Coral bleaching

node · earth · Global tropical reefs

The breakdown of the coral-algal symbiosis under stress, in which the host loses its symbionts, or the symbionts lose their pigments, and the white skeleton becomes visible through the transparent tissue. The dominant trigger on modern reefs is heat: sea temperature held roughly one degree Celsius above the local maximum monthly mean for a period of weeks, with high light making it worse and cloud or turbidity making it better. The mechanism is understood in outline as damage to photosystem II in the symbiont under combined heat and light stress, producing reactive oxygen species that the host expels the symbiont to escape. The crucial point for a reader is that bleaching is not death. A bleached colony is starving, not destroyed; if the heat abates within weeks it can re-acquire symbionts and survive, though often with reduced growth and no reproduction that year. If the heat persists, it starves. Cold shock, freshwater, sedimentation and disease can bleach coral too, which is why an individual bleached colony is not by itself evidence of a marine heatwave.