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Commonly found in oligotrophic environments, diatoms within the genera "Hemiaulus" and "Rhizosolenia" form symbiotic associations with filamentous cyanobacteria in the species "Richelia intracellularis." As an endophyte in 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 an increase in nitrogen fixation from "Richella-Rhizosolenia" symbiosis. A dominant organism in warm oligotrophic waters, five species within the genus "Hemiaulus" receive fixed nitrogen from "R. intracellularis". "Hemiaulus-Richella" symbioses are up to 245 times more abundant than the former, with 80% to 100% of "Hemilalus" cells containing the cyanobiont. Nitrogen fixation in the "Hemiaulus-Richella" symbiosis is 21 to 45 times greater than in the "Richella-Rhizosolenia" symbiosis within the southwestern Atlantic and Central Pacific Gyre, respectively.