coralbleaching

Coral bleaching is a stress response in which corals lose their symbiotic algae, or the algae lose their pigment, causing the coral tissue to turn white. The microscopic algae (dinoflagellates in the family Symbiodiniaceae) living within coral tissues provide a major portion of the coral’s energy through photosynthesis; without them, corals become energy-starved and more vulnerable to mortality.

The principal trigger is elevated sea surface temperature, especially during marine heatwaves associated with climate change and El Niño. Other stressors can increase susceptibility or exacerbate bleaching, including excessive solar radiation, high irradiance, reduced salinity from heavy rainfall, water turbidity from sediment and nutrient pollution, and ocean acidification. Bleaching can be temporary if stress is brief, but prolonged stress raises the risk of death for coral colonies.

During bleaching, corals expel or lose the zooxanthellae, and they may bleach white or pale. If conditions improve, corals can recover by reabsorbing symbionts and resuming energy production; shifts in the symbiont community may also occur to favor more heat-tolerant strains.

Mass bleaching affects reef ecosystems by reducing calcification and growth, lowering resistance to disease, and altering species composition. Consequences include loss of habitat for reef-associated organisms, decreased coastal protection, and potential economic impacts on tourism and fisheries. Recovery depends on stress duration, species, depth, and local conditions, with some reefs slowly regaining cover while others deteriorate.

Monitoring efforts combine satellite and in situ observations to detect and forecast bleaching events, guiding management actions aimed at reducing local stressors, protecting refugia, and supporting reef resilience.