Corals get their energy in part from microscopic symbiotic algae that live inside their cells. These algae, called zooxanthellae, produce sugar and other nutrients through photosynthesis. When ocean temperatures rise beyond a certain threshold, the symbiont's photosynthetic machinery may be damaged and produce harmful reactive oxygen molecules, causing the corals to eject the algae. Without their golden-brown colored symbionts, corals becomes a ghostly white in a process called bleaching and eventually die.