Phylum Cyanobacteria the bluegreen bacteria

The Cyanobacteria are placed in volume 1 of the second edition of Bergey, along with the Archaea (see above), the deeply branching bacteria, the 'Deinococcus-Thermus' group, and the green sulphur and green non-sulphur bacteria (see below). See also Box 7.2.

Members of the Cyanobacteria were once known as blue-green algae because they carry out the same kind of oxygenic photosynthesis as algae and green plants

Box 7.2 Prochlorophyta - a missing link?

It has long been thought that the chloroplasts of eucaryotic cells arose as a result of incorporating unicellular cyanobacteria into their cytoplasm. The photosynthetic pigments of plants and green algae, however, are not the same as those of the blue greens; both contain chlorophyll a, but while the former also have chlorophyll b, the blue greens have a unique group of pigments called phycobilins. In the mid-1970s a group of bacteria was discovered which seemed to offer an explanation for this conundrum. Whilst definitely procaryotic, the Prochlorophyta possess chlorophylls a and band lack phycobilins, making them a more likely candidate for the origins of eucaryotic chloroplasts. Initially, these marine bacteria were placed by taxonomists in a phylum of their own, but in the second edition of Bergey, they are included among the Cyanobacteria.

(Chapter 6). They are the only group of procaryotes capable of carrying out this form of photosynthesis; all the other groups of photosynthetic bacteria to be discussed in this chapter carry out an anoxygenic form. When it became possible to examine cell structure in more detail with the electron microscope, it became clear that the cyanobacteria were in fact procaryotic, and hence quite distinct from the true algae. Old habits die hard, however, and the term 'blue-green algae' is still encountered, particularly in the popular press. Being procaryotic, cyanobacteria do not possess chloroplasts; however they contain lamellar membranes called thylakoids, which serve as the site of photo-synthetic pigments and as the location for both light-gathering and electron transfer processes.

Early members of the Cyanobacteria evolved when the oxygen content of the earth's atmosphere was much lower than it is now, and these organisms are thought to have been responsible for its gradual increase, since photosynthetic eucaryotes did not arise until many millions of years later.

Cyanobacteria are Gram-negative bacteria which may be unicellular or filamentous; in spite of the name by which they were formerly known, they may also appear variously as red, black or purple, according to the pigments they possess. A characteristic of many cyanobacteria is the ability to fix atmospheric nitrogen, that is, to reduce it to ammonium ions (NH4+) for incorporation into cellular constituents (see above). In filamentous forms, this activity is associated with specialised, enlarged cells called heterocysts (Figure 7.10).

The tiny unicellular cyanobacterium Prochlorococcus is found in oceans throughout the tropical and temperate regions and is thought to be the most abundant photosyn-thetic organism on our planet. It has several strains adapted to different light conditions. Some cyanobacteria are responsible for the production of unsightly (and smelly!) 'algal' blooms in waters rich in nutrients such as phosphate. When they die, their decomposition by other bacteria leads to oxygen depletion and the death of other aquatic life forms. Bloom-forming species contain gas vacuoles to aid their buoyancy.

Representative genera: Oscillatoria, Anabaena

Heterocyst

Heterocyst

Figure 7.10 Cyanobacteria. Nitrogen fixation takes place in specialised cells called hetero-cysts, which develop from ordinary cells when supplies of available nitrogen (e.g. ammonia) are depleted. The heterocyst loses its ability to photosynthesise and, therefore, to produce oxygen. This is essential because oxygen is highly inhibitory to the nitrogenase enzyme complex

Figure 7.10 Cyanobacteria. Nitrogen fixation takes place in specialised cells called hetero-cysts, which develop from ordinary cells when supplies of available nitrogen (e.g. ammonia) are depleted. The heterocyst loses its ability to photosynthesise and, therefore, to produce oxygen. This is essential because oxygen is highly inhibitory to the nitrogenase enzyme complex

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