Cyanobacteria

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Cyanobacteria
Temporal range: 3500–0Ma
Tolypothrix (Cyanobacteria).JPG
Tolypothrix sp.
Scientific classification
Domain: Bacteria
Kingdom: Eubacteria
Phylum: Cyanobacteria
Stanier, 1973
Orders

The taxonomy is currently under revision[1][2]

  • Unicellular forms

Chroococcales (suborders-Chamaesiphonales and Pleurocapsales)

  • Filamentous (colonial) forms

Nostocales (= Hormogonales or Oscillatoriales)

  • True-branching (budding over multiple axes)

Stigonematales
Synonyms
  • Myxophyceae Wallroth, 1833
  • Phycochromaceae Rabenhorst, 1865
  • Cyanophyceae Sachs, 1874
  • Schizophyceae Cohn, 1879
  • Cyanophyta Steinecke, 1931
  • Oxyphotobacteria Gibbons & Murray, 1978

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

By producing gaseous oxygen as a by-product of photosynthesis, cyanobacteria are thought to have converted the early reducing 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[edit]

A cyanobacteria bloom near Fiji

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 in lichens, 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[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, allo-phycocyanine and erythro-phycocyanine. Their thallus varies 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]

Colonies of Nostoc pruniforme
Cylindrospermum sp.

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, except for those having [endo]symbiotic nitrogen-fixing bacteria, especially the Fabaceae family, among others).

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 eukaryotic photosynthesis. Cyanobacteria fulfill vital ecological functions in the world's oceans, being important contributors to global carbon and nitrogen budgets.

– Stewart and Falconer[10]

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. 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[edit]

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. 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.

Diagram of a typical cyanobacterial cell

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 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), corals, pteridophytes (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 like purple 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 (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]

Relationship to chloroplasts[edit]

Gloeobacter




Prochlorococcus


Synechococcus




plastids


all other cyanobacteria




Cladogram showing plastids (chloroplasts
and similar) and basal cyanobacteria[16]

Chloroplasts found in eukaryotes (algae and plants) have been noted to appear to have evolved from an endosymbiotic relation with cyanobacteria. This endosymbiotic 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, yet this does not necessitate origin from cyanobacteria themselves, microbiology is still undergoing profound classification changes and entire domains (such as Archaea) are poorly mapped and understood. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion.

Classification[edit]

Tree of Life in Generelle Morphologie der Organismen (1866). Note the location of the genus Nostoc with algae and not with bacteria (kingdom "Monera")
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.[18] then in the phylum Monera 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[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 – 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, 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:

Earth history[edit]

Stromatolites left behind by cyanobacteria are the oldest known fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in Montana.
Oncolites from the Late Devonian Alamo bolide impact in Nevada

Stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria.[21] Stromatolites provide ancient records of life on Earth by fossil remains which might date from more than 3.5 billion years ago.

Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by cyanobacterial growth. Oncolites are similar to stromatolites, but instead of forming columns they form approximately spherical structures.[22] The oncoids often form around a central nucleus, such as a shell fragment,[23] and a calcium carbonate structure is deposited by encrusting microbes. Oncolites are indicators of warm waters in the photic zone, but are also known in contemporary freshwater environments.[24] These structures rarely exceed 10 cm in diameter.

Biotechnology and applications[edit]

Cyanobacteria cultured in specific media. Cyanobacteria can be helpful in agriculture as they have the capability to fix atmospheric nitrogen to soil.

The unicellular cyanobacterium Synechocystis sp. PCC6803 was the third prokaryote and first photosynthetic organism whose genome was completely sequenced.[25] It continues to be an important model organism.[26] The smallest genomes have been found in Prochlorococcus spp. (1.7 Mb)[27][28] and the largest in Nostoc punctiforme (9 Mb).[29] Those of Calothrix spp. are estimated at 12–15 Mb,[30] as large as yeast.

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.[31] Currently efforts are underway to commercialize algae-based fuels such as diesel, gasoline and jet fuel.[12][32][33]

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.[34] 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.[35]

Cyanobacteria may possess the ability to produce substances that could one day serve as anti-inflammatory agents and combat bacterial infections in humans.[36]

Spirulina's extracted blue color is used as a natural food coloring in gum and candy.[37]

Health risks[edit]

Main article: Cyanotoxin

Cyanobacteria can produce neurotoxins, cytotoxins, endotoxins and hepatotoxins (i.e. the microcystin producing bacteria species microcystis), and are called cyanotoxins.

Specific toxins include, anatoxin-a, anatoxin-as, aplysiatoxin, cyanopeptolin, cylindrospermopsin, domoic acid, nodularin R (from Nodularia), neosaxitoxin and saxitoxin. Cyanobacteria reproduce explosively under certain conditions. This results in algal blooms, which can become harmful to other species, and pose a danger to humans and animals, if the cyanobacteria involved produce toxins. Several cases of human poisoning have been documented but a lack of knowledge prevents an accurate assessment of the risks.[38][39][40] Recent studies suggest that significant exposure to high levels of some species of cyanobacteria producing toxins such as BMAA can cause amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's Disease). The Lake Mascoma ALS cluster [41] and Gulf War veterans' cluster are two notable examples.[39][40][42]

Dietary supplementation[edit]

Some cyanobacteria are sold as food, notably Aphanizomenon flos-aquae and Arthrospira platensis (Spirulina).[43]

Spirulina tablets

Microalgae contain substances of high biological value, such as polyunsaturated fatty acids, amino acids (proteins), pigments, antioxidants, vitamins and minerals.[44] Edible blue-green algae reduce the production of pro-inflammatory cytokines by inhibiting NF-κB pathway in macrophages and splenocytes.[45] Consumption of edible blue green algae may also reduce risks of cataracts and age related macular degeneration.[46] It has also shown mitigative effects in animal models of non-alcohol related liver disease, such as steatohepatitis,[47] and Parkinson's disease.[48] Sulfate polysaccharides exhibit immunomodulatory, antitumor, antithrombotic, anticoagulant, anti-mutagenic, anti-inflammatory, antimicrobial, and even antiviral activity against HIV, herpes, and hepatitis.[49] One pilot study concluded that Spirulina supplements may reduce insulin resistance caused by anti-HIV medications.[50] Some studies have found that Spirulina mitigates aflatoxin and cisplatin chemotherapy induced liver damage in rodents.[51][52] These positive health benefits must be distinguished from non-edible species of algae, which are detrimental to health.[53]

See also[edit]

References[edit]

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About plankton
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