An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis (Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living"). Examples are nitrogen-fixing bacteria (called rhizobia) which live in root nodules on legume roots, single-celled algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10–15% of insects.
Many instances of endosymbiosis are obligate; that is, either the endosymbiont or the host cannot survive without the other, such as the gutless marine worms of the genus Riftia, which get nutrition from their endosymbiotic bacteria. The most common examples of obligate endosymbiosis are mitochondria and chloroplasts. Some human parasites, e.g. : Wucherichia bancrofti and Mansonella perstans thrive in their hosts because of an obligate endosymbiosis with Wolbachi spp.. They can both be eliminated from their host by treatments that target this bacterium. However, not all endosymbioses are obligate. Also, some endosymbioses can be harmful to either of the organisms involved.
It is generally agreed that certain organelles of the eukaryotic cell, especially mitochondria and plastids such as chloroplasts, originated as bacterial endosymbionts. This theory is called the endosymbiotic theory, and was first articulated by the Russian botanist Konstantin Mereschkowski in 1905.
Read more about Endosymbiont: Endosymbiosis Theory and Mitochondria and Chloroplasts, Bacterial Endosymbionts in Marine Invertebrates, Symbiodinium Dinoflagellate Endosymbionts in Marine Metazoa and Protists, Endosymbionts in Protists, Bacterial Endosymbionts in Insects, Viral Endosymbionts and Endogenous Retrovirus