Coral Reef Bleaching - What is Coral Reef Bleaching?

Coral bleaching is the term given to the event where the zooxanthellae residing within the corals die or leave the cells, either by expulsion or by their own accord. The coral appears bleached because of the loss of pigmented zooxanthellae. The corals themselves have no colour as the underlying calcium carbonate framework beneath and surrounding the polyps is a white colour, and the polyps on the surface layer are translucent. Bleaching can also occur when the zooxanthellae simply lose their pigment but remain in the host cells. Figure 4 shows a coral in the process of bleaching; the central polyps have lost their zooxanthellae but some outer polyps are still healthy, with a yellow-green colouration. The phenomenon occurs at times when the corals are under unsustainable environmental stress. Environmental factors include extreme temperatures, UV radiation, aerial exposure, nutrient imbalance, sedimentation and chemical contamination.