Coral reefs are complicated ecosystems, filled with many different types of corals and other organisms. The main structure of most reefs is controlled by stony coral colonies that grow into large, boulder-like formations. Healthy corals host tiny photosynthetic creatures called “zooxanthellae” (zoo-zan-thell-ee) in their skeletons. Corals and the zooxanthellae are symbiotic, meaning that the host organism (the coral) and the symbionts (the zooxanthellae) both benefit by cooperating and living together. However, the symbiotic pair live in a delicate balance.
Under stressful conditions, corals can lose their symbionts. Some of the main causes for coral stress are: 1) surface water temperatures that are too hot or too cold; 2) changes in the water’s salinity; and 3) runoff from land carrying dirty, cloudy, or contaminated water into the reef. When the zooxanthellae get stressed out, they start producing damaging oxidizing substances (similar to the compounds that cause aging in humans), and the corals expel them.
Zooxanthellae are often responsible for giving the corals their color. When the zooxanthellae are expelled, the coral loses its source of pigmentation, and all that’s left behind is the coral’s white calcium carbonate skeleton: this white, symbiont-free coral is “bleached.”
Corals can get up to ~90% of their energy from their symbionts. Bleached corals are greatly weakened and susceptible to further stress. If corals are not promptly recolonized by zooxanthellae, they can die from starvation, or fall victim to disease.