Bleached corals on a reef. The bleached corals appear pale and white compared to the golden and pink healthy corals around them. Coral becomes bleached when the tiny coral organisms eject the symbiotic algae that live in their tissues. The algae normally make food for the coral, so lack of them impairs the functioning of the coral, and can be fatal. Coral bleach due to stress, which is usually caused by a rise in water temperature or acidity. Coral bleaching events are closely monitored by researchers investigating the effects of global warming and ocean acidification. Filmed in Palau.