Sericytochromatia, the proposed name of theparaphyletic and most basal group, is the ancestor of both the non-photosynthetic groupMelainabacteria and the photosynthetic cyanobacteria, also called Oxyphotobacteria.[19]
The cyanobacteriaSynechocystis andCyanothece are important model organisms with potential applications in biotechnology forbioethanol production, food colorings, as a source of human and animal food, dietary supplements and raw materials.[20] Cyanobacteria produce a range of toxins known ascyanotoxins that can cause harmful health effects in humans and animals.
Cyanobacteria are found almost everywhere.Sea spray containingmarine microorganisms, including cyanobacteria, can be swept high into the atmosphere where they becomeaeroplankton, and can travel the globe before falling back to earth.[21]
Cyanobacteria are a large and diverse phylum ofphotosyntheticprokaryotes.[22] They are defined by their unique combination ofpigments and their ability to performoxygenic photosynthesis. They often live incolonial aggregates that can take on a multitude of forms.[23] Of particular interest are thefilamentous species, which often dominate the upper layers ofmicrobial mats found in extreme environments such ashot springs,hypersaline water, deserts and the polar regions,[24] but are also widely distributed in more mundane environments as well.[25] They are evolutionarily optimized for environmental conditions of low oxygen.[26] Some species arenitrogen-fixing and live in a wide variety of moist soils and water, either freely or in a symbiotic relationship with plants orlichen-formingfungi (as in the lichen genusPeltigera).[27]
Prochlorococcus, an influential marine cyanobacterium which produces much of the world's oxygen
Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to globalbiogeochemical cycles.[28] They are the only oxygenic photosynthetic prokaryotes, and prosper in diverse and extreme habitats.[29] They are among the oldest organisms on Earth with fossil records dating back at least 2.1 billion years.[30] Since then, cyanobacteria have been essential players in the Earth's ecosystems. Planktonic cyanobacteria are a fundamental component ofmarine food webs and are major contributors to globalcarbon andnitrogen fluxes.[31][32] Some cyanobacteria formharmful algal blooms causing the disruption of aquatic ecosystem services and intoxication of wildlife and humans by the production of powerful toxins (cyanotoxins) such asmicrocystins,saxitoxin, andcylindrospermopsin.[33][34] Nowadays, cyanobacterial blooms pose a serious threat to aquatic environments and public health, and are increasing in frequency and magnitude globally.[35][28]
Cyanobacteria are ubiquitous in marine environments and play important roles asprimary producers. They are part of the marinephytoplankton, which currently contributes almost half of the Earth's total primary production.[36] About 25% of the global marine primary production is contributed by cyanobacteria.[37]
Within the cyanobacteria, only a few lineages colonized the open ocean:Crocosphaera and relatives,cyanobacterium UCYN-A,Trichodesmium, as well asProchlorococcus andSynechococcus.[38][39][40][41] From these lineages, nitrogen-fixing cyanobacteria are particularly important because they exert a control onprimary productivity and theexport of organic carbon to the deep ocean,[38] by converting nitrogen gas into ammonium, which is later used to make amino acids and proteins. Marinepicocyanobacteria (Prochlorococcus andSynechococcus) numerically dominate most phytoplankton assemblages in modern oceans, contributing importantly to primary productivity.[40][41][42] While some planktonic cyanobacteria are unicellular and free living cells (e.g.,Crocosphaera,Prochlorococcus,Synechococcus); others have established symbiotic relationships withhaptophyte algae, such ascoccolithophores.[39] Amongst the filamentous forms,Trichodesmium are free-living and form aggregates. However, filamentous heterocyst-forming cyanobacteria (e.g.,Richelia,Calothrix) are found in association withdiatoms such asHemiaulus,Rhizosolenia andChaetoceros.[43][44][45][46]
Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all,Prochlorococcus, is just 0.5 to 0.8 micrometres across.[47] In terms of numbers of individuals,Prochlorococcus is possibly the most plentiful genus on Earth: a single millilitre of surface seawater can contain 100,000 cells of this genus or more. Worldwide there are estimated to be severaloctillion (1027, a billion billion billion) individuals.[48]Prochlorococcus is ubiquitous between latitudes 40°N and 40°S, and dominates in theoligotrophic (nutrient-poor) regions of the oceans.[49] The bacterium accounts for about 20% of the oxygen in the Earth's atmosphere.[50]
Cyanobacteria are variable in morphology, ranging fromunicellular andfilamentous tocolonial forms. Filamentous forms exhibit functional cell differentiation such asheterocysts (for nitrogen fixation),akinetes (resting stage cells), andhormogonia (reproductive, motile filaments). These, together with the intercellular connections they possess, are considered the first signs of multicellularity.[52][53][54][28]
Many cyanobacteria form motile filaments of cells, calledhormogonia, that travel away from the main biomass to bud and form new colonies elsewhere.[55][56] 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.[citation needed]
Some filamentous species can differentiate into several differentcell types:
Vegetative cells – the normal, photosynthetic cells that are formed under favorable growing conditions
Akinetes – climate-resistant spores that may form when environmental conditions become harsh
Each individual cell (each single cyanobacterium) typically has a thick, gelatinouscell wall.[60] They lackflagella, but hormogonia of some species can move about bygliding along surfaces.[61] Many of the multicellular filamentous forms ofOscillatoria are capable of a waving motion; the filament oscillates back and forth. In water columns, some cyanobacteria float by forminggas vesicles, as inarchaea.[62] These vesicles are notorganelles as such. They are not bounded bylipid membranes, but by a protein sheath.
Some cyanobacteria can fix atmosphericnitrogen in anaerobic conditions by means of specialized cells calledheterocysts.[58][59] 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 intoammonia (NH3),nitrites (NO−2) ornitrates (NO−3), which can be absorbed by plants and converted to protein and nucleic acids (atmospheric nitrogen is notbioavailable to plants, except for those having endosymbioticnitrogen-fixing bacteria, especially the familyFabaceae, amongothers). Nitrogen fixation commonly occurs on a cycle of nitrogen fixation during the night because photosynthesis can inhibit nitrogen fixation.[63]
Free-living cyanobacteria are present in the water ofrice paddies, and cyanobacteria can be found growing asepiphytes on the surfaces of the green alga,Chara, where they may fix nitrogen.[64] Cyanobacteria such asAnabaena (a symbiont of the aquatic fernAzolla) can provide rice plantations withbiofertilizer.[65]
Outer and plasma membranes are in blue, thylakoid membranes in gold, glycogen granules in cyan,carboxysomes (C) in green, and a large dense polyphosphate granule (G) in pink
Thethylakoids of cyanobacteria use the energy ofsunlight to drivephotosynthesis, a process where the energy of light is used to synthesizeorganic compounds from carbon dioxide. Because they are aquatic organisms, they typically employ several strategies which are collectively known as a "CO2 concentrating mechanism" to aid in the acquisition of inorganic carbon (CO2 orbicarbonate). Among the more specific strategies is the widespread prevalence of the bacterial microcompartments known ascarboxysomes,[67] which co-operate with active transporters of CO2 and bicarbonate, in order to accumulate bicarbonate into the cytoplasm of the cell.[68] Carboxysomes areicosahedral structures composed of hexameric shell proteins that assemble into cage-like structures that can be several hundreds of nanometres 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, usingmetabolic channeling to enhance the local CO2 concentrations and thus increase the efficiency of the RuBisCO enzyme.[69]
In contrast topurple bacteria and other bacteria performinganoxygenic photosynthesis, thylakoid membranes of cyanobacteria are not continuous with the plasma membrane but are separate compartments.[70] The photosynthetic machinery is embedded in thethylakoid membranes, withphycobilisomes acting aslight-harvesting antennae attached to the membrane, giving the green pigmentation observed (with wavelengths from 450 nm to 660 nm) in most cyanobacteria.[71]
While most of the high-energyelectrons 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 viaelectrogenic activity.[72]
Respiration in cyanobacteria can occur in the thylakoid membrane alongside photosynthesis,[73] with their photosyntheticelectron transport sharing the same compartment as the components of respiratory electron transport. While the goal of photosynthesis is to store energy by building carbohydrates from CO2, respiration is the reverse of this, with carbohydrates turned back into CO2 accompanying energy release.
Cyanobacteria appear to separate these two processes with their plasma membrane containing only components of the respiratory chain, while the thylakoid membrane hosts an interlinked respiratory and photosynthetic electron transport chain.[73] Cyanobacteria use electrons fromsuccinate dehydrogenase rather than fromNADPH for respiration.[73]
Cyanobacteria only respire during the night (or in the dark) because the facilities used for electron transport are used in reverse for photosynthesis while in the light.[74]
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 ofphotosystem (PS) II and I (Z-scheme). In contrast togreen sulfur bacteria which only use one photosystem, the use of water as an electron donor is energetically demanding, requiring two photosystems.[75]
Attached to the thylakoid membrane,phycobilisomes act aslight-harvesting antennae for the photosystems.[76] The phycobilisome components (phycobiliproteins) are responsible for the blue-green pigmentation of most cyanobacteria.[77] The variations on this theme are due mainly tocarotenoids andphycoerythrins that give the cells their red-brownish coloration. In some cyanobacteria, the color of light influences the composition of the phycobilisomes.[78][79] In green light, the cells accumulate more phycoerythrin, which absorbs green light, whereas in red light they produce morephycocyanin which absorbs red. Thus, these bacteria can change from brick-red to bright blue-green depending on whether they are exposed to green light or to red light.[80] This process of "complementary chromatic adaptation" is a way for the cells to maximize the use of available light for photosynthesis.
A few genera lack phycobilisomes and havechlorophyll b instead (Prochloron,Prochlorococcus,Prochlorothrix). These were originally grouped together as theprochlorophytes 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.[81][82]
In general, photosynthesis in cyanobacteria uses water as anelectron donor and producesoxygen as a byproduct, though some may also usehydrogen sulfide[83] a process which occurs among other photosynthetic bacteria such as thepurple sulfur bacteria.
Environmental impact of cyanobacteria and other photosynthetic microorganisms in aquatic systems. Different classes of photosynthetic microorganisms are found in aquatic and marine environments where they form the base of healthy food webs and participate in symbioses with other organisms. However, shifting environmental conditions can result in community dysbiosis, where the growth of opportunistic species can lead to harmful blooms and toxin production with negative consequences to human health, livestock and fish stocks. Positive interactions are indicated by arrows; negative interactions are indicated by closed circles on the ecological model.[93]
Aquatic cyanobacteria are known for their extensive and highly visibleblooms that can form in bothfreshwater and marine environments. The blooms can have the appearance of blue-green paint or scum. These blooms can betoxic, and frequently lead to the closure of recreational waters when spotted.Marine bacteriophages are significantparasites of unicellular marine cyanobacteria.[96]
Cyanobacterial growth is favoured in ponds and lakes where waters are calm and have little turbulent mixing.[97] Their lifecycles are disrupted when the water naturally or artificially mixes from churning currents caused by the flowing water of streams or the churning water of fountains. For this reason blooms of cyanobacteria seldom occur in rivers unless the water is flowing slowly. Growth is also favoured at higher temperatures which enableMicrocystis species to outcompetediatoms andgreen algae, and potentially allow development of toxins.[97]
Based on environmental trends, models and observations suggest cyanobacteria will likely increase their dominance in aquatic environments. This can lead to serious consequences, particularly the contamination of sources ofdrinking water. Researchers includingLinda Lawton atRobert Gordon University, have developed techniques to study these.[98] Cyanobacteria can interfere withwater treatment in various ways, primarily by plugging filters (often large beds of sand and similar media) and by producingcyanotoxins, which have the potential to cause serious illness if consumed. Consequences may also lie within fisheries and waste management practices. Anthropogeniceutrophication, rising temperatures, vertical stratification and increasedatmospheric carbon dioxide are contributors to cyanobacteria increasing dominance of aquatic ecosystems.[99]
Diagnostic Drawing: Cyanobacteria associated with tufa:Microcoleus vaginatus
Cyanobacteria have been found to play an important role in terrestrial habitats and organism communities. It has been widely reported that cyanobacteriasoil crusts help to stabilize soil to preventerosion and retain water.[100] An example of a cyanobacterial species that does so isMicrocoleus vaginatus.M. vaginatus stabilizes soil using apolysaccharide sheath that binds to sand particles and absorbs water.[101]M. vaginatus also makes a significant contribution to the cohesion ofbiological soil crust.[102]
Some of these organisms contribute significantly to global ecology and theoxygen cycle. The tiny marine cyanobacteriumProchlorococcus was discovered in 1986 and accounts for more than half of the photosynthesis of the open ocean.[103]Circadian rhythms were once thought to only exist in eukaryotic cells but many cyanobacteria display abacterial circadian rhythm.
"Cyanobacteria are arguably the most successful group ofmicroorganisms 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[104]
Symbiosis with land plants[105] Leaf and root colonization by cyanobacteria
(1) Cyanobacteria enter the leaf tissue through thestomata and colonize the intercellular space, forming a cyanobacterial loop. (2) On the root surface, cyanobacteria exhibit two types of colonization pattern; in theroot hair, filaments ofAnabaena andNostoc species form loose colonies, and in the restricted zone on the root surface, specificNostoc species form cyanobacterial colonies. (3) Co-inoculation with2,4-D andNostoc spp. increases para-nodule formation and nitrogen fixation. A large number ofNostoc spp. isolates colonize the rootendosphere and form para-nodules.[105]
Some cyanobacteria, the so-calledcyanobionts (cyanobacterial symbionts), have asymbiotic relationship with other organisms, both unicellular and multicellular.[106] As illustrated on the right, there are many examples of cyanobacteria interactingsymbiotically withland plants.[107][108][109][110] Cyanobacteria can enter the plant through thestomata and colonize the intercellular space, forming loops and intracellular coils.[111]Anabaena spp. colonize the roots of wheat and cotton plants.[112][113][114]Calothrix sp. has also been found on the root system of wheat.[113][114]Monocots, such as wheat and rice, have been colonised byNostoc spp.,[115][116][117][118] In 1991, Ganther and others isolated diverseheterocystous nitrogen-fixing cyanobacteria, includingNostoc,Anabaena andCylindrospermum, from plant root and soil. Assessment of wheat seedling roots revealed two types of association patterns: loose colonization of root hair byAnabaena and tight colonization of the root surface within a restricted zone byNostoc.[115][105]
Live cyanobionts (cyanobacterial symbionts) belonging toOrnithocercusdinoflagellate host consortium (a)O. magnificus with numerous cyanobionts present in the upper and lower girdle lists (black arrowheads) of the cingulum termed the symbiotic chamber. (b)O. steinii with numerous cyanobionts inhabiting the symbiotic chamber. (c) Enlargement of the area in (b) showing two cyanobionts that are being divided by binary transverse fission (white arrows).
The relationships betweencyanobionts (cyanobacterial symbionts) and protistan hosts are particularly noteworthy, as some nitrogen-fixing cyanobacteria (diazotrophs) play an important role inprimary production, especially in nitrogen-limitedoligotrophic oceans.[119][120][121] Cyanobacteria, mostlypico-sizedSynechococcus andProchlorococcus, are ubiquitously distributed and are the most abundant photosynthetic organisms on Earth, accounting for a quarter of all carbon fixed in marine ecosystems.[42][122][49] In contrast to free-living marine cyanobacteria, some cyanobionts are known to be responsible for nitrogen fixation rather than carbon fixation in the host.[123][124] However, the physiological functions of most cyanobionts remain unknown. Cyanobionts have been found in numerous protist groups, includingdinoflagellates,tintinnids,radiolarians,amoebae,diatoms, andhaptophytes.[125][126] Among these cyanobionts, little is known regarding the nature (e.g., genetic diversity, host or cyanobiont specificity, and cyanobiont seasonality) of the symbiosis involved, particularly in relation to dinoflagellate host.[106]
Collective behaviour and buoyancy strategies in single-celled cyanobacteria [127]
Some cyanobacteria – even single-celled ones – show striking collective behaviours and form colonies (orblooms) that can float on water and have important ecological roles. For instance, billions of years ago, communities of marinePaleoproterozoic cyanobacteria could have helped create thebiosphere as we know it by burying carbon compounds and allowing the initial build-up of oxygen in the atmosphere.[128] On the other hand,toxic cyanobacterial blooms are an increasing issue for society, as their toxins can be harmful to animals.[35] Extreme blooms can also deplete water of oxygen and reduce the penetration of sunlight and visibility, thereby compromising the feeding and mating behaviour of light-reliant species.[127]
As shown in the diagram on the right, bacteria can stay in suspension as individual cells, adhere collectively to surfaces to form biofilms, passively sediment, or flocculate to form suspended aggregates. Cyanobacteria are able to produce sulphatedpolysaccharides (yellow haze surrounding clumps of cells) that enable them to form floating aggregates. In 2021, Maeda et al. discovered that oxygen produced by cyanobacteria becomes trapped in the network of polysaccharides and cells, enabling the microorganisms to form buoyant blooms.[129] It is thought that specific protein fibres known aspili (represented as lines radiating from the cells) may act as an additional way to link cells to each other or onto surfaces. Some cyanobacteria also use sophisticated intracellulargas vesicles as floatation aids.[127]
Light microscope view of cyanobacteria from amicrobial mat
The diagram on the left above shows a proposed model of microbial distribution, spatial organization, carbon and O2 cycling in clumps and adjacent areas. (a) Clumps contain denser cyanobacterial filaments and heterotrophic microbes. The initial differences in density depend on cyanobacterial motility and can be established over short timescales. Darker blue color outside of the clump indicates higher oxygen concentrations in areas adjacent to clumps. Oxic media increase the reversal frequencies of any filaments that begin to leave the clumps, thereby reducing the net migration away from the clump. This enables the persistence of the initial clumps over short timescales; (b) Spatial coupling between photosynthesis and respiration in clumps. Oxygen produced by cyanobacteria diffuses into the overlying medium or is used for aerobic respiration.Dissolved inorganic carbon (DIC) diffuses into the clump from the overlying medium and is also produced within the clump by respiration. In oxic solutions, high O2 concentrations reduce the efficiency of CO2 fixation and result in the excretion of glycolate. Under these conditions, clumping can be beneficial to cyanobacteria if it stimulates the retention of carbon and the assimilation of inorganic carbon by cyanobacteria within clumps. This effect appears to promote the accumulation ofparticulate organic carbon (cells, sheaths and heterotrophic organisms) in clumps.[130]
It has been unclear why and how cyanobacteria form communities. Aggregation must divert resources away from the core business of making more cyanobacteria, as it generally involves the production of copious quantities of extracellular material. In addition, cells in the centre of dense aggregates can also suffer from both shading and shortage of nutrients.[131][132] So, what advantage does this communal life bring for cyanobacteria?[127]
Types of cell death according to theNomenclature Committee on Cell Death (upper panel;[133] and proposed for cyanobacteria (lower panel). Cells exposed to extreme injury die in an uncontrollable manner, reflecting the loss of structural integrity. This type of cell death is called "accidental cell death" (ACD). "Regulated cell death (RCD)" is encoded by a genetic pathway that can be modulated by genetic or pharmacologic interventions.Programmed cell death (PCD) is a type of RCD that occurs as a developmental program, and has not been addressed in cyanobacteria yet. RN, regulated necrosis.
New insights into how cyanobacteria form blooms have come from a 2021 study on the cyanobacteriumSynechocystis. These use a set of genes that regulate the production and export of sulphatedpolysaccharides, chains of sugar molecules modified withsulphate groups that can often be found in marine algae and animal tissue. Many bacteria generate extracellular polysaccharides, but sulphated ones have only been seen in cyanobacteria. InSynechocystis these sulphated polysaccharide help the cyanobacterium form buoyant aggregates by trapping oxygen bubbles in the slimy web of cells and polysaccharides.[129][127]
Previous studies onSynechocystis have showntype IV pili, which decorate the surface of cyanobacteria, also play a role in forming blooms.[134][131] These retractable and adhesive protein fibres are important for motility, adhesion to substrates and DNA uptake.[135] The formation of blooms may require both type IV pili and Synechan – for example, the pili may help to export the polysaccharide outside the cell. Indeed, the activity of these protein fibres may be connected to the production of extracellular polysaccharides in filamentous cyanobacteria.[136] A more obvious answer would be that pili help to build the aggregates by binding the cells with each other or with the extracellular polysaccharide. As with other kinds of bacteria,[137] certain components of the pili may allow cyanobacteria from the same species to recognise each other and make initial contacts, which are then stabilised by building a mass of extracellular polysaccharide.[127]
The bubble flotation mechanism identified by Maeda et al. joins a range of known strategies that enable cyanobacteria to control their buoyancy, such as using gas vesicles or accumulating carbohydrate ballasts.[138] Type IV pili on their own could also control the position of marine cyanobacteria in the water column by regulating viscous drag.[139] Extracellular polysaccharide appears to be a multipurpose asset for cyanobacteria, from floatation device to food storage, defence mechanism and mobility aid.[136][127]
The hypothetical conceptual model coupling programmed cell death (PCD) and the role of microcystins (MCs) in Microcystis. (1) The extracellular stressor (e.g., ultraviolet radiation) acts on the cell. (2) Intracellular oxidative stress increases; the intracellular reactive oxygen species (ROS) content exceeds the antioxidative capacity of the cell (mediated mostly by an enzymatic system involving a superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPX)) and causes molecular damage. (3) The damage further activates the caspase-like activity, and apoptosis-like death is initiated. Simultaneously, intracellular MCs begin to be released into the extracellular environment. (4) The extracellular MCs have been significantly released from dead Microcystis cells. (5) They act on the remaining Microcystis cells, and exert extracellular roles, for example, extracellular MCs can increase the production of extracellular polysaccharides (EPS) that are involved in colony formation. Eventually, the colonial form improves the survival of the remaining cells under stressful conditions.[140]
One of the most critical processes determining cyanobacterial eco-physiology iscellular death. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in cyanobacteria is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive.[28] However, reports on cell death of marine and freshwater cyanobacteria indicate this process has major implications for the ecology of microbial communities/[141][142][143][144] Different forms of cell demise have been observed in cyanobacteria under several stressful conditions,[145][146] and cell death has been suggested to play a key role in developmental processes, such as akinete and heterocyst differentiation, as well as strategy for population survival.[140][147][148][52][28]
Cyanophages are viruses that infect cyanobacteria. Cyanophages can be found in both freshwater and marine environments.[149] Marine and freshwater cyanophages haveicosahedral heads, which contain double-stranded DNA, attached to a tail by connector proteins.[150] The size of the head and tail vary among species of cyanophages.Cyanophages, like otherbacteriophages, rely onBrownian motion to collide with bacteria, and then use receptor binding proteins to recognize cell surface proteins, which leads to adherence. Viruses with contractile tails then rely on receptors found on their tails to recognize highly conserved proteins on the surface of the host cell.[151]
Cyanophages infect a wide range of cyanobacteria and are key regulators of the cyanobacterial populations in aquatic environments, and may aid in the prevention of cyanobacterial blooms in freshwater and marine ecosystems. These blooms can pose a danger to humans and other animals, particularly ineutrophic freshwater lakes. Infection by these viruses is highly prevalent in cells belonging toSynechococcus spp. in marine environments, where up to 5% of cells belonging to marine cyanobacterial cells have been reported to contain mature phage particles.[152]
It has long been known thatfilamentous cyanobacteria perform surface motions, and that these movements result fromtype IV pili.[154][136][155] Additionally,Synechococcus, a marine cyanobacteria, is known to swim at a speed of 25 μm/s by a mechanism different to that of bacterial flagella.[156] Formation of waves on the cyanobacteria surface is thought to push surrounding water backwards.[157][158] Cells are known to bemotile by a gliding method[159] and a novel uncharacterized, non-phototactic swimming method[160] that does not involve flagellar motion.
Many species of cyanobacteria are capable of gliding.Gliding is a form of cell movement that differs from crawling or swimming in that it does not rely on any obvious external organ or change in cell shape and it occurs only in the presence of asubstrate.[161][162] Gliding in filamentous cyanobacteria appears to be powered by a "slime jet" mechanism, in which the cells extrude a gel that expands quickly as it hydrates providing a propulsion force,[163][164] although someunicellular cyanobacteria usetype IV pili for gliding.[165][25]
Cyanobacteria have strict light requirements. Too little light can result in insufficient energy production, and in some species may cause the cells to resort to heterotrophic respiration.[24] Too much light can inhibit the cells, decrease photosynthesis efficiency and cause damage by bleaching. UV radiation is especially deadly for cyanobacteria, with normal solar levels being significantly detrimental for these microorganisms in some cases.[23][166][25]
Filamentous cyanobacteria that live in microbial mats often migrate vertically and horizontally within the mat in order to find an optimal niche that balances their light requirements for photosynthesis against their sensitivity to photodamage. For example, the filamentous cyanobacteriaOscillatoria sp. andSpirulina subsalsa found in the hypersaline benthic mats ofGuerrero Negro, Mexico migrate downwards into the lower layers during the day in order to escape the intense sunlight and then rise to the surface at dusk.[167] In contrast, the population ofMicrocoleus chthonoplastes found in hypersaline mats inCamargue, France migrate to the upper layer of the mat during the day and are spread homogeneously through the mat at night.[168] An in vitro experiment usingPhormidium uncinatum also demonstrated this species' tendency to migrate in order to avoid damaging radiation.[23][166] These migrations are usually the result of some sort of photomovement, although other forms of taxis can also play a role.[169][25]
Photomovement – the modulation of cell movement as a function of the incident light – is employed by the cyanobacteria as a means to find optimal light conditions in their environment. There are three types of photomovement: photokinesis, phototaxis and photophobic responses.[170][171][172][25]
Photokinetic microorganisms modulate their gliding speed according to the incident light intensity. For example, the speed with whichPhormidium autumnale glides increases linearly with the incident light intensity.[173][25]
Phototactic microorganisms move according to the direction of the light within the environment, such that positively phototactic species will tend to move roughly parallel to the light and towards the light source. Species such asPhormidium uncinatum cannot steer directly towards the light, but rely on random collisions to orient themselves in the right direction, after which they tend to move more towards the light source. Others, such asAnabaena variabilis, can steer by bending thetrichome.[174][25]
Finally, photophobic microorganisms respond to spatial and temporal light gradients. A step-up photophobic reaction occurs when an organism enters a brighter area field from a darker one and then reverses direction, thus avoiding the bright light. The opposite reaction, called a step-down reaction, occurs when an organism enters a dark area from a bright area and then reverses direction, thus remaining in the light.[25]
Cyanobacteria likely first evolved in a freshwater environment.[13] During thePrecambrian, stromatolite communities of microorganisms grew in most marine and non-marine environments in thephotic zone. After the Cambrian explosion of marine animals, grazing on the stromatolite mats by herbivores greatly reduced the occurrence of the stromatolites in marine environments. Since then, they are found mostly in hypersaline conditions where grazing invertebrates cannot live (e.g.Shark Bay, Western Australia). Stromatolites provide ancient records of life on Earth by fossil remains which date from 3.5Ga ago.[176] The oldest undisputed evidence of cyanobacteria is dated to be 2.1 Ga ago, but there is some evidence for them as far back as 2.7 Ga ago.[30] Cyanobacteria might have also emerged 3.5 Ga ago.[177] Oxygen concentrations in the atmosphere remained around or below 0.001% of today's level until 2.4 Ga ago (theGreat Oxygenation Event).[178] The rise in oxygen may have caused a fall in the concentration ofatmospheric methane, and triggered theHuronian glaciation from around 2.4 to 2.1 Ga ago. In this way, cyanobacteria may have killed off most of the other bacteria of the time.[179]
Oncolites aresedimentary 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 that were not attached to the underlying substrate as they formed.[180] The oncoids often form around a central nucleus, such as a shell fragment,[181] and acalcium carbonate structure is deposited by encrustingmicrobes. Oncolites are indicators of warm waters in thephotic zone, but are also known in contemporary freshwater environments.[182] These structures rarely exceed 10 cm in diameter.
One former classification scheme of cyanobacterial fossils divided them into theporostromata and thespongiostromata. These are now recognized asform taxa and considered taxonomically obsolete; however, some authors have advocated for the terms remaining informally to describe form and structure of bacterial fossils.[183]
Stromatolites left behind by cyanobacteria are the oldest known fossils of life on Earth. This fossil is one billion years old.
Oncolitic limestone formed from successive layers of calcium carbonate precipitated by cyanobacteria
Primary chloroplasts are cell organelles found in someeukaryotic lineages, where they are specialized in performing photosynthesis. They are considered to have evolved fromendosymbiotic cyanobacteria.[189][190] After some years of debate,[191] it is now generally accepted that the three major groups of primary endosymbiotic eukaryotes (i.e.green plants,red algae andglaucophytes) form one largemonophyletic group calledArchaeplastida, which evolved after one unique endosymbiotic event.[192][193][194][195]
Themorphological similarity between chloroplasts and cyanobacteria was first reported by German botanistAndreas Franz Wilhelm Schimper in the 19th century[196] Chloroplasts are only found inplants andalgae,[197] thus paving the way for Russian biologistKonstantin Mereschkowski to suggest in 1905 the symbiogenic origin of the plastid.[198]Lynn Margulis brought this hypothesis back to attention more than 60 years later[199] but the idea did not become fully accepted until supplementary data started to accumulate. The cyanobacterial origin of plastids is now supported by various pieces ofphylogenetic,[200][192][195]genomic,[201] biochemical[202][203] and structural evidence.[204] The description of another independent and more recent primary endosymbiosis event between a cyanobacterium and a separate eukaryote lineage (therhizarianPaulinella chromatophora) also gives credibility to the endosymbiotic origin of the plastids.[205]
The chloroplasts ofglaucophytes have apeptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.[206]
In addition to this primary endosymbiosis, many eukaryotic lineages have been subject tosecondary or eventertiary endosymbiotic events, that is the "Matryoshka-like" engulfment by a eukaryote of another plastid-bearing eukaryote.[207][189]
Chloroplasts have many similarities with cyanobacteria, including a circularchromosome, prokaryotic-typeribosomes, and similar proteins in the photosynthetic reaction center.[208][209] Theendosymbiotic theory suggests that photosynthetic bacteria were acquired (byendocytosis) by earlyeukaryotic cells to form the firstplant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Likemitochondria, chloroplasts still possess their own DNA, separate from thenuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those in cyanobacteria.[210] DNA in chloroplasts codes forredox proteins such as photosynthetic reaction centers. TheCoRR hypothesis proposes this co-location is required for redox regulation.
Timing and trends in cell diameter, loss of filamentous forms and habitat preference within cyanobacteria
Based on data: nodes (1–10) and stars representing common ancestors from Sánchez-Baracaldo et al., 2015,[46] timing of theGreat Oxidation Event (GOE),[211] theLomagundi-Jatuli Excursion,[212] andGunflint formation.[213] Green lines represent freshwater lineages and blue lines represent marine lineages are based on Bayesian inference of character evolution (stochastic character mapping analyses).[46]
Taxa are not drawn to scale – those with smaller cell diameters are at the bottom and larger at the top
Cyanobacteria have fundamentally transformed the geochemistry of the planet.[214][211] Multiple lines of geochemical evidence support the occurrence of intervals of profound global environmental change at the beginning and end of theProterozoic (2,500–542 Mya).[215][216][217] While it is widely accepted that the presence of molecular oxygen in the early fossil record was the result of cyanobacteria activity, little is known about how cyanobacteria evolution (e.g., habitat preference) may have contributed to changes inbiogeochemical cycles through Earth history. Geochemical evidence has indicated that there was a first step-increase in the oxygenation of the Earth's surface, which is known as theGreat Oxidation Event (GOE), in the earlyPaleoproterozoic (2,500–1,600 Mya).[214][211] A second but much steeper increase in oxygen levels, known as the Neoproterozoic Oxygenation Event (NOE),[216][85][218] occurred at around 800 to 500 Mya.[217][219] Recentchromium isotope data point to low levels of atmospheric oxygen in the Earth's surface during the mid-Proterozoic,[215] which is consistent with the late evolution of marine planktonic cyanobacteria during theCryogenian;[220] both types of evidence help explain the late emergence and diversification of animals.[221][46]
Understanding the evolution of planktonic cyanobacteria is important because their origin fundamentally transformed thenitrogen andcarbon cycles towards the end of thePre-Cambrian.[219] It remains unclear, however, what evolutionary events led to the emergence of open-ocean planktonic forms within cyanobacteria and how these events relate to geochemical evidence during the Pre-Cambrian.[216] So far, it seems that ocean geochemistry (e.g.,euxinic conditions during the early- to mid-Proterozoic)[216][218][222] and nutrient availability [223] likely contributed to the apparent delay in diversification and widespread colonization of open ocean environments by planktonic cyanobacteria during theNeoproterozoic.[219][46]
Cyanobacteria are capable of natural genetictransformation.[224][225][226] Natural genetic transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous DNA from its surroundings. For bacterial transformation to take place, the recipient bacteria must be in a state ofcompetence, which may occur in nature as a response to conditions such as starvation, high cell density or exposure to DNA damaging agents. In chromosomal transformation, homologous transforming DNA can be integrated into the recipient genome byhomologous recombination, and this process appears to be an adaptation forrepairing DNA damage.[227]
Tree of Life inGenerelle Morphologie der Organismen (1866). Note the location of the genusNostoc with algae and not with bacteria (kingdom "Monera")
Historically, bacteria were first classified as plants constituting the class Schizomycetes, which along with the Schizophyceae (blue-green algae/Cyanobacteria) formed the phylum Schizophyta,[235] then in the phylumMonera in the kingdomProtista byHaeckel in 1866, comprisingProtogens, Protamaeba, Vampyrella, Protomonae, andVibrio, but notNostoc and other cyanobacteria, which were classified with algae,[236] later reclassified as theProkaryotes byChatton.[237]
The cyanobacteria were traditionally classified by morphology into five sections, referred to by the numerals I–V. The first three –Chroococcales,Pleurocapsales, andOscillatoriales – are not supported by phylogenetic studies. The latter two –Nostocales andStigonematales – are monophyletic as a unit, and make up the heterocystous cyanobacteria.[238][239]
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).[240] In Nostocales and Stigonematales, the cells have the ability to develop heterocysts in certain conditions. Stigonematales, unlike Nostocales, include species with truly branched trichomes.[238]
Most taxa included in the phylum or division Cyanobacteria have not yet been validly published underThe International Code of Nomenclature of Prokaryotes (ICNP)[241] and are instead validly published under theInternational Code of Nomenclature for algae, fungi, and plants. These exceptions are validly published under ICNP:
Recent research has suggested the potential application of cyanobacteria to the generation ofrenewable energy by directly converting sunlight into electricity. Internal photosynthetic pathways can be coupled to chemical mediators that transfer electrons to externalelectrodes.[251][252] In the shorter term, efforts are underway to commercializealgae-based fuels such asdiesel,gasoline, andjet fuel.[72][253][254] Cyanobacteria have been also engineered to produce ethanol[255] and experiments have shown that when one or two CBB genes are being over expressed, the yield can be even higher.[256][257]
Cyanobacteria may possess the ability to produce substances that could one day serve as anti-inflammatory agents and combat bacterial infections in humans.[258] Cyanobacteria's photosynthetic output of sugar and oxygen has been demonstrated to have therapeutic value in rats with heart attacks.[259] While cyanobacteria can naturally produce various secondary metabolites, they can serve as advantageous hosts for plant-derived metabolites production owing to biotechnological advances in systems biology and synthetic biology.[260]
Spirulina's extracted blue color is used as a natural food coloring.[261]
Researchers from several space agencies argue that cyanobacteria could be used for producing goods for human consumption in future crewed outposts on Mars, by transforming materials available on this planet.[262]
Somemicroalgae contain substances of high biological value, such aspolyunsaturated fatty acids,amino acids,proteins, pigments,antioxidants,vitamins, and minerals.[264] Edible blue-green algae reduce the production of pro-inflammatory cytokines by inhibiting NF-κB pathway in macrophages and splenocytes.[265] Sulfate polysaccharides exhibit immunomodulatory, antitumor, antithrombotic, anticoagulant, anti-mutagenic, anti-inflammatory, antimicrobial, and even antiviral activity againstHIV,herpes, andhepatitis.[266]
Recent studies suggest that significant exposure to high levels of cyanobacteria producing toxins such asBMAA can causeamyotrophic lateral sclerosis (ALS). People living within half a mile of cyanobacterially contaminated lakes have had a 2.3 times greater risk of developing ALS than the rest of the population; people around New Hampshire'sLake Mascoma had an up to 25 times greater risk of ALS than the expected incidence.[272] BMAA from desert crusts found throughout Qatar might have contributed to higher rates of ALS inGulf War veterans.[268][273]
Several chemicals can eliminate cyanobacterial blooms from smaller water-based systems such as swimming pools. They includecalcium hypochlorite,copper sulphate, Cupricide (chelated copper), andsimazine.[274] The calcium hypochlorite amount needed varies depending on the cyanobacteria bloom, and treatment is needed periodically. According to the Department of Agriculture Australia, a rate of 12 g of 70% material in 1000 L of water is often effective to treat a bloom.[274] Copper sulfate is also used commonly, but no longer recommended by the Australian Department of Agriculture, as it kills livestock, crustaceans, and fish.[274] Cupricide is a chelated copper product that eliminates blooms with lower toxicity risks than copper sulfate. Dosage recommendations vary from 190 mL to 4.8 L per 1000 m2.[274] Ferric alum treatments at the rate of 50 mg/L will reduce algae blooms.[274][275] Simazine, which is also a herbicide, will continue to kill blooms for several days after an application. Simazine is marketed at different strengths (25, 50, and 90%), the recommended amount needed for one cubic meter of water per product is 25% product 8 mL; 50% product 4 mL; or 90% product 2.2 mL.[274]
Climate change favours cyanobacterial blooms both directly and indirectly.[35] Many bloom-forming cyanobacteria can grow at relatively high temperatures.[280] Increasedthermal stratification of lakes and reservoirs enables buoyant cyanobacteria to float upwards and form dense surface blooms, which gives them better access to light and hence a selective advantage over nonbuoyant phytoplankton organisms.[281][97] Protracted droughts during summer increase water residence times in reservoirs, rivers and estuaries, and these stagnant warm waters can provide ideal conditions for cyanobacterial bloom development.[282][279]
The capacity of the harmful cyanobacterial genusMicrocystis to adapt to elevated CO2 levels was demonstrated in both laboratory and field experiments.[283]Microcystis spp. take up CO2 andHCO− 3 and accumulateinorganic carbon incarboxysomes, and strain competitiveness was found to depend on the concentration of inorganic carbon. As a result,climate change and increased CO2 levels are expected to affect the strain composition of cyanobacterial blooms.[283][279]
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![Cyanobacterial remains of an annulated tubular microfossil Oscillatoriopsis longa [184] Scale bar: 100 μm](/mt/?noimg=&dark=on&url=http%3a%2f%2fwww.wikipedia.org%2fwiki%2fFile%3aOscillatoriopsis_longa_fossil.jpg)
