Coral Bleaching

So why does temperature have such a big impact on corals? To understand this question it is important to know a little background information about coral reefs.  It seems like the increase of a few degrees would be negligible, however, due to a series of very specialized mutualistic relationships (or the ability to help each other out), there is a very sensitive balance that is about to be disrupted.

This figure shows how zooanthella live within the tissues of the coral polyp 

Corals are widely appreciated for their dramatic and varied coloring; however, these colors are not produced by the coral itself. corals host symbiotic algae referred to as zooxanthellae (Donner, 2005). A symbiotic relationship is when two organisms work together and help each other out. Zooxanthellae do this by assisting coral in producing energy through photosynthesis; they also increase the rate of calcification by removing CO2 from immediate waters ("Microdocs...", 2012). Corals return the favor by sheltering the algae and providing them with CO2 and other necessary nutrients. Calcification is the hardening of tissues through lime deposits that protects the organism. Unfortunately, as the temperature rises, it causes stress that can ultimately lead to the expulsion of the zooxanthellae ("When Corals...", 2011). With no more algae hosted in their tissue this leads to a loss of pigmentation, causing them to have the appearance of being bleached as the calcareous skeleton becomes visible ("When Corals...", 2011). This event where coral gets rid of the zooxanthellae  is referred to coral bleaching because of the color loss experienced. Bleaching of coral does not lead to immediate death of the coral, however, the species will not be able to live in the conditions very long and usually develops high mortality rates. This affect is reversible. When temperatures are returned to normal, the algae can return to the corals who were able to survive and the reefs are able to make a recovery ("When Corals...", 2011). However, the longer and more intense the event is, the more likely coral death will occur (Donner, 2005).

http://www.telegraph.co.uk/earth/earthnews/7896403/Coral-reefs-suffer-mass-bleaching.html

http://minivannews.com/environment/maldives-suffering-worst-coral-bleaching-since-1998-8132