Coral Bleaching

The seventh Status of Coral Reefs of the World report, released by the Global Coral Reef Monitoring Network (GCRMN), highlighted that coral bleaching events are becoming more frequent and that recovery periods between events have shortened dramatically.

What Is Coral Bleaching?

Coral bleaching occurs when corals experience environmental stress due to changes in ocean temperature, pollution, or ocean acidity. Under these conditions, corals lose the symbiotic algae responsible for providing much of their food and colour, causing them to appear pale or white.

Process of Coral Bleaching

Under stressful conditions, the zooxanthellae, or food-producing algae living inside coral polyps, can produce harmful reactive oxygen species (ROS).

In response, corals expel these algae from their tissues. This exposes the pale white calcium carbonate skeleton beneath the coral’s transparent tissues, giving the coral a bleached appearance.

The expulsion disrupts the symbiotic relationship between corals and zooxanthellae. As a result, corals lose a major source of food and become more vulnerable to disease and death if stressful conditions persist.

Major Causes of Coral Bleaching

Change in Ocean Temperature

The rise in ocean temperature due to climate change is the leading cause of mass coral bleaching. Even a prolonged increase of a few degrees above the usual seasonal temperature can cause significant stress to corals.

Runoff and Pollution

Stormwater runoff can carry pollutants, sediments and agricultural chemicals into coastal waters. Changes in water salinity caused by heavy rainfall can also stress nearshore corals and contribute to bleaching.

Overexposure to Sunlight

During periods of high ocean temperatures, intense solar radiation can increase stress on corals, particularly those living in shallow waters.

Extremely Low Tides

During exceptionally low tides, shallow-water corals may become exposed to air. This exposure can cause temperature and desiccation stress, leading to bleaching.

What Are Corals?

Corals are marine invertebrates belonging to the phylum Cnidaria. Individual corals are called polyps, which are small, soft-bodied organisms. Thousands of polyps may live together in a colony, and colonies grow as polyps reproduce and build calcium carbonate skeletons.

Symbiotic Relationship Between Corals and Zooxanthellae

Corals share a mutually beneficial relationship with single-celled algae known as zooxanthellae.

• Benefits to corals: Zooxanthellae use sunlight for photosynthesis and provide corals with food and nutrients, supporting their growth and survival.

• Benefits to algae: Corals provide the algae with shelter and access to nutrients.

• Colouration: Zooxanthellae contribute to the bright colours of many corals.

When this relationship breaks down during bleaching, corals lose a major source of energy and may struggle to survive.