Coral Bleaching

1 min read
Coral Bleaching Blog Image


Scientists working on Australia’s Great Barrier Reef have successfully trialled a new method for freezing and storing coral larvae they say could eventually help rewild reefs threatened by climate change. The Great Barrier Reef has suffered four bleaching events in the last seven years, including the first ever bleach during a La Nina phenomenon, which typically brings cooler temperatures.

What are Coral reefs?

  • Corals are marine invertebrates or animals not possessing a spine.
  • Each coral is called a polyp and thousands of such polyps live together to form a colony, which grows when polyps multiply to make copies of themselves.
  • Corals share a symbiotic relationship with single-celled algae called zooxanthellae.
  • The algae provides the coral with food and nutrients, which they make through photosynthesis, using the sun’s light.
  • In turn, the corals give the algae a home and key nutrients. The zooxanthellae also give corals their bright colour.
  • Australia’s Great Barrier Reef is the world’s largest reef system stretching across 2,300 km.
    • It hosts 400 different types of coral, gives shelter to 1,500 species of fish and 4,000 types of mollusc.

Coral Bleaching:

  • Coral Bleaching happens when corals experience stress in their environment due to changes in temperature, pollution or high levels of ocean acidity.
  • Under stressed conditions, the zooxanthellae or food-producing algae living inside coral polyps start producing reactive oxygen species, which are not beneficial to the corals.
  • So, the corals expel the colour-giving zooxanthellae from their polyps, which exposes their pale white exoskeleton, giving the corals a bleached appearance.
  • This also ends the symbiotic relationship that helps the corals to survive and grow.

What Causes Coral Bleaching?

  • Change in Ocean Temperature: Increased Ocean temperature caused by climate change is the leading cause of coral bleaching.
  • Runoff and Pollution: Storm generated precipitation can rapidly dilute ocean water and runoff can carry pollutants, which can bleach near shore corals.
  • Overexposure to sunlight: When temperatures are high, high solar irradiance contributes to bleaching in shallow water corals.
  • Extremely low tides: Exposure to the air during extremely low tides can cause bleaching in shallow corals.


Q1) What are the effects of coral bleaching?

Bleached corals are likely to have reduced growth rates, decreased reproductive capacity, increased susceptibility to diseases and elevated mortality rates. Changes in coral community composition can occur when more susceptible species are killed by bleaching events.

Source: Scientists freeze Great Barrier Reef coral in world-first trial