The bleaching effect on the corals, a threat to the reefs

Why do coral reefs bleach?

In the tissues of marine corals live microscopic plants called zooxanthellae. These tiny plants are what provide colour to the coral, and through photosynthesis provide 90% of the energy that the coral requires to live. When corals are stressed they expel the zooxanthellae from their interior and this results in coral bleaching. Because the polyps, the animals that together form the coral, are colourless, the disappearance of the plants that live in their tissues causes coral bleaching.

What causes coral bleaching? 

This coral bleaching effect is also known as "bleaching" and does not mean that the coral has died. In order to survive, the coral needs the presence of the micro-plants that live inside it, which is why, if the conditions that have caused coral bleaching do not change and the zooxanthellae do not return to the coral tissues, the coral will die after a certain period of time.

Corals are very sensitive to temperature, living in water temperatures between 18ºC and 30ºC. An increase in ocean temperature of as little as 1 degree Celsius above the maximum summer water temperature can trigger a coral bleaching episode. High temperatures cause zooxanthellae to become hypersensitised to light (photoinhibition) and leave the coral tissues, causing this loss of colour and leading to bleaching.

The continued increase in carbon dioxide emissions, along with other greenhouse gases, has caused tropical sea temperatures to rise by 1ºC over the last 100 years. This increase in ocean water temperature is accelerating and could increase by up to 2ºC in the next century. 

Massive coral bleaching episode in Papua New Guinea. Credits: Jurgen Freund/NPL/Minden Pictures.

Another factor that causes corals to become stressed and consequently bleach is the pollution of the seas with chemical substances that cause an increase in the acidity of the oceans. The main causes are industrial activities that discharge their waste water without being previously filtered, the use of chemical fertilisers used in the agricultural sector that after filtering into the ground end up in the seas, and finally the use of sun creams whose components are harmful to coral reefs as well as other marine species such as dolphins or cetaceans.

Would coral bleaching episodes lead to their extinction?

Reef bleaching is a phenomenon that has been observed since 1979. Over the last 20 years, coral bleaching episodes have become more frequent and intense. As the measures taken to combat climate change have so far been insufficient, coral reefs in the Caribbean and Southeast Asia are already beginning to suffer from annual mass bleaching events. The fact that bleaching episodes are becoming more and more frequent prevents the coral from recovering, condemning to death those reefs exposed to continuous stress.

So far, there has been no sign that the coral is able to acclimatise to the new ocean temperatures, or at least its ability to adapt is not fast enough compared to the increase in sea surface temperature.

We are the last generation capable of taking action to prevent the extinction of coral reefs in the near future. It is estimated that, if drastic measures are not taken, between 75% and 90% of corals could be dead by 2050. This would seriously threaten the survival of thousands of marine species. 

If you want to contribute and save the reefs you can ADOPT a coral or as many as you want so that our team of biologists can recover the most damaged areas.