Coral Bleaching

Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement (Brown and Ogden, 1993). However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.

Corals tolerate a narrow temperature range between 25 degrees Celsius and 29 degrees Celsius depending on location. Corals bleach in response to prolonged temperature change and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach when water reaches a constant 32 degrees Celsius (Brown and Ogden, 1993).

Experiments have shown that an increase in UV light causes coral bleaching. UV light experienced by corals can increase in calm waters. The amount of mycosporine-like amino acids in a coral's tissues helps to determine how much UV it can withstand without bleaching (Gleason, 1993).

The exact mechanism by which corals bleach or the trigger that induces bleaching is unknown. However, there are a number of hypotheses that attempt to answer this question. Laboratory experiments have shown that the zooxanthellae are released into the gut of the polyp and then are expelled from the polyp through the mouth (Brown and Ogden, 1993). However, this has not been observed in nature. Another hypothesis is that stressed corals give algae fewer nutrients and thus the algae leave the polyp. Algae may produce oxide toxicity under stress, and these toxins may affect the polyps.

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