How corals lose their colour

by Shreya Yadav


Hot and polluted waters lead to a breakdown in the symbiotic relationship between the coral animal and algal plant, resulting in coral ‘bleaching’

The coral animal and the algae that live inside it evolved together for millions of years to form the reefs we see today. Even though they were nearly wiped out several times in their long history because of changes in the chemistry and biology of the Earth, they always managed to recover and re-build themselves, modifying themselves to get stronger. But the association between the algal symbiont and its coral host remains fragile, and is a relationship that can break down easily.

Warming waters

Since 1900, the rate at which the Earth heats up has been higher than what it had been 1000 years ago. This has been mainly because of people — the Earth has never seen this many cars, or so much pollution, or so little forest and natural land. The sea has also become warmer, and in the shallow waters where the world’s tropical coral reefs thrive, hot waters upset the process of photosynthesis being performed by the algae inside the coral.

Bleaching and adaptation

Normally, algae use sunlight and carbon dioxide to make food for the coral and get a safe place to live in exchange. Oxygen is also produced as part of photosynthesis, but the coral usually recycles it because it can be very toxic unless it is combined with other elements. But when sea temperatures rise over 30°C, too much oxygen begins to build up inside the coral tissue. After a point, it becomes so harmful that the coral is forced to expel all the algae inside it, thereby becoming ‘bleached’. A once-all-colourful reef turns a dull ghostly white.

Some corals have learnt to adapt to these events. They are able to exchange the algae that make them bleach with others that can tolerate warmer water. Sometimes, if the seawater cools down enough, bleached coral may regain algae that it expelled earlier and come alive again. But if the sea heats up too much, or too often, corals stop being able to cope. Just like you might get sunburnt if you stood in the sun for too long without any protection, corals too get really uncomfortable in the heat. But unlike you, corals can’t take shelter indoors!

Protecting reefs

Bleached reefs are unhealthy reefs, with corals in a sort of coma. They get diseases faster and become weaker, breaking more easily. Then if a big storm comes, an entire reef can crumble. And when a reef collapses, all the animals that live here — fish and crabs and sharks and rays — also lose their home.

230 million years ago, when coral reefs first appeared, there was no one around to see this spectacular world unfurling. Now, people are worried that they might be disappearing. Bits and pieces of reefs in the ocean are being protected, and they seem to stay much healthier when they’re away from human interference. But if we want to see coral reefs flourishing far into the future, we need to protect more of them, and make sure we’re not polluting or damaging the rest. After all, colourful things are much nicer to have around, don’t you think?

This article appeared in the Hindu in School on 5 November 2014.

Picture: A healthy coral.
Credit: Wikimedia Commons