Endosymbiont

An endosymbiont or endobiont^[1] is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects.^[2][3]

Endosymbiosis played key roles in the development of eukaryotes and plants. Roughly 2.2 billion years ago an archaea absorbed a bacterium through phagocytosis that eventually became the mitochondria that provide energy to all living cells. Approximately 1 billion years ago, other cells absorbed cyanobacteria that eventually became chloroplasts, organelles that produce energy from sunlight.^[4] Some 100 million years ago, UCYN-A, a nitrogen-fixing bacteria became an endosymbiont of marine algae Braarudosphaera bigelowii, eventually evolving into a nitroplast.^[5] Similarly, Diatoms in the family Rhopalodiaceae have evolved a diazoplast, a nitrogen-fixing organelle.^[6]

Symbionts are either obligate (require their host to survive) or facultative (can survive independently).^[7] The most common examples of obligate endosymbiosis are mitochondria and chloroplasts, which reproduce via mitosis in tandem with their host cells. Some human parasites, e.g. Wuchereria bancrofti and Mansonella perstans, thrive in their intermediate insect hosts because of an obligate endosymbiosis with Wolbachia spp.^[8] They can both be eliminated by treatments that target their bacterial host.^[9]