Cyanobacteria
From Freepedia
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| Image:Anabaena sperica.jpeg Anabaena sphaerica (Nostocales) | ||||
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The taxonomy of the |
Cyanobacteria (Greek: cyanos = blue) are a phylum of bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae, even though it is now known that they are not related to any of the other algal groups, which are all eukaryotes. Nonetheless, the description is still sometimes used to reflect their appearance and ecological role. Fossil traces of cyanobacteria are claimed to have been found from around 3.8 billion years ago, but recent evidence has sparked controversy over this assertion. See: Stromatolite
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Forms
Cyanobacteria include unicellular, colonial and filamentous forms. Some filaments form differentiated cells that are specialized for nitrogen fixation, called heterocysts, and resting cells called akinetes. Each individual cell typically has a thick, gelatinous cell wall, which has a gram-negative stain. They lack flagella, but may move about by gliding along surfaces. Most are found in freshwater, but some are marine, occur in damp soil or even temporarily moistured rocks in deserts. A few are endosymbionts in lichens, plants, or of various protists, and provide energy for their host. Some even live in the fur of sloths, giving them a form of camoflage.
Photosynthesis
Photosynthesis in cyanobacteria generally uses water as an electron donor and produces oxygen as a by-product, though some may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via 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 symbionts with 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) I and II. 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 archaebacterial property - the ability to reduce elemental sulfur by anaerobic respiration in dark. Probable 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 thylakoid membrane, phycobilisomes act as light harvesting antennae for photosystem II. The phycobilisome components (phycobilin)are responsible for the blue-green pigmentation of most cyanobacteria. The variations to this theme is mainly due to carotenoids and phycoerythrins which 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
Chloroplasts found in eukaryotes (algae and higher plants) most likely represent reduced endosymbiotic cyanobacteria. This endosymbiotic theory is supported by various structural and genetic similarities. Primary chloroplasts are found among the green plants, where they contain chlorophyll b, and among the red algae and 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 - Nostocales and 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, Hormogoneae and Gloeobacteria
- The orders Chroococcales, Gloeobacterales, Nostocales, Oscillatoriales, Pleurocapsales and Stigonematales
- The families Prochloraceae and Prochlorotrichaceae
- The genera Halospirulina, Planktothricoides, Prochlorococcus, Prochloron, Prochlorothrix.
Other
Certain cyanobacteria produce cyanotoxins like Anatoxin-a, Anatoxin-as, Aplysiatoxin, Cylindrospermopsin, Domoic acid, Microcystin LR, Nodularin R (from Nodularia), or Saxitoxin. Sometimes a mass-reproduction of cyanobacteria results in algal blooms. Some are marketed as having nutritional value, such as Aphanizomenon flos-aquae (E3live) or Spirulina.
See hypolith for an example of cyanobacteria living in extreme conditions.
