Cyanobacteria
| Cyanobacteria Temporal range: 3500–0Ma | |
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| Tolypothrix sp. | |
| Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Eubacteria |
| Phylum: | Cyanobacteria Stanier, 1973 |
| Orders | |
Chroococcales (suborders-Chamaesiphonales and Pleurocapsales)
Nostocales (= Hormogonales or Oscillatoriales)
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| Synonyms | |
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Cyanobacteria/saɪˌænoʊbækˈtɪəriə/, also known asCyanophyta, is a phylumof bacteria that obtain their energy throughphotosynthesis.[3] The name "cyanobacteria" comes from the color of the bacteria (Greek:κυανός (kyanós) = blue). They are often calledblue-green algae (but some consider that name a misnomer, as cyanobacteria are prokaryotic andalgae should be eukaryotic,[4] although other definitions of algae encompass prokaryotic organisms).[5]
By producing gaseous oxygen as a byproduct of photosynthesis, cyanobacteria are thought to have converted the early reducing atmosphere into an oxidizing one, causing the "rusting of the Earth"[6] anddramatically changing the composition of life forms on Earth by stimulatingbiodiversity and leading to the near-extinction of oxygen-intolerant organisms. According toendosymbiotic theory, the chloroplastsfound in plants and eukaryotic algaeevolved from cyanobacterial ancestors via endosymbiosis.
Contents
[hide]Ecology[edit]
Cyanobacteria can be found in almost every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks indeserts, bare rock and soil, and evenAntarctic rocks. They can occur asplanktonic cells or form phototrophic biofilms. They are found in almost every endolithic ecosystem.[7] A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form ofcamouflage.[8]
Aquatic cyanobacteria are known for their extensive and highly visible blooms that can form in bothfreshwater and marine environments. The blooms can have the appearance of blue-green paint or scum. These blooms can be toxic, and frequently lead to the closure of recreational waters when spotted. Marine bacteriophages are significantparasites of unicellular marine cyanobacteria.[9]
Characteristics[edit]
Cyanobacteria are a photosynthetic nitrogen fixing group that survive in wide variety of habitats, soils, and water. In this group, photosynthetic pigments are cyanophycin, allophycocyanine and erythrophycocyanine. Their thalli vary from unicellular to filamentous and filamentous heterocystous. They fix atmospheric nitrogen in aerobic conditions by heterocyst, specialized cells, and in anaerobic conditions.
Nitrogen fixation[edit]
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Cyanobacteria include unicellular and colonialspecies. Colonies may form filaments, sheets, or even hollow balls. Some filamentous colonies show the ability to differentiate into several different celltypes: vegetative cells, the normal, photosynthetic cells that are formed under favorable growing conditions;akinetes, the climate-resistant spores that may form when environmental conditions become harsh; and thick-walled heterocysts, which contain the enzyme nitrogenase, vital for nitrogen fixation. Heterocysts may also form under the appropriate environmental conditions (anoxic) when fixed nitrogen is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia (NH3), nitrites (NO−
2) ornitrates (NO−
3), which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen is not bioavailable to plants, except for those having [endo]symbiotic nitrogen-fixing bacteria, especially theFabaceae family, among others).
2) ornitrates (NO−
3), which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen is not bioavailable to plants, except for those having [endo]symbiotic nitrogen-fixing bacteria, especially theFabaceae family, among others).
Rice plantations use healthy populations of nitrogen-fixing cyanobacteria (Anabaena, as symbiotes of the aquatic fern Azolla) for use as rice paddy fertilizer.[10] Free-living cyanobacteria are present in the water column in rice paddies, and cyanobacteria can be found growing as epiphytes on the surfaces of the green alga, Chara, where they may fix nitrogen.[11]
Morphology[edit]
Many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. To break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium.
Each individual cell of a cyanobacterium typically has a thick, gelatinous cell wall. They lack flagella, but hormogonia of some species can move about by gliding along surfaces. Many of the multicellular filamentous forms of Oscillatoria are capable of a waving motion; the filament oscillates back and forth. In water columns, some cyanobacteria float by forming gas vesicles, as in archaea. These vesicles are not organelles as such. They are not bounded by lipid membranes, but by a protein sheath.
Some of these organisms contribute significantly to global ecology and theoxygen cycle. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.[13] Many cyanobacteria even display the circadian rhythms that were once thought to exist only in eukaryotic cells (see bacterial circadian rhythms).
Photosynthesis[edit]
While contemporary cyanobacteria are linked to the plant kingdom as descendants of the endosymbiotic progenitor of the chloroplast, there are several features which are unique to this group. (At the same time, a majority of features are remarkably conserved among the plants, cyanobacteria, and algal oxygenic phototrophs).
Carbon fixation[edit]
Cyanobacteria use the energy of sunlight to drive photosynthesis, a process where the energy of light is used to split water molecules into oxygen, protons, and electrons. Because they are aquatic organisms, they typically employ several strategies which are collectively known as a "carbon concentrating mechanism" to aid in the acquisition of inorganic carbon (CO2 or bicarbonate). Among the more unique strategies is the widespread prevalence of the bacterial microcompartments known as carboxysomes.[14] Theseicosahedral structures are composed of hexameric shell proteins that assemble into cage-like structures that can be several hundreds of nanometers in diameter. It is believed that these structures tether the CO2-fixing enzyme, RuBisCO, to the interior of the shell, as well as the enzymecarbonic anhydrase, using the paradigm of metabolic channeling to enhance the local CO2 concentrations and thus increase the efficiency of the RuBisCO enzyme.[15]
Electron transport[edit]
In contrast to chloroplast-containing eukaryotes, cyanobacteria lack compartmentalization of their thylakoid membranes, which have been demonstrated to be contiguous with the plasma membrane. Thus, the various protein complexes involved in respiratory energy metabolism share several of the mobile energy carrier pools (e.g., the Quinone pool, cytochrome c, ferredoxins), thereby affording interactions between photosynthetic and respiratory metabolism. Furthermore, there is a tremendous diversity among the respiratory components between species. Thus cyanobacteria can be said to have a "branched electron transport chain", analogous to the situation in purple bacteria.
While most of the high-energy electrons derived from water are used by the cyanobacterial cells for their own needs, a fraction of these electrons may be donated to the external environment via electrogenicactivity.[16]
Metabolism and organelles[edit]
As with any prokaryotic organism, cyanobacteria do not have nuclei or an internal membrane system. However, many species of cyanobacteria have folds on their external membranes that function in photosynthesis. Cyanobacteria get their colour from the bluish pigmentphycocyanin, which they use to capture light for photosynthesis. In general, photosynthesis in cyanobacteria uses water as anelectron donor and produces oxygenas a byproduct, though some may also use hydrogen sulfide[17] a process which occurs among other photosynthetic bacteria such as thepurple sulfur bacteria. Carbon dioxideis 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. They are often found assymbionts with a number of other groups of organisms such as fungi (lichens), corals, pteridophytes (Azolla),angiosperms (Gunnera), etc.
Many cyanobacteria are able to reduce nitrogen and carbon dioxide underaerobic conditions, a fact that may be responsible for their evolutionary and ecological success. The water-oxidizing photosynthesis is accomplished by coupling the activity of photosystem (PS) II and I (Z-scheme). In anaerobic conditions, they are also able to use only PS I—cyclic photophosphorylation—with electron donors other than water (hydrogen sulfide, thiosulphate, or even molecular hydrogen[18]) just like purple photosynthetic bacteria. Furthermore, they share an archaeal property, the ability to reduce elemental sulfur byanaerobic respiration in the dark. Their photosynthetic electron transport shares the same compartment as the components of respiratory electron transport. Their plasma membrane contains only components of the respiratory chain, while the thylakoidmembrane hosts an interlinked respiratory and photosynthetic electron transport chain.[citation needed]The terminal oxidases in the thylakoid membrane respiratory/photosynthetic electron transport chain are essential for survival to rapid light changes, although not for dark maintenance under conditions where cells are not light stressed.[19]
Attached to thylakoid membrane,phycobilisomes act as light-harvesting antennae for the photosystems. The phycobilisome components (phycobiliproteins) are responsible for the blue-green pigmentation of most cyanobacteria. The variations on this theme are due mainly to carotenoidsand phycoerythrins that give the cells their red-brownish coloration. In some cyanobacteria, the color of light influences the composition of phycobilisomes. In green light, the cells accumulate more phycoerythrin, whereas in red light they produce more phycocyanin. Thus, the bacteria appear green in red light and red in green light. This process is known as complementary chromatic adaptation, and is a way for the cells to maximize the use of available light for photosynthesis.
A few genera, however, lack phycobilisomes and have chlorophyll binstead (Prochloron, Prochlorococcus,Prochlorothrix). These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria. For this reason, they are now considered as part of the cyanobacterial group.[citation needed]
There are also some groups capable ofheterotrophic growth,[20] while others are parasitic, causing diseases in invertebrates or eukaryotic algae (e.g., the black band disease).[21][22][23]
Relationship to chloroplasts[edit]
|
Cladogram showing plastids (chloroplasts
and similar) and basal cyanobacteria[24]
and similar) and basal cyanobacteria[24]
Chloroplasts found ineukaryotes (algae and plants) appear to have evolved from an endosymbiotic relation with cyanobacteria. This endosymbiotic theory is supported by various structural and genetic similarities.[25]Primary chloroplasts are found among the "true plants" or green plants – species ranging from sea lettuce toevergreens and flowers that contain chlorophyll b – as well as among thered algae and glaucophytes, marine species that contain phycobilins. It now appears that these chloroplasts probably had a single origin, in an ancestor of the clade calledArchaeplastida, yet this does not necessitate origin from cyanobacteria themselves, microbiology is still undergoing profound classification changes and entire domains (such asArchaea) are poorly mapped and understood. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.
Classification[edit]
See also: Bacterial taxonomy
Historically, bacteria were first classified as plants constituting the class Schizomycetes, which along with the Schizophyceae (blue-green algae/Cyanobacteria) formed the phylum Schizophyta.[26] then in the phylum Monera in the kingdomProtista by Haeckel in 1866, comprising Protogens, Protamaeba, Vampyrella, Protomonae, and Vibrio, but not Nostoc and other cyanobacteria, which were classified with algae.[27]later reclassified as the Prokaryotes byChatton.[28]
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.
The members of Chroococales are unicellular and usually aggregate in colonies. The classic taxonomic criterion has been the cell morphology and the plane of cell division. In Pleurocapsales, the cells have the ability to form internal spores (baeocytes). The rest of the sections include filamentous species. In Oscillatoriales, the cells are uniseriately arranged and do not form specialized cells (akinetes and heterocysts). In Nostocales and Stigonematales, the cells have the ability to develop heterocysts in certain conditions. Stigonematales, unlike Nostocales, include species with truly branched trichomes.
Most taxa included in the phylum or division Cyanobacteria have not yet been validly published under theBacteriological Code, except:
- The classes Chroobacteria,Hormogoneae, and Gloeobacteria
- The orders Chroococcales,Gloeobacterales, Nostocales,Oscillatoriales, Pleurocapsales, andStigonematales
- The families Prochloraceae andProchlorotrichaceae
- The genera Halospirulina,Planktothricoides, Prochlorococcus,Prochloron, and Prochlorothrix
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