|Bleached (left) and non-bleached (right) colonies of the coral Porites compressa.|
About Coral Bleaching: Many reef-building corals harbor tiny algae that live inside their tissues. These algae use light energy to make sugars, a process known as photosynthesis. A portion of these sugars are given to the coral animal and the coral uses the sugars for energy.
Corals that lose their algae are affected in the same way. This is the reason why widespread coral bleaching is so problematic. Not all corals die if they are bleached. Some corals are capable of recovering from a bleaching event, while others cannot. Why do you think this would be?
Coral bleaching occurs when the tiny brown colored algae are expelled or removed from the coral animal's tissue. This causes the corals to turn a white color, effectively causing the coral animal to look bleached. When corals lose their algae (also called Symbiodinium), they also lose the source of energy provided by algae photosynthesis. This would be analogous to if, all of a sudden, you lost the capacity to digest and gain energy from a fraction of the food you ate everyday. As you can see, this would be problematic for you and you might become stressed and more susceptible to diseases.