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FIRST the bad news. The planet’s coral reefs are up against it. They face a barrage of threats: burial by sediment eroding from deforested lands, strangulation by algae thriving on fertiliser runoff, overfishing of species vital for reef health, damage by anchors, toxic pollution – the list goes on and on.

As if that wasn’t bad enough, soaring levels of carbon dioxide are making seawater more acidic, which will make it harder and harder for coral polyps to build their rigid skeletons.

Worst of all, when seawater gets abnormally hot, the brightly coloured symbiotic algae that live within tropical corals and produce most of their food disappear, leaving their hosts vulnerable to starvation and disease. With global warming kicking in, such “bleaching” episodes are becoming more and more common.

As waters warm, some tropical coral species are at least expanding their ranges northwards and southwards. However, the amount of suitable habitat, for instance areas with clear water, is limited.

Catastrophic

Overall, the long-term outlook for corals is nothing less than . If CO₂ levels and temperatures continue to rise, half of all coral-associated species could become rare or extinct. And it’s not just the incredible diversity of life that will vanish. As reef growth slows, erosion will start to outstrip reef building, and existing reefs will crumble away.

For those who view reefs as merely beautiful scenery, this may not sound like much to fuss over. They are far more than that, though. Hundreds of millions of people depend on reefs for protection from storms and tsunamis, as well as for their livelihoods, through fishing or tourism.

“I want to save coral reefs not just because they’re pretty,” says Simon Donner, an ecologist and climate modeller at the University of British Columbia in Vancouver, Canada. “I want to save them because people need them.”

Mass extinction

Coral reefs have survived dramatic climate change in the past, but there have also been five occasions when no reefs were built anywhere on Earth for millions of years. With the oceans warming and becoming more acidic – and doing so at an unprecedented speed – some think we’re heading towards a sixth mass extinction.

The good news? While the reefs are suffering, they are not dead yet – there is still a chance to save them. It will be several decades before they pass the point of no return, giving us a window of opportunity (see “50 years to save the reefs”).

If we can help some reefs survive this period intact, many species will survive and could eventually recolonise other areas. To achieve this, biologists are exploring some outlandish ideas, including installing mega-cooling systems for reefs and engaging in “gardening” on a massive scale. Could such extraordinary ideas work?

Cooling system

Yes, in the case of reef cooling, claims inventor Phil Kithil, founder of in Santa Fe, New Mexico. He has developed a simple pumping system for bringing cool, deep water up to the surface.

“Pumps could cool reefs by bringing up deep water, using only wave power”

Kithil originally designed the pumps with the idea of by deploying thousands of them to cool surface waters. He later considered the possibility of using the pumps to bring up nutrient-rich water to fertilise plankton growth, leading to increased drawdown of atmospheric CO₂. While those applications are still being explored, more recently, in collaboration with the Oregon-based Climate Foundation, he began looking at reefs.

Each pump is a large plastic tube at least 100 metres long, held in place by a weight at the bottom and a large buoy at the top. Pumping is powered by wave action: as the buoy drops down into the trough of a wave, the tube also drops, pushing water through a valve at the bottom of the tube. When the next wave lifts the buoy, the valve closes.

Feasible

As bleaching is triggered by temperature increases of just 1 or 2 °C and deeper water can be more than 20 °C cooler, only a relatively small amount of water would have to be pulled up to keep the reef healthy. “When you do the math, it’s feasible,” says Mark Eakin, head of the US National Oceanic and Atmospheric Administration’s Coral Reef Watch, who has taken an interest in the technology.

One challenge would be ensuring the cooler water actually reaches corals, as in many locations the pumps would have to be placed in deep water some distance from the reefs. Another issue is that deeper waters tend to have more nutrients, which could be just as bad for reefs as fertiliser run-off. However, the pumps can be deactivated when not needed, which should minimise the problem. “It’s kind of like chemotherapy: you only do it long enough to get rid of the cancer, and that’s what we’re talking about here,” says Eakin.

Then there is the cost. With a single pump costing around $10,000, large-scale deployments would not come cheap. “It’s a bit of a kooky idea,” says William Skirving, also with Coral Reef Watch, “but only because of scale. The basic notion is not so dumb and possibly even doable.”

Huge bonuses

The scale issue is one that plagues all ideas of physically manipulating reefs, as they cover such extensive areas. There is no doubt that, given billions, huge protective projects could be accomplished, but this kind of funding is unlikely to be forthcoming.

If local communities that depend on reefs can be persuaded to invest in measures to save them, however, much could still be achieved. An island that succeeds in protecting its reefs during major bleaching incidents could reap huge bonuses in tourism, for instance.

Beyond reef cooling, most other ideas for protecting reefs focus on corals’ symbiotic tenants, the zooxanthellae. These species of single-celled algae can live freely, but prefer to pay for secure accommodation inside the coral by providing food.

Symbiotic relationship

The relationship is thought to begin when a coral captures an algal cell. In some cases, the digestive process is halted and the algae continue to live inside the coral polyp’s cells (pictured below). Only a few species are saved, but no one knows how corals select their symbionts. “It’s one of the black boxes of the field,” says Mark Warner at the University of Delaware in Lewes.

Any food the algae produce that is not quickly used, the coral takes as rent. This arrangement works fine in normal circumstances, but it leaves the algae with little to spare. As temperatures rise, the algae photosynthesise faster, producing more food but also suffering greater stress and damage. Most plants spend the night repairing this damage using energy stored during the day, but zooxanthellae are robbed of that luxury by their landlords. So they can only perform repairs in the morning and evening, when their photosynthetic machinery does not have to run at full blast.

Coral bleaching occurs when the damage from photosynthesising outstrips an alga’s repair rate, leading to deterioration. The process is not well understood, but algal cells may give off toxic chemicals that cause their coral hosts to either eject or digest them. When enough algae are lost, the corals turn a stark white and can die if the algae are not replaced.

Ocean acidification

A more recently appreciated problem only magnifies the threat. The oceans are helping to stave off global warming by soaking up much of the excess CO₂ we are producing. However, dissolved CO₂ forms a weak acid that converts carbonate ions, which corals use to build their calcium carbonate skeletons, into bicarbonate ions, which they cannot use. Extra CO₂ causes ocean acidification, making it harder and harder for corals to build reefs.

Bleaching can also block reef building, even when it stops short of killing corals. After losing algae, corals have to rely solely on their energy reserves or on the plankton they catch with their tentacles. by Andrea Grottoli at Ohio State University in Columbus has shown that while some corals do not get enough food this way, others can increase their feeding rate fivefold after bleaching, providing more than enough to meet their daily energy requirements.

It had been assumed that when bleached corals can catch enough food, they continue to grow and build their calcium carbonate skeletons, maintaining reefs. Unpublished work by Grottoli, though, has revealed a surprising catch. In the coral species she has studied, the carbon for building their skeletons apparently has to come from zooxanthellae. “It’s a complete revision of coral physiology,” she says. “We always thought carbon was carbon, but it’s not.”

Maintaining good relations

So if we want to maintain protective reefs we need to ensure that the coral-zooxanthellae relationship does not break down. One possible ray of hope comes from the Persian Gulf, where corals thrive even at temperatures higher than those predicted for most other reefs by 2100 (although massive bleaching is occurring even here as temperatures rise yet higher). While the reasons for this unusual tolerance are not yet clear, one factor may be the species of algae the coral choose as their symbionts, which belong to a group known as clade D.

Andrew Baker of the University of Miami in Florida is studying whether such heat tolerance could be transferred to other reefs. He and his colleagues many corals host clade D species, even when other species dominate, and that in some cases bleaching can lead to clade D species gaining a competitive advantage and taking over.

Baker is now exploring whether corals can be induced to switch to clade D species, perhaps through seeding with cultivated algae. If so, this could become part of a wider coral-rescue strategy. Many groups are already raising corals to place on damaged reefs, and reintroducing species to areas where they are no longer found. Seeding such corals with clade D algae before reintroduction might reduce losses during bleaching events.

Wild idea

“I think it’s a wild idea with a very low chance of success, and that’s putting it politely,” says Ove Hoegh-Guldberg, a coral researcher at the University of Queensland in Brisbane, Australia. It is not even clear if corals are capable of switching to new algal symbiont species, he adds.

On this point, though, he and Baker actually agree. Baker only recently decided the seeding scheme was worth trying, as he believes the situation facing coral is so dire. “I think it would be a shame if we opted not to attempt this because we intellectually argued ourselves into a corner,” he says. “It’s definitely a high-risk experiment, but if there’s a small chance of it working I’ll take that chance.”

Even if seeding is successful, there could be trade-offs, as with most things in life. Preliminary work by Grottoli’s group suggests clade D symbionts do not photosynthesise as well as others, so seeding might slow growth.

Global gardening

Hoegh-Guldberg does see promise in a different method of capitalising on natural heat tolerance: assisted colonisation. The idea is to move corals with the greatest resilience to bleaching into areas with less resilient corals. “Because of the extreme conditions coming up, humans might want to start gardening at a grand level in hopes of minimising losses,” he says.

“Humans might want to start coral gardening at a grand level”

Another idea is to shade coral from the sun to prevent water temperatures rising too high. There is a natural precedent for this idea, as reefs shaded by Papua New Guinea’s mountains fared better than their fully lit neighbours during a recent bleaching event. To test whether this can work elsewhere, Hoegh-Guldberg has collaborated with tour operators to drag shade cloth over small sections of the Great Barrier Reef. The team has had encouraging results in small-scale experiments, and is planning to test water sprinklers that break up the surface as another way to shade reefs.

As with all the strategies, doing this on a massive scale would be extremely costly, but there may be ways to raise the money. At the Great Barrier Reef, for instance, tour companies are assigned specific reef sections, which gives them an incentive to invest in measures to keep their section healthy. If this management model were adopted by other countries, more of the billions raised from tourism might be spent protecting reefs.

Predicting bleaching

The challenge for researchers is to explore all these ideas, to perfect techniques and identify the drawbacks. It is becoming possible to predict bleaching events months in advance, giving reef managers time to intervene.

“People ask what is the solution, X or Y, but it’s X, Y and Z,” says Donner. “I think we should try every sort of adaptive mechanism and adaptive management technique we can come up with.”

Not everyone agrees that extraordinary measures such as pumping up cool water are even worth exploring. There is, however, near unanimous support for a dramatic expansion in protected marine areas to ensure a few reefs at least are as healthy as possible as bleaching events increase. “We have very strong evidence that recovery after something like bleaching is much, much faster when we treat reefs well,” says Hoegh-Guldberg.

Intelligent conservation

We should also be smart about picking areas to protect, says Tim McClanahan of the Wildlife Conservation Society in Mombasa, Kenya. There are reefs where ocean currents keep waters cooler, and reefs dominated by corals that are more tolerant of higher temperatures or better able to feed after bleaching. Sites that combine as many of these characteristics as possible should be selected for new marine protected areas, McClanahan says. “Let’s do this more intelligently.”

There is no doubt about the most crucial measure, though. “It will all go for naught if we don’t reduce greenhouse-gas emissions,” says Donner. “We are frittering away time. This all has to start now.”

Endangered species – Learn more about the conservation battle in our comprehensive special report.