Endosymbiosis

Endosymbiosis occurs when a symbiont lives inside the body or the cells of another organism. It is a very widespread phenomenon in living things.^[1]

Examples are:

• Rhizobia: nitrogen-fixing bacteria which live in root nodules on plants of the pea family.
• Singly-celled forams which include a single-celled alga inside the cell. This is 'facultative', which means they may or may not do it. A more far-reaching version is idioplastic endosymbiosis. Here, the foram consumes the alga, but keeps its chloroplasts in working order.
• Green algae inside marine polychaete worms.
• Single-celled algae inside reef-building corals.
• Gut flora (often flagellates) inside the digestive systems of insects and vertebrates which digest plant cellulose. These are 'obligate' symbionts, which means the host must have them.^[1]
• Lichen: fungus+alga or bacterium.
• Mycorrhiza: fungal hyphae and roots of trees.

1. ↑ ^{1.0 1.1} Buchner P. 1965. Endosymbioses of animals with plant microorganisms. Wiley N.Y.