Introduction

The cyanophages are a group of DNA viruses that attack host organisms found within the cyanobacteria or blue-green algae, a group of oxygenic photo-synthetic procaryotes. A continuing dichotomy has occurred as to the nomenclature and taxonomy of these microorganisms. Many of the existing genera and species were mainly described on the basis of morphological characteristics obtained from field-collected samples, and not from axenic cultures. It has been argued that in cell size and morphological complexity the blue-green algae more closely resemble the green algae and other microalgae than the other bacteria and that their dual photosystems are almost identical with that of the eucaryotic algae and higher green plants rather than with anything known in the anoxygenic photosynthetic bacteria.

Detailed physiological and genetic studies have been used to delineate this group of microorganisms more completely. Their multilayered cell wall closely resembles the cell wall structure of Gram-negative eubacteria. On the basis of 16S and 5S rRNA nucleotide base sequence data, the blue-green algal procaryotes have been assigned to the eubacteria, a group distinct from eucaryotes (e.g. algae) and the archaebacteria. Other characteristics that have been analyzed include analysis of pigments (chlorophyll a and phytobilins), chemoheterotrophic/photohetero-trophic capabilities, nitrogen-fixation ability, DNA base composition, DNA/DNA hybridization and modes of asexual and sexual reproduction. Of considerable importance is the fact that the basic cell structure of this group is procaryotic, which is synonymous with bacteria. Therefore, while blue-green algae and the cyanobacteria are reasonably compatible terms, cyanobacteria will be used here. In Volume 3 of Bergey's Manual of Systematic Bacteriology, Castenholz and Waterbury have divided the cyanobacteria into five morphological groups, which are summarized in Table 1. This is the classification scheme that will be used for this article.

Table 1 Taxa of the cyanobacteria

Order

Distinguishing characteristics

Representative genera

Chroococcales Unicellular, reproduce by binary fission in one, two or three planes or by budding. Cells are coccoid to rod-shaped

Pleurocapsales Reproduce by the formation of small spherical cells (baeocytes) produced through multiple fission, often colonial

Oscillatoriales All filamentous forms which undergo binary fission in a single plane and produce 'vegetative' cells only. Heterocysts and akinetes do not occur Nostocales Filamentous organisms dividing exclusively by binary fission in one plane only. Have the potential to produce heterocysts. Some possibility of false branching Stigonematales Exhibit the highest degree of morphological complexity and differentiation. Filamentous with branching and heterocysts

Chamaesiphon, Gioeobacter, Synechococcus, Synechocystis, (Microcystis), (Anacystis)

Dermocarpa, Xenococcus, Myxosarcina, Pleurocapsa

Spirulina, Oscillatoria, Lyngbya, (Phormidium), (Plectonoma)

Anabaena, Aphanizomenon, Nostoc, Nodularia

Fischerella, Chlorogloeopsis

Summarized from Castenholz RW and Waterbury JG (1989) Cyanobacteria. In: Staley JT (ed.) Bergey's Manual of Systematic Bacteriology, vol. 3, p. 1710. Baltimore: Williams and Wilkins.

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