Endosymbiont

Endosymbiont refers to an organism that lives inside the body or cells of another organism, the host, in a close and long-term biological interaction. Endosymbionts are often bacteria or archaea, and their relationship with the host can be mutualistic, commensal, or parasitic. In many cases, the endosymbiont provides a benefit such as nutrients or defense, while the host provides a protected environment and resources.

A prominent concept is the endosymbiotic theory, which proposes that key organelles of eukaryotic cells—mitochondria and plastids (including chloroplasts)—originated from free-living bacteria that entered ancestral cells and became integrated as intracellular organelles. Evidence includes circular genomes, ribosomes similar to bacterial ones, double membranes, and the ability to reproduce by division within the host cell. Over time, many endosymbiont genes have been transferred to the host nucleus, and the endosymbiont's genome has become reduced.

Contemporary examples of endosymbionts include Buchnera aphidicola, which provides essential amino acids to aphids, and Wolbachia, a widespread bacterium that infects many arthropods and can influence reproduction. In plants, Rhizobium bacteria form nitrogen-fixing symbioses in root nodules. In corals and other marine organisms, photosynthetic dinoflagellates and other microbes act as endosymbionts, contributing to host nutrition.

Endosymbiotic relationships are a major force in biology, affecting metabolism, development, ecology, and evolution. They can be stable and heritable through generations or acquired anew through environmental interactions.