Cyanobacteria

{{Taxobox | color = lightgrey | name = Cyanobacteria | image = Anabaena sperica.jpeg | image_width = 200px | image_caption = Anabaena sphaerica (Nostocales) | regnum = Bacteria | divisio = Cyanobacteria | subdivision_ranks = Orders | subdivision = The taxonomy of the
Cyanobacteria is currently
under revision. see [1] }} Cyanobacteria (Greek: cyanos = blue) are a phylumof bacteriathat obtain their energy through photosynthesis. They are often referred to as blue-green algae, even though it is now known that they are not directly related to any of the other algal groups, which are all eukaryotes. Nonetheless, the description is still sometimes used to reflect their appearance and ecologicalrole. Fossiltraces of cyanobacteria are claimed to have been found from around 3.8 billionyears ago, but recent evidence has sparked controversy over this assertion. See: Stromatolite

Forms

Cyanobacteria include unicellular, colonial, and filamentous forms. Some filamentous cyanophytes form differentiated cells, called heterocysts, that are specialized for nitrogen fixation, and resting cells called akinetes. Each individual cell typically has a thick, gelatinous cell wall, which stains gram-negative. The cyanophytes lack flagella, but may move about by glidingalong surfaces. Most are found in freshwater, but many are marine, occur in damp soil, or even temporarily moistened rocks in deserts. A few are endosymbiontsin lichens, plants, various protists, or spongesand provide energy for the host. Some even live in the fur of sloths, providing a form of camouflage.

Photosynthesis

Photosynthesisin cyanobacteria generally uses water as an electron donorand produces oxygenas a by-product, though some may also use hydrogen sulfideas occurs among other photosynthetic bacteria. Carbon dioxideis reduced to form carbohydratesvia the Calvin cycle. In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoids. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. Due to their abilities to fix nitrogen in aerobic conditions they are often found as symbiontswith a number of other groups of organisms as fungi (lichens), corals, pteridophytes(Azolla), angiosperms(Gunnera) etc.

The water-oxidizing photosynthesis is accomplished by coupling the activity of photosystem(PS) II and I. They are the only group of organisms that are able to fix nitrogen and carbon in aerobic environment which could account for their evolutionary and ecological success. Moreover, they are able to use in anaerobic conditions only PS I—cyclic photophosphorylation—with electron donors other than water (hydrogen sulfide, thiosulphate, or even molecular hydrogen) just like purple photosynthetic bacteria. Also they share an archaebacterialproperty—the ability to reduce elemental sulfur by anaerobic respiration in the dark. Probably the most intriguing thing about these organisms is that their photosynthetic electron transport shares the same compartment (the thylakoid) and components of the respiratory electron transport. Actually, their plasma membrane contains only components of the respiratory chain, while the thylakoid membrane hosts both respiratory and photosynthetic electron transport.

Attached to thylakoidmembrane, phycobilisomesact as light harvesting antennae for either photosystem II or I (see state transition). The phycobilisome components (phycobilin)are responsible for the blue-green pigmentation of most cyanobacteria. The variations to this theme is mainly due to carotenoidsand phycoerythrinswhich give the cells the red-brownish coloration.

A few genera, however, lack phycobilins and have chlorophyll b as well as chlorophyll a, giving them a bright green colour. These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria.

Relationship to chloroplasts

Chloroplastsfound in eukaryotes (algae and higher plants) most likely represent reduced endosymbiotic cyanobacteria. This endosymbiotic theoryis supported by various structural and genetic similarities. Primary chloroplasts are found among the green plants, where they contain chlorophyll b, and among the red algaeand glaucophytes, where they contain phycobilins. It now appears that these chloroplasts probably had a single origin. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

Classification

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three - Chroococcales, Pleurocapsales, and Oscillatoriales- are not supported by phylogenetic studies. However, the latter two - Nostocalesand Stigonematales- are monophyletic, and make up the heterocystous cyanobacteria.

Most taxa included in the phylum or division Cyanobacteria have not been validly published under the Bacteriological Code. Except:

• The classes Chroobacteria, Hormogoneaeand Gloeobacteria
• The orders Chroococcales, Gloeobacterales, Nostocales, Oscillatoriales, Pleurocapsalesand Stigonematales
• The families Prochloraceaeand Prochlorotrichaceae
• The genera Halospirulina, Planktothricoides, Prochlorococcus, Prochloron, Prochlorothrix.

Other

Certain cyanobacteria produce cyanotoxinslike Anatoxin-a, Anatoxin-as, Aplysiatoxin, Cylindrospermopsin, Domoic acid, Microcystin LR, Nodularin R(from Nodularia), or Saxitoxin. Sometimes a mass-reproductionof cyanobacteria results in algal blooms. Some are marketed as having nutritional value, such as Aphanizomenon flos-aquae (E3live) or Spirulina.

The unicellular cyanobacterium Synechocystis sp. PCC 6803 was the first photosynthetic organism whose genome was completely sequenced (in 1996, by the Kazusa Research Institute, Japan). It continues to be an important model organism.

At least one secondary metabolite, cyanovirin, has shown to possess anti-HIV activity.

See hypolithfor an example of cyanobacteria living in extreme conditions.

Superfood

Some (Gillian Cribbs, 1997 and Marshall Savage, 1992 & 1994) have suggested that cyanobacteria would make a good source of food for the 21st century and beyond.

References

• Gillian Cribbs (1997) Nature's Superfood, the Blue-Green Algae Revolution. Newleaf. ISBN 0-752-20569-2
• Marshall Savage, (1992, 1994) The Millennial Project: Colonizing the Galaxy in Eight Easy Steps. Little, Brown. ISBN 0-316-77163-5cs:Sinice

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It uses material from the http://en.wikipedia.org/wiki/Cyanobacteria Wikipedia article Cyanobacteria.