Rising ocean temperatures can cause coral bleaching.  Bleaching is an event in which zooxanthellae, combinations of cynobacteria and photosynthesizing algae, abandon corals as result of higher temperatures.  Zooxanthellae are responsible for the coral’s pretty color, which is why their exodus is called bleaching.  These organisms also provide up to 95% of the coral’s food.  A coral can survive a bleaching, but not sustained bleaching.