Endosymbiont:
An endosymbiont or endobiont 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. [Wikipedia]
Noted in “A bacterium has evolved into a new cellular structure inside algae,” Michael Le Page, NewScientist (20 April 2024, paywall):
It is quite common for one species to live inside the cells of another in a mutually beneficial relationship called endosymbiosis. For instance, cells in the roots of legumes such as peas host nitrogen-fixing bacteria. The success of cockroaches is partly due to endosymbiotic bacteria that produce essential nutrients. Some cells even host multiple endosymbionts.
