Zooxanthellae and the phenomenon of symbiosis

Zooxanthellae are unicellular algae capable of living in the endoderm of many corals and molluscs (e.g.: the Benetids).
They are also photosynthetic dinoflagellates.
Sheltered from disturbances, within the polyps of a coral, they protect themselves from the variation of living conditions, from sedimentation and from predators.

To feed themselves, they use the nitrogen and phosphate waste of the polyp as a source of nutrients which are locally more concentrated than in the external environment.
Thanks to these nutrients and to the energy of the light, the zooxanthellae elaborate organic matter essentially carbohydrates. Part of this material produced is used as a food source by the polyp. This makes the algae a primary producer for coral ecosystems. As for the polyp, it benefits from the organic products elaborated by the zooxanthellae.

The polyp feeds the alga in CO2 and carbonated elements taken from the external environment and from the cellular respiration to stimulate photosynthesis and the production of carbohydrates.
The phenomenon quite known today is the symbiosis. This process allows in this example a mutual exchange between the two organisms, the polyp and the zooxanthella. Thanks to photosynthesis, the alga produces sugars and oxygen which are essential for the corals. Conversely, the corals bring carbon dioxide and nitrogenous compounds to the zooxanthellae.

The zooxanthellae give them access to lipids, protides or carbohydrates.

Finally, their relationship is complementary and essential to their survival, this symbiosis is mutualism.