Paleo Slide Set: Coral Paleoclimatology:Natural Record of Climate change for High School Student
Bleached (Pocillopera) 1983, shows how an El Niņo year can affect coral growth.

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. 

Photo Credits:
Peter W. Glynn 
Department of Biology, University of Houston