Keeping it Great

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 (see page 56). We know the invaders collectively as zooxanthellae, but scientifically they are algae species belonging to the genus Symbiodinium. 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, glycerol 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 Symbiodinium. This is a bad long-term decision. But sometimes, 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 Symbiodinium. The Symbiodinium 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.