Cyanobacteria

Cyanobacteria

Overview
Cyanobacteria is a phylum of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that obtain their energy through photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. The name "cyanobacteria" comes from the color of the bacteria ( = blue).

The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 atmosphere into an oxidizing one, which dramatically changed
Great Oxygenation Event
The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...

 the composition of life forms on Earth by stimulating biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 and leading to the near-extinction of oxygen-intolerant organisms
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

.
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Encyclopedia
Cyanobacteria is a phylum of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 that obtain their energy through photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. The name "cyanobacteria" comes from the color of the bacteria ( = blue).

The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 atmosphere into an oxidizing one, which dramatically changed
Great Oxygenation Event
The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...

 the composition of life forms on Earth by stimulating biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 and leading to the near-extinction of oxygen-intolerant organisms
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

. According to endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

, chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s in plants and eukaryotic algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 have evolved from cyanobacterial ancestors via endosymbiosis.

Ecology


Cyanobacteria can be found in almost every terrestrial and aquatic habitat, from oceans to fresh water to bare rock to soil. They can occur as planktonic cells or form phototrophic biofilms
Phototrophic biofilms
Phototrophic biofilms occur on contact surfaces in a range of terrestrial and aquatic environments. Phototrophic biofilms can best be described as surface attached microbial communities mainly driven by light as the energy source with phototrophic organisms clearly present...

 in fresh water and marine environments, they occur in damp soil, or even on temporarily moistened rocks in desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s. A few are endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...

s in lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s, plants, various protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s, or sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...

s and provide energy for the host
Host (biology)
In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

. Some live in the fur of sloth
Sloth
Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...

s, providing a form of camouflage
Camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

.

Aquatic cyanobacteria are probably best known for the extensive and highly visible blooms that can form in both freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 and the marine environment and can have the appearance of blue-green paint or scum. The association of toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 with such blooms has frequently led to the closure of recreational waters when blooms are observed. Marine bacteriophage
Marine bacteriophage
Marine bacteriophages or marine phages are viruses that live as obligate parasitic agents in marine bacteria such as cyanobacteria. Their existence was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of...

s are a significant parasite of unicellular marine cyanobacteria. When they infect cells, they lyse
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....

 them, releasing more phages into the water.

Nitrogen fixation



Cyanobacteria include unicellular and colonial
Colony (biology)
In biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in colonies...

 species. Colonies may form filaments
Filamentation
Filamentation is the anomalous growth of certain bacteria, such as E. coli, in which cells continue to elongate but do not divide . Bacterial filamentation is often observed as a result of bacteria responding to various stresses, including DNA damage or inhibition of replication...

, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into several different cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 types: vegetative cells, the normal, photosynthetic cells that are formed under favorable growing conditions; akinete
Akinete
An akinete is a thick-walled dormant cell derived from the enlargement of a vegetative cell. It serves as a survival structure. It is a resting cell of cyanobacteria and unicellular and filamentous green algae. Under magnification, akinetes appear thick walled with granular-looking cytoplasms....

s, the climate-resistant spores that may form when environmental conditions become harsh; and thick-walled heterocysts, which contain the enzyme nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...

, vital for nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

. Heterocysts may also form under the appropriate environmental conditions (anoxic) when fixed nitrogen
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

 is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 , nitrites  or nitrates  which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen is not bioavailable to plants).

Rice plantations utilize healthy populations of nitrogen-fixing cyanobacteria (Anabaena
Anabaena
Anabaena is a genus of filamentous cyanobacteria that exists as plankton. It is known for its nitrogen fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to...

, as symbiotes of the aquatic fern Azolla) for use as rice paddy fertilizer.

Cyanobacteria are arguably the most successful group of microorganisms on earth. They are the most genetically diverse; they occupy a broad range of habitats across all latitudes, widespread in freshwater, marine and terrestrial ecosystems, and they are found in the most extreme niches such as hot springs, salt works, and hypersaline bays. Photoautotrophic, oxygen-producing cyanobacteria created the conditions in the planet's early atmosphere that directed the evolution of aerobic metabolism and eukarotic photosynthesis. Cyanobacteria fulfill vital ecological functions in the world's oceans, being important contributors to global carbon and nitrogen budgets.
– Stewart and Falconer

Ecology


Many cyanobacteria also form motile filaments, called hormogonia
Hormogonium
Hormogonia are motile filaments of cells formed by some cyanobacteria in the family Nostoceae. They are formed during asexual reproduction in unicellular, filamentous cyanobacteria some contain heterocysts and akinetes....

, 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. In order 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
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

. They lack flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

, but hormogonia and some species may move about by gliding
Bacterial gliding
Bacterial gliding is a process whereby a bacterium can move under its own power. This process does not involve the use of flagella, which is a more common means of motility in bacteria...

 along surfaces. Many of the multi-cellular 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, like in archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

. These vesicles are not organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s as such. They are not bounded by lipid membranes but by a protein sheath.

Some of these organisms contribute significantly to global ecology and the oxygen cycle
Oxygen cycle
The Oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within its three main reservoirs: the atmosphere , the total content of biological matter within the biosphere , and the lithosphere...

. The tiny marine cyanobacterium Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....

was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean. Many cyanobacteria even display the circadian rhythms that were once thought to exist only in eukaryotic cells (see bacterial circadian rhythms
Bacterial circadian rhythms
Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: in constant conditions Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three...

).

Photosynthesis




Carbon fixation


Cyanobacteria account for 20–30% of Earth's photosynthetic productivity and convert solar energy into biomass-stored chemical energy at the rate of ~450 TW. Cyanobacteria utilize the energy of sunlight to drive photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, a process where the energy of light is used to split water molecules into oxygen, protons, and electrons. While most of the high-energy electrons derived from water are utilized by the cyanobacterial cells for their own needs, a fraction of these electrons are donated to the external environment via electrogenic activity. Cyanobacterial electrogenic activity is an important microbiological conduit of solar energy into the biosphere.

Metabolism and Organelles


Cyanobacteria have an elaborate and highly organized system of internal membranes which function in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Cyanobacteria get their name from the bluish pigment phycocyanin
Phycocyanin
Phycocyanin is a pigment from the light-harvesting phycobiliprotein family, along with allophycocyanin and phycoerythrin. It is an accessory pigment to chlorophyll. All phycobiliproteins are water-soluble and therefore cannot exist within the membrane as do carotenoids, but aggregate forming...

, which they use to capture light for photosynthesis. Photosynthesis in cyanobacteria generally uses water as an electron donor
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 and produces oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 as a by-product, though some may also use hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 as occurs among other photosynthetic bacteria. Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 is reduced to form carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s via the Calvin cycle
Calvin cycle
The Calvin cycle or Calvin–Benson-Bassham cycle or reductive pentose phosphate cycle or C3 cycle or CBB cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms...

. In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoid
Thylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...

s. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria. Due to their ability to fix nitrogen in aerobic conditions they are often found as symbionts with a number of other groups of organisms such as fungi (lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...

s), coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

s, pteridophyte
Pteridophyte
The pteridophytes are vascular plants that produce neither flowers nor seeds, and are hence called vascular cryptogams. Instead, they reproduce and disperse only via spores. Pteridophytes include horsetails, ferns, club mosses, and quillworts...

s (Azolla), angiosperms (Gunnera
Gunnera
Gunnera is a genus of herbaceous flowering plants, some of them gigantic. The genus is the only member of the family Gunneraceae.The 40-50 species vary enormously in leaf size...

) etc.

Many cyanobacteria are able to reduce nitrogen and carbon dioxide under aerobic 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
Photosystem
Photosystems are functional and structural units of protein complexes involved in photosynthesis that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons...

 (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
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

, thiosulphate, or even molecular hydrogen) just like purple photosynthetic bacteria
Purple bacteria
Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...

. Furthermore, they share an archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

l property, the ability to reduce elemental sulfur by anaerobic respiration
Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...

 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 thylakoid
Thylakoid
A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as...

 membrane hosts both respiratory and photosynthetic electron transport.

Attached to thylakoid membrane, phycobilisome
Phycobilisome
Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes.- General structure :Phycobilisomes are protein complexes anchored to thylakoid membranes. They are made of stacks of chromophorylated proteins, the phycobiliproteins, and their associated...

s act as light harvesting antennae for the photosystems. The phycobilisome components (phycobiliprotein
Phycobiliprotein
Phycobiliproteins are water-soluble proteins present in cyanobacteria and certain algae that capture light energy, which is then passed on to chlorophylls during photosynthesis. Phycobiliproteins are formed of a complex between proteins and covalently bound phycobilins that act as chromophores...

s) are responsible for the blue-green pigmentation of most cyanobacteria. The variations on this theme are mainly due to carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s and phycoerythrin
Phycoerythrin
Phycoerythrin is a red protein from the light-harvesting phycobiliprotein family, present in cyanobacteria, red algae and cryptomonads.Like all phycobiliproteins, phycoerythrin is composed of a protein part, organised in a hexameric structure of alpha and beta chains, covalently binding...

s which give the cells the 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 b instead (Prochloron
Prochloron
Prochloron is a unicellular oxygenic photosynthetic prokaryote commonly found as symbionts in coral reefs, particularly in didemnid ascidians...

, Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....

, 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.

Relationship to chloroplasts



Chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s found in eukaryotes (algae and plants) likely evolved from an endosymbiotic relation with cyanobacteria. This endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

 is supported by various structural and genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 similarities Primary chloroplasts are found among the "true plants" or green plant
Viridiplantae
Viridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g. Viridiplantae, Chlorobionta, or simply Plantae, the latter expanding the traditional Plant Kingdom to include the green algae...

s – species ranging from sea lettuce
Sea lettuce
The sea lettuces comprise the genus Ulva, a group of edible green algae that is widely distributed along the coasts of the world's oceans. The type species within the genus Ulva is Ulva lactuca Linnaeus, "lactuca" meaning lettuce...

 to evergreen
Evergreen
In botany, an evergreen plant is a plant that has leaves in all seasons. This contrasts with deciduous plants, which completely lose their foliage during the winter or dry season.There are many different kinds of evergreen plants, both trees and shrubs...

s and flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

s which contain chlorophyll b – as well as among the red algae and glaucophyte
Glaucophyte
The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and green algae plus land plants , they form the Archaeplastida...

s, marine species which contain phycobilins. It now appears that these chloroplasts probably had a single origin, in an ancestor of the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 called Primoplantae. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

It was once thought that the mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

 in eukaryotes also developed from an endosymbiotic relationship with cyanobacteria; however, it is now suspected that this evolutionary event occurred when aerobic bacteria were engulfed by anaerobic host cells. Mitochondria are believed to have originated not from cyanobacteria but from an ancestor of Rickettsia
Rickettsia
Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...

.

Relationship to Earth history


Stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....

s of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

ized oxygen-producing cyanobacteria have been found from 2.8 billion years ago, possibly as old as 3.5 billion years ago.

The biochemical capacity to use water as the source for electrons in photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 evolved once, in a common ancestor of extant cyanobacteria. The geologic record
Geologic record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...

 indicates that this transforming event took place early in our planet's history, at least 2450-2320 million years ago (mya), and probably much earlier. Geobiological interpretation of Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...

 (>2500 mya) sedimentary rocks remains a challenge; available evidence indicates that life existed 3500 mya, but the question of when oxygenic photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 evolved continues to engender debate and research.

A clear paleontological window on cyanobacterial evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 opened about 2000 mya, revealing an already diverse biota
Biota (ecology)
Biota are the total collection of organisms of a geographic region or a time period, from local geographic scales and instantaneous temporal scales all the way up to whole-planet and whole-timescale spatiotemporal scales. The biota of the Earth lives in the biosphere.-See...

 of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

 (2500-543 mya), in part because the redox structure of the oceans favored photoautotrophs capable of nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...

.

The most common cyanobacterial structures in the fossil record are the mound-producing stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....

s and related oncolite
Oncolite
Oncolites are sedimentary structures formed out of oncoids, which are layered spherical growth structures formed by cyanobacterial growth. Oncolites are very similar to stromatolites, but instead of forming columns they form approximately spherical structures...

s. Indeed, these fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 colonies are so common that paleobiology
Paleobiology
Paleobiology is a growing and comparatively new discipline which combines the methods and findings of the natural science biology with the methods and findings of the earth science paleontology...

, micropaleontology
Micropaleontology
Micropaleontology is the branch of paleontology that studies microfossils.-Microfossils:...

 and paleobotany
Paleobotany
Paleobotany, also spelled as palaeobotany , is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments , and both the evolutionary history of plants, with a...

 cite the Pre-Cambrian and Cambrian period as an "age of stromatolites" and an "age of algae."

Green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

 joined the blue-greens as major primary producers on continental shelves near the end of the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...

, but only with the Mesozoic era (251-65 mya) radiations of dinoflagellate
Dinoflagellate
The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...

s, coccolithophorids, and diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

s did primary production in marine shelf waters take modern form.

Today, the blue-green bacteria remain critical to marine ecosystems as primary producers in oceanic gyre
Gyre
A gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis Effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl...

s, as agents of biological nitrogen fixation, and—in modified form—as the plastids of marine algae.

Classification


Historically, Bacteria were first classified as plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s constituting the class Schizomycetes, which along with the Schizophyceae (blue green algae/Cyanobacteria) formed the phylum Schizophyta.then in the phylum Monera
Monera
Monera is a superseded kingdom that contains unicellular organisms without a nucleus , such as bacteria....

in the kingdom Protista by Haeckel in 1866, comprising Protogens, Protamaeba, Vampyrella, Protomonae and Vibrio, but not Nostoc and other cyanobacteria, which were classified with algae
later reclassified as the Prokaryotes by Chatton.

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three – Chroococcales
Chroococcales
Chroococcales is an order of cyanobacterial life which includes the harmful algal bloom Microcystis aeruginosa.-Characteristics:The order is characterized by single, floating cells or colonies which are embedded to a matrix...

, Pleurocapsales, and Oscillatoriales – are not supported by phylogenetic studies. However, the latter two – Nostocales
Nostocales
The Nostocales order contains most of the species of cyanobacteria. It includes filamentous forms, both simple or branched, and both those occurring as single strands or multiple strands within a sheath. Some but not all members show a decrease in width from the base...

 and Stigonematales
Stigonematales
Stigonematales is the order of cyanobacteria that includes filamentous strains that produce hormogonia and exhibit true branching. Under the more recent classification system devised by Rippka et al., these strains are listed under section V .-References:...

 – 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, includes species with truly branched trichomes.
Most taxa included in the phylum or division Cyanobacteria have not yet been validly published under the Bacteriological Code
International Code of Nomenclature of Bacteria
The International Code of Nomenclature of Bacteria or Bacteriological Code governs the scientific names for bacteria, including Archaea. It denotes the rules for naming taxa of bacteria, according to their relative rank...

. Except:
  • The classes Chroobacteria, Hormogoneae and Gloeobacteria
  • The orders Chroococcales
    Chroococcales
    Chroococcales is an order of cyanobacterial life which includes the harmful algal bloom Microcystis aeruginosa.-Characteristics:The order is characterized by single, floating cells or colonies which are embedded to a matrix...

    , Gloeobacterales, Nostocales
    Nostocales
    The Nostocales order contains most of the species of cyanobacteria. It includes filamentous forms, both simple or branched, and both those occurring as single strands or multiple strands within a sheath. Some but not all members show a decrease in width from the base...

    , Oscillatoriales, Pleurocapsales and Stigonematales
    Stigonematales
    Stigonematales is the order of cyanobacteria that includes filamentous strains that produce hormogonia and exhibit true branching. Under the more recent classification system devised by Rippka et al., these strains are listed under section V .-References:...

  • The families Prochloraceae and Prochlorotrichaceae
  • The genera Halospirulina, Planktothricoides, Prochlorococcus
    Prochlorococcus
    Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....

    , Prochloron
    Prochloron
    Prochloron is a unicellular oxygenic photosynthetic prokaryote commonly found as symbionts in coral reefs, particularly in didemnid ascidians...

    , Prochlorothrix.

Biotechnology and applications


The unicellular cyanobacterium Synechocystis
Synechocystis
Synechocystis sp. PCC6803 is a freshwater cyanobacterium capable of both phototrophic growth by oxygenic photosynthesis in sunlight and heterotrophic growth by glycolysis and oxidative phosphorylation during dark periods...

sp. PCC6803 was the third prokaryote and first photosynthetic organism whose genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 was completely sequenced
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....

. It continues to be an important model organism. The smallest genomes have been found in Prochlorococcus
Prochlorococcus
Prochlorococcus is a genus of very small marine cyanobacteria with an unusual pigmentation . These bacteria belong to the photosynthetic picoplankton and are probably the most abundant photosynthetic organism on Earth....

spp. (1.7 Mb) and the largest in Nostoc
Nostoc
Nostoc is a genus of cyanobacteria found in a variety of environmental niches that forms colonies composed of filaments of moniliform cells in a gelatinous sheath.The name "Nostoc" was invented by Paracelsus...

 punctiforme
(9 Mb). Those of Calothrix spp. are estimated at 12–15 Mb, as large as yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

.

Some cyanobacteria are sold as food, notably Aphanizomenon flos-aquae
Aphanizomenon flos-aquae
Aphanizomenon flos-aquae is a freshwater species of cyanobacteria. Cyanobacteria are an ancient clade of bacterial microphyte, part of the cyanobacteria phylum.AFA as a species has both toxic and nontoxic forms...

and Arthrospira platensis (Spirulina).

Recent research has suggested the potential application of cyanobacteria to the generation of Clean and Green Energy via converting sunlight into electricity. Currently efforts are underway to commercialize algae-based fuels such as diesel, gasoline and jet fuel.

Health risks


Some cyanobacteria produce toxin
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s, called cyanotoxin
Cyanotoxin
Cyanotoxins are toxins produced by bacteria called cyanobacteria . Cyanobacteria are found almost everywhere, but particularly in lakes and in the ocean where, under certain conditions, they reproduce exponentially to form blooms. Blooming cyanobacteria can produce cyanotoxins in such...

s. These include anatoxin-a
Anatoxin-a
Anatoxin-a, also known as Very Fast Death Factor, is a secondary, bicyclic amine alkaloid and cyanotoxin with acute neurotoxicity. The toxin is produced by at least four different genera of cyanobacteria and has been reported in North America, Europe, Africa, Asia, and New Zealand. The toxin is...

, anatoxin-as, aplysiatoxin
Aplysiatoxin
Aplysiatoxin is a cyanotoxin produced by certain cyanobacteria species. It is used as a defensive secretion to protect these cyanobacteria from predation by fish, being a potent irritant and carcinogen, by acting as a powerful activator of Protein kinase C...

, cylindrospermopsin
Cylindrospermopsin
Cylindrospermopsin is a cyanotoxin produced by a variety of freshwater cyanobacteria. CYN is a polycyclic uracil derivative containing guanidino and sulfate groups. It is also zwitterionic, making it highly water soluble. CYN is toxic to liver and kidney tissue and is thought to inhibit protein...

, domoic acid
Domoic acid
Domoic acid , the neurotoxin that causes amnesic shellfish poisoning , also known as zombie acid, is a kainic acid analog, heterocyclic amino acid associated with certain harmful algal blooms.-Occurrence:...

, microcystin LR, nodularin R (from Nodularia
Nodularia
Nodularia is a genus of filamentous nitrogen-fixing cyanobacteria, or blue-green algae. They occur mainly in brackish or salinic waters, such as the hypersaline Makgadikgadi Pans, the Peel-Harvey Estuary in Western Australia or the Baltic Sea. Nodularia cells occasionally form heavy algal blooms...

), or saxitoxin
Saxitoxin
Saxitoxin is a neurotoxin naturally produced by certain species of marine dinoflagellates and cyanobacteria Saxitoxin (STX) is a neurotoxin naturally produced by certain species of marine dinoflagellates (Alexandrium sp., Gymnodinium sp., Pyrodinium sp.) and cyanobacteria Saxitoxin (STX) is a...

. Cyanobacteria reproduce explosively under certain conditions. This results in algal bloom
Algal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...

s, which can become harmful to other species if the cyanobacteria involved produce toxins.

These toxins can be neurotoxins, hepatotoxins, cytotoxins, and endotoxins, and can be toxic and dangerous to humans as well as other animals and marine life in general. Several cases of human poisoning have been documented but a lack of knowledge prevents an accurate assessment of the risks. Recent studies suggest that significant exposure to high levels of some species of cyanobacteria causes amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...

 (ALS, also known as Lou Gehrig's Disease). The Lake Mascoma ALS cluster and Gulf War veteran's cluster are two notable examples.

See also

  • Anatoxin
    Anatoxin-a
    Anatoxin-a, also known as Very Fast Death Factor, is a secondary, bicyclic amine alkaloid and cyanotoxin with acute neurotoxicity. The toxin is produced by at least four different genera of cyanobacteria and has been reported in North America, Europe, Africa, Asia, and New Zealand. The toxin is...

  • Archean Eon of earth's prehistory
  • Bacterial phyla
    Bacterial phyla
    The bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...

    , the other major lineages of domain Bacteria
  • Biofertilizer
    Biofertilizer
    A bio-fertilizer is a substance which contains living microorganisms which, when applied to seed, plant surfaces, or soil, colonizes the rhizosphere or the interior of the plant and promotes growth by increasing the supply or availability of primary nutrients to the host plant.Bio-fertilizers add...

  • Cyanobiont
    Cyanobiont
    A cyanobiont is a cyanobacterium that lives in symbiosis with an eukaryote, sometimes inside the cells of the eukaryote. The cyanobiont fixes nitrogen, and sometimes also performs photosynthesis for the host organism. The cyanobiont is often vertically transmitted, this is the case for cyanobionts...

  • Geological history of oxygen
    Geological history of oxygen
    Before photosynthesis evolved, Earth's atmosphere had no free oxygen . Oxygen was first produced by photosynthetic prokaryotic organisms that emitted O2 as a waste product. These organisms lived long before the first build-up of oxygen in the atmosphere, perhaps as early as...

  • Hypolith
    Hypolith
    In Arctic and Antarctic ecology, a hypolith is a photosynthetic organism that livesunderneath rocks in climatically extreme deserts such as Cornwallis Island and Devon Island in the Canadian high Arctic. The community itself is the hypolithon....

  • Microbial mats
  • Proterozoic Eon of earth's prehistory
  • Stromatolite
    Stromatolite
    Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....

  • The Great Oxygenation Event
    Great Oxygenation Event
    The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...


Further reading

  • Gillian Cribbs (1997) Nature's Superfood, the Blue-Green Algae Revolution. Newleaf. ISBN 0-7522-0569-2
  • Marshall Savage
    Marshall Savage
    Marshall Thomas Savage is an advocate of space travel who wrote The Millennial Project: Colonizing the Galaxy in Eight Easy Steps and founded the Living Universe Foundation, which was designed to make plans for stellar exploration over the next 1,000 years....

    , (1992, 1994) The Millennial Project: Colonizing the Galaxy in Eight Easy Steps
    The Millennial Project: Colonizing the Galaxy in Eight Easy Steps
    The Millennial Project: Colonizing the Galaxy in Eight Easy Steps by Marshall T. Savage is a book in the field of Exploratory engineering that gives a series of concrete stages the author believes will lead to interstellar colonization...

    . Little, Brown. ISBN 0-316-77163-5
  • Fogg, G.E., Stewart, W.D.P., Fay, P.and Walsby, A.E. 1973. The Blue-green Algae. Academic Press, London and New York. ISBN 0-12-261650-2
  • "Architects of the earth's atmosphere." Introduction to the Cyanobacteria. University of California, Berkeley
    University of California, Berkeley
    The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...

    . 03 Feb. 2006.
  • Whitton, B.A. Phylum Cyanophyta (Cyanobacteria). in The Freshwater Algal Flora of the British Isles. Cambridge University Press
    Cambridge University Press
    Cambridge University Press is the publishing business of the University of Cambridge. Granted letters patent by Henry VIII in 1534, it is the world's oldest publishing house, and the second largest university press in the world...

    , Cambridge ISBN 0-521-77051-3
  • Whitton BA and Potts M (2000) The ecology of cyanobacteria: their diversity in time and space, Springer, ISBN 0792347358.

External links