Cyanobionts are cyanobacteria that live in symbiosis with a wide range of organisms such as terrestrial or aquatic plants; as well as, algal and fungal species.[1] They can reside within extracellular or intracellular structures of the host.[2] In order for a cyanobacterium to successfully form a symbiotic 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.[2] The most well-known plant-associated cyanobionts belong to the genus Nostoc.[3] With the ability to differentiate into several cell types that have various functions, members of the genus Nostoc have the morphological plasticity, flexibility and adaptability to adjust to a wide range of environmental conditions, contributing to its high capacity to form symbiotic relationships with other organisms.[4] Several cyanobionts involved with fungi and marine organisms also belong to the genera Richelia, Calothrix, Synechocystis, Aphanocapsa and Anabaena, as well as the species Oscillatoria spongeliae.[4] Although there are many documented symbioses between cyanobacteria and marine organisms, little is known about the nature of many of these symbioses.[5] The possibility of discovering more novel symbiotic relationships is apparent from preliminary microscopic observations.[5]
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, many of which have significance in maintaining the biogeochemistry of both open ocean and coastal waters.[5] 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.[2] Most cyanobacteria-host symbioses are found in oligotrophic areas where limited nutrient availability may limit the ability of the hosts to acquire carbon (DOC), in the case of heterotrophs and nitrogen in the case of phytoplankton, although a few occur in nutrient-rich areas such as mudflats.[5]