Here’s a great video describing coral bleaching. Of course it’s a simplified description of a bleaching event, but it’s very informative. Zooxanthellae, a symbiotic algae, take up residence in a coral for protection and a place in which they can soak up sunlight for photosynthesis. In return, the coral receives vital nutrients which are needed for survival. Zooxanthellae can provide up to 95% of of a coral’s nutrition. When temperatures in the ocean get too high, the Zooxanthellae leave the coral to find a more suitable residence. The coral turns white, or bleaches, as the symbiotic algae leave its tissue. The coral will then starve to death if temperatures don’t return to normal, allowing the algae to return. The result is massive coral death, followed by the invasion of other algae types that blanket the reef.