconciled with observed Trichodesmium abundances. It is curious that there are so few known N2-fixing microorganisms in oligotrophic oceans when it is clearly ecologically advantageous. Here we show that there are unicellular cyanobacteria in the open ocean that are expressing nitrogenase, and are abundant enough to potentially have a significant role in N dynamics.