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Cyanobacteria

Article Index

Cyanobacteria /sˌænbækˈtɪəriə/, also known as Cyanophyta, is a phylum of bacteria that obtain their energy throughphotosynthesis.[3] The name "cyanobacteria" comes from the color of the bacteria (Greekκυανός (kyanós) = blue). They are often called blue-green algae, but some consider that name a misnomer as cyanobacteria are prokaryotic and algaeshould be eukaryotic,[4] although other definitions of algae encompass prokaryotic organisms.[5]

By producing oxygen as a gas as a by-product of photosynthesis, cyanobacteria are thought to have converted the earlyreducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, the chloroplasts found in plants and eukaryotic algae evolved from cyanobacterial ancestors via endosymbiosis.

Ecology

Cyanobacteria can be found in almost every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil, and even Antarctic rocks. They can occur as planktonic cells or form phototrophic biofilms. They are found in almost every endolithic ecosystem.[6] A few are endosymbionts inlichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage.[7]

Aquatic cyanobacteria are known for their extensive and highly visible blooms that can form in both freshwater 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 significant parasites of unicellular marine cyanobacteria.[8]

Characteristics

Cyanobacteria are a photosynthetic nitrogen fixing group that survive in wide variety of habitat, soil and water. In this group photosynthetic pigments are cyanophycin, allo-phycocyanine and erythro-phycocyanine. Their thallus varies from unicellular to filamentous, filamentous heterocystous. They fix atmospheric nitrogen in aerobic condition by heterocyst, specialized cell, and in anaerobic condition.

Nitrogen fixation

Cyanobacteria include unicellular and colonial species. Colonies may form filaments, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into several different cell types: 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
) or nitrates (NO
3
), 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, as symbiotes of the aquatic fern Azolla) for use as rice paddy fertilizer.[9]

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[10]

Ecology

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. 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. They lack flagella, but hormogonia of some species can move about by gliding 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, 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 the oxygen cycle. The tiny marine cyanobacterium Prochlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.[11] Many cyanobacteria even display the circadian rhythms that were once thought to exist only in eukaryotic cells (see bacterial circadian rhythms).

Photosynthesis

Carbon fixation

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. 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 are donated to the external environment via electrogenic activity.[12] Cyanobacterial electrogenic activity is an important microbiological conduit of solar energy into the biosphere

Metabolism and organelles

As with any prokaryotic organism, cyanobacteria do not have nuclei or aninternal membrane system. However, many species of cyanobacteria have folds on their external membranes that function in photosynthesis. Cyanobacteria get their colour from the bluish pigment phycocyanin, which they use to capture light for photosynthesis. In general, photosynthesis in cyanobacteria uses water as an electron donor and produces oxygen as a by-product, though some may also use hydrogen sulfide[13] a process which occurs among other photosynthetic bacteria such as the purple sulfur 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. They are often found as symbionts with a number of other groups of organisms such as fungi (lichens), coralspteridophytes (Azolla), angiosperms (Gunnera) 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 (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[14]) just likepurple photosynthetic bacteria. Furthermore, they share an archaeal property, the ability to reduce elemental sulfur by anaerobic 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 thylakoid membrane 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.[15]

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 carotenoids and phycoerythrins that 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 (ProchloronProchlorococcusProchlorothrix). 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]

Relationship to chloroplast

Chloroplasts found in eukaryotes (algae and plants) likely evolved from an endosymbiotic relation with cyanobacteria. Thisendosymbiotic theory is supported by various structural and genetic similarities.[17] Primary chloroplasts are found among the "true plants" or green plants – species ranging from sea lettuce to evergreens and flowers that contain chlorophyll b – as well as among the red 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 called Archaeplastida. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

Classification

Historically, bacteria were first classified as plants constituting the class Schizomycetes, which along with the Schizophyceae (blue green algae/Cyanobacteria) formed the phylum Schizophyta.[18] then in the phylum Monera in the kingdom Protista by Haeckelin 1866, comprising ProtogensProtamaebaVampyrellaProtomonae and Vibrio, but not Nostoc and other cyanobacteria, which were classified with algae[19] later reclassified as the Prokaryotes by Chatton.[20]

The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I-V. The first three –ChroococcalesPleurocapsales, 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, 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. Except:

Biotechnology applications

The unicellular cyanobacterium Synechocystis sp. PCC6803 was the third prokaryote and first photosynthetic organism whosegenome was completely sequenced.[21] It continues to be an important model organism.[22] The smallest genomes have been found in Prochlorococcus spp. (1.7 Mb)[23][24] and the largest in Nostoc punctiforme (9 Mb).[25] Those of Calothrix spp. are estimated at 12–15 Mb,[26] as large as yeast.

Some cyanobacteria are sold as food, notably Aphanizomenon flos-aquae and Arthrospira platensis (Spirulina).[27]

Recent research has suggested the potential application of cyanobacteria to the generation of renewable energy via converting sunlight into electricity. Internal photosynthetic pathways can be coupled to chemical mediators that transfer electrons to external electrodes.[28] Currently efforts are underway to commercialize algae-based fuels such as dieselgasoline and jet fuel.[12][29][30]

Researchers from a company called Algenol have cultured genetically modified cyanobacteria in sea water inside a clear plastic enclosure so that they first make sugar (pyruvate) from CO2 and the water via photosynthesis. Then, the bacteria secrete ethanol from the cell into the salt water. As the day progresses, and the solar radiation intensifies, ethanol concentrations build up and the ethanol itself evaporates onto the roof of the enclosure. As the sun recedes, evaporated ethanol and water condenses into droplets, which run along the plastic walls and into ethanol collectors, from where it is extracted from the enclosure with the water and ethanol separated outside the enclosure. As of March 2013, Algenol was claiming to have tested its technology in Florida and to have achieved yields of 9,000 US gallons per acre per year.[31] This could potentially meet US demands for ethanol in gasoline in 2025, assuming a B30 blend, from an area of around half the size of California’s San Bernardino County, requiring less than one tenth of the area than ethanol from other biomass, such as corn, and only very limited amounts of fresh water.[32]

Cyanobacteria may possess the ability to produce substances that could one day serve as anti-inflammatory agents and combat bacterial infections in humans.[33]

Spirulina's extracted blue color is used as a natural food coloring in gum and candy.[34]

Health Risks

Some cyanobacteria produce toxins, called cyanotoxins. These include anatoxin-aanatoxin-asaplysiatoxincylindrospermopsindomoic acidmicrocystin LRnodularin R (from Nodularia), neosaxitoxin and saxitoxin. Cyanobacteria reproduce explosively under certain conditions. This results in algal blooms, which can become harmful to other species if the cyanobacteria involved produce toxins.

These toxins can be neurotoxinshepatotoxinscytotoxins, 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.[35][36][37] Recent studies suggest that significant exposure to high levels of some species of cyanobacteria producing toxins such asBMAA can cause amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's Disease). The Lake Mascoma ALS cluster [38] and Gulf War veteran's cluster are two notable examples.[36][37][39]

Dietary Supplementation

Microalgae contain substances of high biological value, such as polyunsaturated fatty acids, amino acids (proteins), pigments, antioxidants, vitamins and minerals.[40]Edible blue-green algae reduce the production of pro-inflammatory cytokines by inhibiting NF-κB pathway in macrophages and splenocytes.[41] Consumption of edible blue green algae may also reduce risks of cataracts and age related macular degeneration.[42] It has also shown mitigative effects in animal models of non-alcohol related liver disease, such as steatohepatitis,[43] and Parkinson's disease.[44] Sulfate polysaccharides exhibit immunomodulatory, antitumor, antithrombotic, anticoagulant, anti-mutagenic, anti-inflammatory, antimicrobial, and even antiviral activity against HIV, herpes, and hepatitis.[45] They also improve insulin resistance in HIV.[46] They also protect against aflatoxin and cisplatin chemotherapy induced liver damage.[47][48] These positive health benefits must be distinguished from non-edible species of algae, which are detrimental to health.[49]