Endosymbiont #

An endosymbiont is an organism that lives within the body or cells of another organism. This relationship is typically mutualistic, meaning both organisms benefit from it.

Examples of endosymbionts include nitrogen-fixing bacteria (called rhizobia) that live in the root nodules of legumes, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to insects.

Endosymbiosis has played key roles in the development of eukaryotes and plants. For instance, about 2.2 billion years ago, an archaea absorbed a bacterium through phagocytosis that eventually became the mitochondria, which 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.

Endosymbionts can be either obligate (require their host to survive) or facultative (can survive independently). The most common examples of obligate endosymbiosis are mitochondria and chloroplasts, which reproduce via mitosis in tandem with their host cells.