Back in 1842, Charles Darwin (Mr Evolution himself) wondered about coral reefs. They grew in tropical waters that contained very few nutrients, and yet they flourished like an oasis in the desert. How could they do this? We now know that they form an incredibly efficient food web and ecosystem that extracts the maximum energy from the environment.
Coral reefs cover less than one-tenth of one per cent of the area of the world’s oceans — but they support more than 25 per cent of all creatures that live in the oceans.
They are the ‘rainforests’ of the seas. Unfortunately, coral reefs worldwide are in trouble. The Great Barrier Reef lost half of its coral cover in the 27 years between 1985 and 2012. If the trend continues, it will lose half of what is left by the year 2022.
The Great Barrier Reef stretches some 2600 km off the coast of Queensland, with over 2900 individual reefs and some 900 islands.
This enormous structure is made of, and was built by, trillions of tiny animals known as ‘coral polyps’. Each polyp is a cylindrical animal usually only a few millimetres in diameter and a few centimetres long. They sit on rock, or on the remains of previous coral, and excrete a carbonate exoskeleton at their base. At the other end they have a set of tentacles surrounding a central mouth. This is how they get their food.
But they cannot survive by themselves. They live in a very cosy mutually beneficial relationship with some microscopic algae known as ‘zooxanthella’ or more commonly as ‘Symbiodinium’.
The relationship is so close that the Symbiodinium actually live inside the flesh of the host coral polyp. The polyps give them carbon dioxide and ammonium, and in return the Symbiodinium give the polyps essential carbohydrates and nutrients so they can grow. Indeed, the Symbiodinium provide more than 95 per cent of the metabolic needs of the coral polyps inside which they live.
The Symbiodinium are really quite weird and unlike any other lifeform. They have 100 times more DNA than we humans have — and we have no idea why. They do photosynthesis, but by a completely different process from all other light harvesting organisms. They are also one of the sources of colour of coral, thanks to their inherent brown-yellow colouration.
When coral is under stress, for example due to abnormally high temperatures in the water, the polyps actually expel their friends (the Symbiodinium) from their body. The polyps lose the colouration — and this event is called ‘coral bleaching’. If they don’t take up some Symbiodinium within a few days, the happy marriage is doomed and the coral dies.
Worldwide coral bleaching began happening in the late 1970s when atmospheric carbon dioxide levels reached around 320 parts per million. Since then we have seen them become distressingly more frequent.
As I said, recent research has shown the Great Barrier Reef lost half of its coral, in a 27-year window. About 10 per cent of that was directly due to coral bleaching. About 48 per cent was caused by storm damage, and about 42 per cent was due to attack by the infamous crown of thorns starfish.
Certainly with global warming raising the temperature of both the atmosphere and the oceans, there’s more energy in those systems and we would expect more storm damage. And as the environment along the Great Barrier Reef has changed, the crown of thorns starfish has increased in numbers. They sit on top of the coral polyps, turn their stomach inside out and then liquefy and absorb the flesh of the coral.
According to Professor Ove Hoegh-Guldberg from the Centre for Marine Studies at the University of Queensland, if we don’t do something about it, then: “Reefs will become rapidly eroding rubble banks such as those seen in some inshore regions of the Great Barrier Reef, where dense populations of corals have vanished over the past 50 to 100 years.”
We need our coral reefs. On one hand they are a major source of protein for over one billion people in Asia, via the food chain. On the other hand, diving tourism in the Caribbean alone generates over $100 billion each year. In Australia, the income from international tourism to the Great Barrier Reef is around $7 billion each year.
Unfortunately, the coral polyp has exactly the wrong characteristics to deal with a rapidly changing environment. It has a very long life cycle, and often reproduces asexually by cloning so that the babies have exactly the same properties as the parents. In sexual reproduction, the babies are different from the parents and have a better chance of dealing with changes.
So if the coral polyps can’t change their ways in a hurry, maybe we have to — so that the Great Barrier Reef doesn’t become the Mediocre Barrier Reef.
Read article at ABC Science website