Most reef-building corals support an algae called zooxanthellae, which lives within the fleshy parts of the coral polyps. These algae give living coral their brownish color. Zooxanthellae grow and therefore supply the coral polyps with food and oxygen by a process called photosynthesis. In turn, the food the corals filter feed through their tentacles supplies both the corals and the zooxanthellae with the crucial nutrients, phosphorous and nitrogen. These elements are then cycled back and forth between the two organisms. How else might coral help the algae? Coral provides a habitat for the algae. Without algae populations, the corals would be unable to produce substantial reef structures. This interdependent relationship between the coral and the algae is an example of symbiosis, a relationship in which two organisms work together to survive.
A dramatic sign that a coral is living under stressful conditions is coral bleaching. The algae are expelled (kicked out) and the coral loses its color causing the polyps to look clear and the
colony to turn white hence, the term "bleaching". (No bleach is involved.) Corals might bleach when they face water that is too hot or cold, live above water for too long, have a nutrient deficiency or when they encounter waters that are clouded with
silt or pollution. Adverse organism reactions to the environment can often be the first sign that something has gone wrong in terms of temperature, amount of nutrients or an imbalance in the food chain.
Peter W. Glynn
Department of Biology, University of Houston
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