Why do corals turn white, and can they recover?

A coral turns white because it has lost its partners. Corals live with tiny algae called zooxanthellae that provide the coral with nutrients through photosynthesis. In exchange, the coral gives the algae the carbon dioxide and ammonium they need. This is coral bleaching: the loss of the symbiotic algae and their photosynthetic pigments.

Stressors such as changes in water temperature, light, salinity or nutrients can set it off. Under stress, the coral polyp consumes or expels the algae, and the coral becomes pale or white. The expulsion is meant for short-term survival. But the algae provide up to 90 percent of the coral's energy needs, so after expelling them the coral may begin to starve.

A white coral is therefore not necessarily dead. Coral can survive short-term disturbances, but if the conditions persist its chances of survival diminish. To recover, algae have to re-enter the coral's tissues and restart photosynthesis. Some bleached corals even look bright blue, pink or purple: they make sun-screening pigments, because blue light that the algae used to absorb is now reflected by the white skeleton.

Marine heatwaves caused by El Niño have been found to be one of the main causes of widespread bleaching and coral death, and the IPCC's Sixth Assessment Report found that the frequency and severity of mass bleaching have increased sharply worldwide since the early 1980s. In 2016 alone, bleaching killed 29 to 50 percent of the coral on the Great Barrier Reef.

After a bleaching event, a reef can go one of two ways. It can recover, recolonised by zooxanthellae, or it can undergo a regime shift in which thick layers of macroalgae take over.