Cyanobiont

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.

[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.

[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.

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.

[2] 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.

As free living organisms, cyanobacteria typically divide more frequently compared to eukaryotic cells, but as symbionts, cyanobionts slow down division times so they do not overwhelm their host.

[10] 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'.

Cyanobionts provide benefit through dissolved organic carbon (DOC) production or nitrogen fixation but vary in function depending on their host.

[12] Organisms that depend on cyanobacteria often live in nitrogen-limited, oligotrophic environments and can significantly alter marine composition leading to blooms.

[12][13] Commonly found in oligotrophic environments, diatoms within the genera Hemiaulus and Rhizosolenia form symbiotic associations with filamentous cyanobacteria in the species Richelia intracellularis.

Abundant marine cyanobacteria in the genus Synechococcus form symbionts with dinoflagellates in the genera Ornithocercus, Histionesis and Citharistes, where it is hypothesized to benefit its host through the provision of fixed nitrogen in oligotrophic, subtropical waters.

has provided evidence that symbionts belonging to the genus Synechococcus, supply organic carbon rather than nitrogen, due to the absence of nitrogenase within these cyanobacteria.

[26] One hundred species within the classes Calcarea and Demospongiae form symbioses with cyanobacteria in the genera Aphanocapsa, Synechocystis, Oscillatoria and Phormidium.

[12][27] Cyanobacteria benefit their hosts through providing glycerol and organic phosphates through photosynthesis and supply up to half of their required energy and a majority of their carbon budget.

All cyanobionts inhabiting lichens contain heterocysts to fix nitrogen, which can be distributed throughout the host in specific regions (heteromerous) or randomly throughout the thallus (homoiomerous).

[33] In 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.

Members of the cyanobacterial genus Nostoc can become motile through the use of hormogonia, while the host plant excretes chemicals to guide the cyanobacteria via chemotaxis.