Some cyanobacteria, the so-called cyanobionts (cyanobacterial symbionts), have a symbiotic relationship with other organisms, both unicellular and multicellular. As illustrated on the right, there are many examples of cyanobacteria interacting symbiotically with land plants. Cyanobacteria can enter the plant through the stomata and colonize the intercellular space, forming loops and intracellular coils. Anabaena spp. colonize the roots of wheat and cotton plants. Calothrix sp. has also been found on the root system of wheat. Monocots, such as wheat and rice, have been colonised by Nostoc spp., In 1991, Ganther and others isolated diverse heterocystous nitrogen-fixing cyanobacteria, including Nostoc, Anabaena and Cylindrospermum, from plant root and soil. Assessment of wheat seedling roots revealed two types of association patterns: loose colonization of root hair by Anabaena and tight colonization of the root surface within a restricted zone by Nostoc.
The relationships between cyanobionts (cyanobacterial symbionts) and protistan hosts are particularly noteworthy, as some nitrogen-fixing cyanobacteria (diazotrophs) play an important role in primary production, especially in nitrogen-limited oligotrophic oceans. Cyanobacteria, mostly pico-sized Synechococcus and Prochlorococcus, are ubiquitously distributed and are the most abundant photosynthetic organisms on Earth, accounting for a quarter of all carbon fixed in marine ecosystems. In contrast to free-living marine cyanobacteria, some cyanobionts are known to be responsible for nitrogen fixation rather than carbon fixation in the host. However, the physiological functions of most cyanobionts remain unknown. Cyanobionts have been found in numerous protist groups, including dinoflagellates, tintinnids, radiolarians, amoebae, diatoms, and haptophytes. 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.