coral bleaching

Coral bleaching is generally caused by warm water temperatures, but can also be caused due to extreme cold water temperatures as well. While temperatures seem to be the major cause, it can also be caused by sediment, nutrients, and other changes in a reef's environment. The coral will rid itself of zooxanthella, losing a symbiotic relationship with an algae living within its tissue. Then the coral becomes white. It is under great stress, both the hot-stress and cold-stress make the coral susceptible to disease and death.
picture: http://25.media.tumblr.com/tumblr_m0v0qcgRC01r8x2ybo1_400.jpg