The symbiotic algae (zooxanthellae) that live in corals are what give healthy corals their variety of different colors. When the coral host is stressed, its colorful zooxanthellae expels from its tissues, causing the coral to appear pale or white—a process known as coral bleaching. Severe bleaching is usually associated with environmental stress, such as unusually warm (or cold) water temperatures, increased light or solar radiation, changes in salinity, sedimentation, or other pollution from land. Though bleaching does not mean the coral is immediately dead, if it persists it can impair normal functions of the coral host and lead to coral death (mortality), which has devastating impacts on the ecosystem. An intense El Niño in 2015–2016 led to the longest global coral bleaching event on record. Source: NOAA
Photo Credit: Coral cover at Jarvis Island before the bleaching event (Photo: NOAA Fisheries/Paula Ayotte), during the bleaching event in 2015–2016 (Photo: Woods Hole Oceanographic Institution/Cohen Lab), and red turf algae growing over dead and damaged coral after the bleaching event in 2016 (Photo: NOAA Fisheries/Bernardo Vargas-Ángel).