What's wrong with this picture? The blue coral on the right may look pretty, but it's actually an example of coral bleaching in National Marine Sanctuary of American Samoa. When water temperature changes too much, corals expel critical symbiotic algae called zooxanthellae that provide nourishment and protective compounds. Those algae are what give corals their vibrant colors, and without them, the corals turn white (or in this case, blue-ish) and may ultimately die. Although coral bleaching can be caused by a variety of events, the leading cause of coral bleaching is increased ocean temperature caused by climate change.