According to the National Ocean Service, coral bleaching is when coral reefs lose their color after losing their symbiotic algae. Corals and algae depend on each other in a mutual relationship. The algae gives coral the majority of their nutrients and their pigmentation, in return corals give algae shelter inside of their polyps. The polyps need the algae for food, and fish in turn get their food from the polyps. When coral reefs become stressed by external factors like light, temperature, and nutrition intake, they begin to lose their color and turn white, eventually dying in the process.