How do Gulf corals beat the heat?

The waters of the Gulf are hot. If the sea off Australia was as warm, the entire Great Barrier Reef would die.

A microscopic image shows a Porites coral from Abu Dhabi. Scientists believe its fluorescent pigments may play a role in protecting it from harsh temperatures.
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The waters of the Gulf are hot - up to 35°C in summer. If the sea off Australia was that warm, the entire Great Barrier Reef would die.

So how do the corals and reef fish in Abu Dhabi's coastal waters survive?

In geological terms, the emirate's modern coastline is extremely young - just 4,000 years old, according to Prof John Burt, a marine biologist at New York University Abu Dhabi.

In that time, somehow its corals have managed to beat the heat. The question is whether that has been a slow process, creating a stable local stock of heat-resistant corals, or whether each generation in turn finds its own way of coping.

As he puts it: "Have these corals, through natural selection over thousands of years, had enough exposure to those conditions that only the heartiest are surviving?"

Or could it be that as coral larvae float into this region and settle into their new surroundings, they somehow switch on a genetic protective mechanism to handle the heat?

To get some answers, Prof Burt has taken samples of hump coral (Porites lobata), a common reef-builder, from Abu Dhabi and sent them to a lab in Britain.

There, at the University of Southampton, Jorg Wiedenmann subjects them to a range of temperatures to see how well they stand up.

"Accurate predictions of the fate of coral reefs require a profound knowledge of the adaptation capacity of the main reef builders," said Prof Wiedenmann.

The aim is to work out the molecular basis of the corals' acclimatisation to hot water.

The coral live in symbiosis with zooxanthellae, a type of algae that lives inside the coral's tissue. The algae photosynthesises, producing sugars that provide up to 90 per cent of the coral's energy, and in return, the coral provides shelter, nutrients - mostly nitrogen and phosphorus - and carbon dioxide for photosynthesis. "It's a win-win situation," said Prof Burt.

Such a cosy arrangement is not without pitfalls. So co-dependent are the two species, that if one dies the other is unable to feed itself to stay alive, leading to coral bleaching.

At temperatures above 35C, the algae's photosynthesis goes into overdrive, producing oxygen radicals that damage the coral tissue. To protect itself, the coral essentially spits out the zooxanthellae.

Without the green of the algae, the coral's white calcite skeleton becomes visible through its jellylike tissue. The coral can live off its fat reserves for a week, but after that it needs to take the algae back in or it will die.

An extreme case of this bleaching was seen in 1998, when the El Niño weather phenomenon subjected 80 per cent of the world's coral reefs to extreme temperatures. In the Gulf, the water was hotter than 36°C for more than a month. "There was a huge mass mortality that year," says Prof Burt. "Many reefs have still not recovered."

And 2010 was hot, too, with water temperatures in the Gulf exceeding 37°C. Corals were lost on many reefs in the southern basin of the Gulf, the waters between Qatar and Dubai.

Those kind of temperatures could become the norm as climate change sets in. By 2100, the world's oceans are expected to be an average of 2° to 3°C warmer than now.

Not only that, the predicted rise in levels of atmospheric carbon dioxide would have its own corrosive effect.

Coral's white skeleton is made of calcium carbonate - the same substance as human bones - and is sensitive to changes in water chemistry. When carbon dioxide in the atmosphere dissolves in the sea, it forms carbonic acid, which eats away at the skeleton.

Experts fear these effects - polyp death and corrosion - will mean that by the end of this century reefs will be heavily degraded or even lost.

But somehow, so far, the Gulf's corals seem to be coping. Prof Wiedenmann believes it may be something to do with an unusual complement of protective mechanisms.

Such mechanisms could include fluorescent pigments fulfilling some kind of photoprotective function, enzymes that combat oxidative stress or enzymes that synthesise special compounds that help to endure the heat stress.

At his lab in Southampton, he is investigating green fluorescent proteins, using mass spectrometry to work out which compounds are present in the coral and algal cells. The work promises new insight into the functioning of the two partners.

One coral in particular - the table coral, acropora - has managed a particularly impressive recovery along the Abu Dhabi coast, after having been wiped out by the 1998 El Niño.

A little over a decade ago, there was no acropora off Abu Dhabi; now it is abundant on several of the 10 reefs studied by Prof Burt. So where can it have come from?

Corals reproduce in one of two ways: through fragmentation or larval production. Fragmentation, also called vegetative propagation, is when a piece of coral breaks off, rolls across the sand, lands somewhere else and starts growing.

For that, though, there must be an existing population of adult corals - tricky, since some species were all but wiped out in 1998.

That, says Prof Burt, leaves larval production. Coral larvae are produced in massive spawning events, when countless billions of tiny, 1mm-long larval bulbs are released.

They float around the sea for up to a week - a time known as the pelagic larval duration - until they develop the ability to attach themselves to a rocky surface. Once there, they begin secreting their skeleton, becoming the first building blocks of a new reef colony.

Sometimes they settle for a little while and decide they don't like their new neighbourhood. But because they are living off the fat reserves they had as an egg, the larvae only have a limited time to choose their home, which means they can only disperse so far from the reef on which they were born.

Working with an oceanographer from the marine biodiversity section of the Environment Agency-Abu Dhabi, Prof Burt has been developing detailed maps of coastal current patterns in the southern Gulf. They have led him to suspect seeding reefs must be upstream in the Gulf, either off the Saudi Arabian or Qatari coast, which would strengthen calls for cross-boundary environmental protection agreements.

The slow rates of recovery suggest that the problem is not larval production, but more the harsh environments the larvae find when they land. And with the water already so hot, the corals are at the thresholds of their tolerance, so even slight increases in stress can push them over the edge.

Prof Burt believes that the way to speed up the recovery might be to propagate the corals manually, collecting larvae during spawning events, settling them in artificial nurseries, and then planting those juveniles back out onto reefs.

The sooner that happens, he says, the better. "It's going to cost money to bring them back, and it needs to be done soon."

Otherwise, the benefits of the reefs for the Gulf ecosystem could be lost forever. Still, the recovery of the corals so far gives him cause for hope.

If the corals here can survive without bleaching at temperatures higher even than those predicted elsewhere, perhaps the world's reefs have a chance.