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= Cyanobionts = Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plant, algal, and fungal species. They can reside within extracellular or intracellular structures of the host. In order for a cyanobacterium to successfully form a sybiotic relationship, it must be able to exchange signals with the host, overcome defense mounted by the host, be capable of hormogonia formation, chemotaxis, heterocyst formation, as well as possess adequate resilience to reside in host tissue which may present extreme conditions such as low oxygen levels, and/or acidic mucilage. The most well-known plant-associated cyanobionts belong to the Nostoc genus. With the ability to differentiate into several cell types that have various functions, Nostoc’s morphological plasticity, flexibility and adaptability to any environmental condition, are partially responsible for its high capacity to form symbiotic relationships with other organisms. Several cyanobionts involved with fungi and marine organisms also belong to the Richelia, Calothrix, Synechocystis, Aphanocapsa, and Anabaena genera, as well as the Ocillatoria spongeliae species. Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses. The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.

Currently, cyanobionts have been found to form symbiosis with various organisms in marine environments such as diatoms, dinoflagellates, sponges, protozoans, Ascidians, Acadians, and Echiuroid worms, of which have significance in maintaining the biogeochemistry of both open ocean and coastal waters. Specifically, symbioses involving cyanobacteria are mostly mutualistic, in which the cyanobionts are responsible for nutrient provision to the host in exchange for attaining high structural-functional specialization. Most cyanobacteria-host symbioses are found in oligotrophic areas where there is limited availability of nutrients such as carbon (DOC) and nitrogen, although a few occur in nutrient-rich areas such as mudflats.

Role in symbiosis
Cyanobionts play a variety of roles in their symbiotic relationships with the host organism. They function primarily as nitrogen- and carbon-fixers. However, they can also be involved in metabolite exchange, as well as in provision of UV protection to their symbiotic partners since some can produce nitrogen-containing compounds with sunscreen-like properties, such as scytonemin and amino acids similar to mycosporin.

By entering into a symbiosis with nitrogen-fixing cyanobacteria, organisms that otherwise cannot inhabit low-nitrogen environments are provided with adequate levels of fixed nitrogen to carry out life functions. Providing nitrogen is a common role of cyanobionts in many symbiotic relationships, most especially in those with photosynthetic hosts. Formation of an anaerobic envelope (heterocyst) to prevent nitrogenase from being irreversibly damaged in the presence of oxygen is an important strategy employed by nitrogen-fixing cyanobacteria to carry out fixation of dinitrogen in the air, via nitrogenase, into biologically-useful nitrogen. To keep up with the large nitrogen demand of both the symbiotic partner and itself, cyanobionts fix nitrogen at a higher rate, as compared to their free-living counterparts, by increasing frequency of heterocyst formation.

Cyanobacteria are also photosynthetically active and can therefore meet carbon requirements independently. In symbioses involving cyanobacteria, at least one of the partners must be photoautotrophic in order to generate sufficient amounts of carbon for the mutualistic system. This role is usually allocated to cyanobionts in symbiotic relationships with non-photosynthetic partners such as marine invertebrates.

Maintenance of successful symbioses
In order to maintain a successful symbiosis following host infection, cyanobacteria will need to match their life cycles with those of their hosts’. In other words, cyanobacteria cell division must be done at a rate matching their host in order to divide at similar times. As free living organisms, cyanobacteria divide very frequently compared to eukaryotic cells, but as symbionts, cyanobionts must slow down division times as to not overwhelm their host. It is unknown how cyanobionts are able to adjust their growth rates, but it is not a result of nutrient limitation by the host. Instead, cyanobionts somehow know to limit their own nutrient uptake in order to delay cell division, while the excess nutrients will then be diverted to the host for uptake.

As the host continues to grow and reproduce, the cyanobiont will continue to infect and replicate in the new cells. This is known as vertical transmission, where new daughter cells of the host will be quickly infected by the cyanobionts in order to maintain their symbiotic relationship. This is most commonly seen when hosts reproduce asexually. In the water fern Azolla, cyanobacteria colonize the cavities within dorsal leaves. As new leaves form and begin to grow, the new leaf cavities that develop will quickly become colonized by new incoming cyanobacteria.

An alternative mode of transmission is known as horizontal transmission, where hosts acquire and utilize new cyanobacteria from the surrounding environment between each host generation. This mode of transmission is commonly seen when hosts reproduce sexually, as it tends to increase the genetic diversity of both host and cyanobiont. Hosts that utilize horizontal transmission in order to obtain cyanobacteria will typically acquire a large and diverse cyanobiont population. This may be used as a survival strategy in open oceans as indiscriminate uptake of cyanobacteria may guarantee capture of appropriate cyanobionts for each successive generation.

Genetic modifications within host
Following infection and establishment of an endosymbiotic relationship, the new cyanobionts will no longer be free living and autonomous, but rather begin to dedicate their physiological activities in tandem with their hosts'. Over time and evolution, the cyanobiont will begin to lose portions of their genome in a process known as genome erosion. As the relationship between the cyanobacteria and host evolves, the cyanobiont genome will develop signs of degradation, particularly in the form of pseudogenes. A genome undergoing reduction will typically have a large proportion of psuedogenes and transposable elements dispersed throughout the genome. Furthermore, cyanobacteria involved in symbiosis will begin to accumulate these mutations in specific genes, particularly those involved in DNA repair, glycolysis, and nutrient uptake. These gene sets are critical for organisms that live independently, however as cyanobionts living in symbiosis with their hosts, there may not be any evolutionary need to continue maintaining the integrity of these genes. As the major function of a cyanobiont is to provide their host with fixed nitrogen, genes involved in nitrogen fixation or cell differentiation are observed to remain relatively untouched. This may suggest that cyanobacteria involved in symbiotic relationships can selectively stream line their genetic information in order to best perform their functions as cyanobiont-host relationships continue to evolve over time.

Examples of symbioses
Cyanobacteria have been documented to form symbioses with a large range of eukaryotes in both marine and terrestrial environments. Cyanobionts provide benefit through dissolved organic carbon (DOC) production or nitrogen fixation but vary in function depending on their host. Organisms that depend on cyanobacteria often live in Nitrogen-limited, oligotrophic environments and can significantly alter marine composition leading to blooms.

Diatoms
Commonly found in oligotrophic environments, diatoms within the genera Hemiaulus and Rhizosolenia both form a symbiotic association with the filamentous cyanobacteria Richelia intracellularis. As an endophyte of up to 12 species of Rhizosolenia, R. intracellularis provides fixed nitrogen to its host via the terminally-located heterocyst. Richella-Rhizosolenia symbioses have been found to be abundant within the nitrogen-limited waters of the Central-Pacific Gyre. Several field studies have linked the occurrence of phytoplankton blooms within the gyre to a increase in nitrogen fixation from Richella-Rhizosolenia symbiosis. A dominant organism in warm oligotrophic waters, five species within the genus Hemiaulus receives fixed nitrogen from R. intracellularis. Hemiaulus-Richella symbioses are up to 245 times more abundant than the former with 80%-100% of Hemilalus containing the cyanobiont. Nitrogen fixation from Hemiaulus-Richella is 21 to 45 times greater than Richella-Rhizosolenia within the southwestern Atlantic and Central Pacific Gyre, respectively.

Other genera of diatoms can form symbioses with cyanobacteria, however their relationships are less known. Spheroid cyanobacteria have been found within the diatom Rhopalodia gibba which have been found to posses genes for nitrogen fixation but do not possess the proper pigments for photosynthesis.

Dinoflagellates
Heterotrophic dinoflagellates can form symbioses with cyanobacteria (phaeosomes) most often in tropical, marine environments. The function of the cyanobiont varies depending on its host species. The abundant marine cyanobacteria Synechococcus forms symbionts with dinoflagellate generaOrnithocercus, Histionesis and Citharistes where it is hypothesized to benefit its host through the provision of fixed nitrogen in oligotrophic, subtropical waters. Increased instances of phaeosome symbiosis have been documented in a stratified, nitrogen-limited environment and living within a host can provide the desired anaerobic environment for fixation to occur. . However there is conflicting evidence of this. One study on the phaeosomes of Ornithocercus spp. has provided evidence for carbon rather than nitrogen fixation from Synechococcus, due to the absence of nitrogenase within the cyanobacteria.

Sponges
100 species within the classes Calcarea and Demospongiae form symbioses with cyanobacteria genera Aphanocapsa, Synechocystis, Oscillatoria and Phormidium. Cyanobacteria benefit their host through providing glycerol and organic phosphates through photosynthesis and supply up to half of its required energy and a majority of its carbon budget. Two groups of photosynthetic sponges have been described: "cyanosponges" and "phototrophs". Cyanosponges are mixotrophic and therefore obtain energy through heterotrophic feeding as well as photosynthesis. The latter group receives almost all of their energy requirements through photosynthesis and therefore have a larger surface area in order increase exposure to sunlight. The most common cyanobiont found in sponges is Synechococcus with the species Candidatus Synechococcus spongiarum inhabiting a majority of symbiotic sponges within the Caribbean. Another widely distributed species of cyanobacteria Oscillatoria spongeliae is found within Lamellodysidea herbacea along side ten other species. O. spongeliae benefits their host through providing carbon as well as a variety of chlorinated amino derivatives depending on the host strain.

Lichens
Lichens are the result of a symbiosis between a mycobiont and an autotroph, usually green algae or cyanobacteria. About 8% of lichen species contain a cyanobiont, most commonly Nostoc as well as Calothrix, Scytonema and Fischerella. All cyanobionts inhabiting lichens contain heterocysts to fix nitrogen and can be distributed throughout their host in specific regions (heteromerous) or randomly throughout the thallus (homoiomerous). Additionally, some lichen species are tripartate, containing both a cyanobacteria and green algal symbiont.

Bryophytes
Bryophytes are non-vascular plants encompassing mosses, liverworts, and hornworts and most often form symbioses with the cyanobacteria genera Nostoc. Depending on the host, the cyanobiont can be present inside (endophytic) or outside the host (epiphytic). In the mosses, cyanobacteria are major nitrogen fixers and grow mostly epiphytically aside from two species of Sphagnum which protect the cyanobiont from an acidic bog environment. In terrestrial Arctic environments, cyanobionts are the primary supplier of nitrogen to the ecosystem wether free-living or epiphytic with mosses. Cyanobacterial association with liverworts are rare with only four out of 340 genera forming symbionts. Two of the genera Marchantia and Porella, are epiphytic while genera Blasia and Cavicularia are endophytic .In the hornworts however, endophytic cyanbionts have been described in more than triple the amount of genera in liverworts. Bryophytes and cyanobacteria possess different structures depending on the nature of the symbiosis. . For instance, colonies of cyanobacteria in Blasia are present as auricles (small dots) between inner and out papillae near the ventral surface for the liverwort whereas cyanobionts in hornworts Anthoceros and Phaeoceros are present within the thallus' specialized slime cavities. However, cyanobacteria first must locate and physically interact with their host in order to form a symbiotic relationship. In Nostoc, cyanobacteria can become motile through the use of hormogonia, whereas the host plant excretes chemicals to guide the cyanobacteria via chemotaxis. For instance, Blasia can secrete HIF, a strong chemoattractant when nitrogen-starved and symbiotic cyanobacteria Nostoc punctiforme has been shown to posses genes that complement chemotaxis-related proteins within Gunnera.

Ascidians
Filamentous cyanobacteria within the genera Synechocystis and Prochloron has been found within the tunic cavity of didemnid sea squirts. The symbiosis is proposed to have originated through the intake of a combination of sand and cyanobacteria which eventually proliferated. The hosts benefit from receiving fixed carbon from the cyanobiont while the cyanobiont may benefit by protection from harsh environments.

Echiuroid Worms
Little is known about the symbiotic relationship between echiuroid worms and cyanobacteria. Unspecified cyanobacteria has been found within the subepidermal connective tissue of Ikedosoma gogoshimense and Bonellia fuliginosa.

Coral
Unicellular and symbiotic cyanobacteria were discovered in the host cells of the coral Montastraea cavernosa from Caribbean Islands.These cyanobionts are coexisted with the symbiotic dinoflagellate zooxanthellae in the coral and produce the nitrogen-fixing enzyme nitrogenase. Detail of the interaction among these association remains unknown.