Marine spawning acropora coral on the Great Barrier Reef. Image Credit: Mike McCoy

A quick guide to help you understand coral bleaching

  • June 07, 2018

Here, Dr Karl explains the number one problem facing our Great Barrier Reef: coral bleaching caused by climate change.

Dr Karl Kruszelnicki
Contributor
Dr Karl Kruszelnicki

Dr Karl is a prolific broadcaster, author and Julius Sumner Miller fellow in the School of Physics at the University of Sydney. His latest book, Karl the Universe and Everything is published by Pan MacMillan. Follow him on Twitter at @DoctorKarl

CORAL REEFS ARE surprisingly important for people. Worldwide, they occupy only about 0.1 per cent of the surface area of the oceans. But they are the source of 25 per cent of the fish we eat.

Unfortunately, Australia's Great Barrier Reef (GBR) lost about 50 per cent of its coral between 1985 and 2012 - and is tragically on track to lose another 25 per cent by 2022. It suffered major coral bleachings In 1998, 2002, and 2016-2017 and the story is similar for coral reefs around the world.

So what is a coral? It's an animal without a respiratory or circulatory system. It has tentacles surrounding its mouth - which is also how it gets rid of wastes (yup, it eats through its bum!). It lives Inside a hard shell that it manufactures - and can survive only because its soft flesh gets invaded by single-celled photosynthetic algae.

We know the invaders collectively as zooxanthellae, but scientifically they are algae species belonging to the genus Symblodinium. There are about a million of them in each cubic centimetre of a coral animal's flesh.

The Symbiodinium and coral are locked together in a tight symbiotic relationship. The coral gives the algae carbon dioxide, ammonium, sulfur, phosphorus, nitrogen and more. The algae give the coral amino acids, glucose, gtycerol and a home. Indeed, the algae supply more than 90 per cent of the coral's metabolic needs.

Here's the weird part. When times are bad for the coral animal (such as when the local water around the reef gets too hot) it will expel the Symbiodi ream. This Is a bad long-term decision But some-times, in the short term, if (for example) the local heating doesn't last too long, the coral can limp through the bad time, survive, and then bring back in the Syrnbiodinium. The Symbiodirrium helps give the coral its colour. So when coral expels it It becomes much more pale In colour - hence the name 'bleaching.'

There are several threats to the GBR. The overwhelming majority are a direct result of human activity. Unless we do something, the Great Barrier Reef will become the Average, or even Mediocre Barrier Reef.

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