The Serial Endosymbiosis Theory , or endosymbiosis hypothesis, is a theory championed by Lynn Margulis that says some organelles in eukaryotes such as mitochondria and chloroplasts evolved from individual bacteria. There are several very good examples of symbiosis in nature. Coral reefs, Mycorrhizae, termites, and Trichonympha all have beneficial symbiotic relationships with other organisms. They would not be able to survive without their vital symbiotes. Both organisms are better off because of the relationship.
Coral reefs have minuscule dinoflagellate algae living in their tissues.
These photosynthetic protists produce food that the coral uses. The algae is provided a safe place to live, so is also benefited from the relationship. Since the dinoflagellates need sunlight to produce food, corals are restricted to shallow waters. Despites this, corals and dinoflagellates are important to each other s survival. Mycorrhizae are a type of fungus that live with the roots of many plants.