An organism that lives inside another, benefitting both individuals. For example, endosymbiotic cyanobacteria often inhabit living cells of the genus Epithemia.

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Image Credit: Rex Lowe
The diatom Epithemia turgida contains endosymbiotic cyanobacteria in many of the cells. The endosymbionts are visible as round bodies within the cytoplasm. In Epithemia and Rhophalodia, endosymbionts are important in fixing atmospheric nitrogen for use by the cells.