Mechanism of Coral Bleaching

Coral bleaching is the paling of reef-building corals after disruption of their symbiosis with microscopic photosynthetic algae, commonly called zooxanthellae, living within coral tissues. Abnormally warm seawater can cause the coral to lose these algae or their pigments, making the white calcium carbonate skeleton visible through the transparent tissue. Bleached corals remain alive initially but lose a major source of energy.

Normal coral-algal partnership

The coral provides the algae with shelter and raw materials for photosynthesis. In return, the algae transfer much of their photosynthetic energy to the host, support calcification and growth, and give corals much of their colour.

• This partnership enables reef-building corals to thrive in nutrient-poor tropical waters.

Thermal disruption and bleaching cascade

Sustained temperatures above the coral's usual thermal range impair the algae's photosynthetic machinery. Under strong light, damaged photosynthesis produces excess reactive oxygen species, creating oxidative stress in both the algae and the coral host.

• To limit cellular damage, the coral loses or expels the stressed algae and may also lose algal pigments.
• With fewer pigmented algae, the coral tissue becomes translucent and the underlying calcium carbonate skeleton appears white.
• Bleaching severity depends mainly on the intensity and duration of heat stress; high solar irradiance can aggravate it.

Recovery or mortality

If temperatures return to tolerable levels soon enough, surviving corals may regain compatible algae and recover. Prolonged or repeated bleaching reduces energy supply, growth, reproduction and disease resistance, and can ultimately cause coral death and reef degradation.

• Bleaching is therefore a stress response, not immediate death.
• Different coral species and algal partners have unequal thermal tolerance, so bleaching and recovery vary across a reef.