UPSC Express

• Coral Bleaching
  • When coral polyps are too hot, they expel their zooxanthellae.
  • Since zooxanthellae are responsible for the colors of many corals, when coral polyps expel their zooxanthellae, the coral turn white, a phenomenon called coral bleaching.
  • The zooxanthellae return to the polyps when the water cools, but repeated bleaching events lead to the death of the coral polyps.

Result of new study

• Though warm waters as a result of global warming results in coral bleaching, it could be offset by diversity the algae renders to corals.

About coral reefs:

• Coral reefs are large underwater structures composed of the skeletons of coral, which are marine invertebrate animals.
• The coral species that build coral reefs are known as hermatypic or “hard” corals because they extract calcium carbonate from seawater to create a hard, durable exoskeleton that protects their soft, sac-like bodies.
• Each individual coral is referred to as a polyp.

Mutualism between micro-algae and coral

• A species interaction in which both species benefit from the interaction is called as mutualism.
• The relationship between corals and photosynthetic algae living inside their tissues is the best example of mutualism.
• Coral polyps can obtain most of their nutrition from algae, called zooxanthellae.
• Zooxanthellae live symbiotically within the polyps.
• These algae use the carbon dioxide produced by the polyps to conduct photosynthesis.
• In turn zooxanthellae are responsible for the colors and diversity of them to polyps.

Ideal conditions for Coral Reefs

• Sunlight
  • Corals need to grow in shallow water where sunlight can reach them.
  • Corals rarely develop in water deeper than 165 feet (50 meters).
• Clear water
  • Corals need clear water that lets sunlight through; they don’t thrive well when the water is opaque.
  • Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae.
• Warm water temperature:
  • Corals generally live in water temperatures of 20–32° C.
• Clean water
  • Corals are sensitive to pollution and sediments.
  • Sediment can create cloudy water and be deposited on corals, blocking out the sun and harming the polyps.
• Saltwater
  • Corals need saltwater to survive and require a certain balance in the ratio of salt to water.
  • This is why corals don’t live in areas where rivers drain fresh water into the ocean (“estuaries”).

Threats to Coral Reefs:

Coral reefs are being degraded by an accumulation of stresses arising from anthropogenic activities and changes in the natural environment.

• Over-fishing
  • Over-fishing of certain species on or adjacent to coral reefs can affect the reef’s ecological balance and biodiversity.
• Destructive fishing methods
  • Fishing with dynamite, cyanide and other damaging methods can damage entire reefs and is 100% unsustainable.
• Recreational activities
  • Physical damage to the coral reefs can occur through contact from careless swimmers, divers, and poorly placed boat anchors.
• Coastal development
  • Sensitive habitats can be destroyed or disturbed by the dredging of deep-water channels or marinas, and through the dumping of waste materials.
• Pollution
  • Coral reefs need clean water to thrive. From litter to waste oil, pollution is damaging reefs worldwide.
  • Pollution alters the natural flow of water, greater amounts of fresh water, nutrients and sediment can reach the reefs causing further degradation.
  • Nutrient-rich water causes phytoplankton to thrive in coastal areas, often causing algal blooms.
  • It also encourages the growth of algae, which compete with corals for space on the reef.
• Plastic pollution
  • 8 million tones of plastic rubbish enters the world’s oceans every single year.
  • Many discarded plastic is broken down into what is known as microplastics, tiny pieces that are mistaken by coral polyps as food and ingested.
  • After years of campaigning, many countries are now changing policy; banning plastic bags, straws and single-use plastic packaging.