Coral reefs are large underwater structures made up of thousands of coral lifeforms. Their wide range of colors are caused by a number of factors, including life that inhabits them and environmental conditions. Coral can cover the whole spectrum of visible colors and their coloring can indicate whether the coral populating a reef are healthy or unhealthy.
Coral reefs derive their unique coloring from organisms living within their structure. These organisms are known as zooxanthellae. The reefs are actually clear in color with the variety of colors seen on healthy reefs coming from these organisms.
The zooxanthellae have a symbiotic relationship with the reefs. These organisms use photosynthesis to survive, which aids the reefs in nutrient production. The coral reef provides the organisms with a safe structure to live within and also provides carbon dioxide needed for photosynthesis.
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You can determine if a coral reef is healthy or experiencing "coral bleaching," which signals a dying reef. Reefs that are bleach white have lost most of their zooxanthellae and are struggling to survive. Warmer water temperatures and pollution in the water can cause the zooxanthellae to leave or die off, in turn destroying the coral reef.
When a coral reef dies due to lack of nutrients or ideal water conditions, the reef does not always disappear or fall apart. The skeletal structure of the reef remains and becomes part of the rest of the larger structure of healthy coral. This is why when viewing reefs, you may see a wide range of colorful coral mixed with hardened and bleach white dead coral.
The coloring of corals serves a functional purpose as well. Different environmental conditions affect the coloring, including the amount of light the coral receives and the type of light rays it is exposed to. For example, the amount and type of UV rays that the coral is exposed to can damage the coral over time.
Some coral have developed certain colors to protect against higher levels of UV rays. Coral that have colors such as pink, blue and purple often have a protective layer against UV rays. Coral can also change color throughout their existence due to changes in their environment.
Continued pollution of the earth's water as well as overfishing are examples of humanity's impact on the health of the world's coral reefs. Large amounts of bleached coral in a single reef area can be a sign that pollution in that area is high enough to damage the ecosystem.