The Vital Spark That
New Scientist, 06 July 2002
Far out in the Indian Ocean, on a rocky bank just beneath the waves, divers are growing a coral reef. Its skeleton - a framework of steel girders - sits on the seabed a few metres down. They swim around it, attaching small chunks of living coral to the girders with wire. Finally, one of them clips on an electrical cable that dangles from a raft floating above. Seconds later, tiny bubbles of hydrogen start to rise from the steel.
They're making "Biorock". On board the raft is an array of solar cells and as the current generated flows through the metal frame, it triggers a chemical reaction in the seawater that coats the steel with a form of limestone that corals can't resist. Eventually this metal structure should disappear beneath a multicoloured forest of coral.
The experiment began in March at Saya de Malha, a bank on a submerged plateau in the western Indian Ocean, called the Mascarene Ridge. It was organised by marine biologist Thomas Goreau and Wolf Hilbertz, an engineer and architect, who have been growing artificial reefs all over the world for more than a decade.
Goreau and Hilbertz aren't the first to create artificial reefs, but they believe their structures are the best yet. Corals seem to love them, they're cheap to build and grow stronger as they age. And at a time when many reefs - and the creatures they support - are struggling to survive, these coral "arks" could provide a lifeline for human communities that rely on the fish that reefs attract.
Goreau and Hilbertz even claim that corals growing on their artificial reefs are over 10 times as likely to survive stressful environmental changes as naturally formed coral. If they're right, these structures may be one of the few remaining hopes for saving much of the world's coral from extinction.
This has never been more important: reefs all over the planet are under massive assault. The Global Coral Reef Monitoring Network, based in Australia, estimates that more than a quarter of the world's reefs have died in the past few decades and that at least another quarter will perish within twenty years.
The assaults take several forms, including physical destruction caused by fishing and pollution by nutrients leached into the sea from farmland and cities. But global warming looks set to be the gravest threat of all.
Coral polyps feed on food particles in the water, and as they grow they deposit calcium carbonate around themselves to form a protective skeleton. The polyps also capture energy from sunlight, courtesy of photosynthetic algae called zooxanthellae embedded in their outer surfaces. It's the algae that give corals their bright colours. But they are sensitive to small rises in seawater temperature: an increase of just 1 °C somehow forces them out of the polyps, leaving the coral with a "bleached" appearance. Without the energy the algae provide, the coral often dies.
Mass bleaching is becoming a regular event. In 1998, for example, warm seawater killed most corals in the Indian Ocean. Then, in 2002, corals across the South Pacific died, with Australia's Great Barrier Reef hit especially hard. "There is little doubt that current rates of warming in tropical seas will lead to longer and more intense bleaching events," says Ove Hoegh-Guldberg of the Centre for Marine Studies at the University of Queensland, Australia, who chairs the UN Working Group on Coral Bleaching.
Now there's a new threat. As levels of carbon dioxide in the atmosphere rise, the concentration of the gas in seawater is rising too, and researchers believe that this may prevent polyps building their skeletons.
Could artificial reefs save the day? Not traditional designs, says Goreau. Since the 1950s artificial reefs have been built from refuse such as scrap metal and car tyres in the hope of increasing yields of fish and crustaceans. More recently, structures have been built from plastic, steel and concrete, but most remain relatively barren compared with natural reefs.
Biorock reefs, on the other hand, are rather more sophisticated. The concept dates back to the mid-1970s when Hilbertz, working at the University of Texas at Austin, tried using electrodes immersed in seawater to mimic the way seashells and reefs grow.
Electrolyse water and bubbles of hydrogen form at the cathode as electrons bind to hydrogen ions in the seawater. To compensate for the loss of these ions, carbonic acid (H2CO3) in the seawater around the cathode breaks down, releasing hydrogen ions and carbonate ions. Hilbertz noticed that when the carbonate ions reach a critical concentration, a form of limestone made primarily from calcium carbonate, called aragonite, is deposited on the cathode.
He also found that brucite a mineral made from magnesium hydroxide - forms at the cathode. This is softer than aragonite, so to build useful structures, he had to find ways to strengthen the material. Hilbertz discovered he could eliminate the brucite by adjusting the current and by periodically turning it off completely, giving the more soluble brucite a chance to dissolve. The result was a metal cathode coated with a material as strong as concrete. He also found that the best anode was a titanium mesh covered with a layer of ruthenium oxide, which is resistant to corrosion underwater.
Hilbertz reasoned that if you generated the electricity from renewable resources such as solar power, you could create low-cost limestone "reefs" of any shape and size. In experiments, he found he could grow the coatings at up to 5 centimetres a year. The structures would require little maintenance and would even heal themselves when damaged. And since the limestone resembles natural reefs, coral should colonise it rapidly.
However, some marine scientists worried that electrolysis might damage living coral. So Hilbertz began experiments to determine what the effects might be. He attached Elkhorn coral to two steel structures in seawater and passed a small electric current through one but not the other. "Nothing remarkable took place on the control," he says. "In contrast, corals thrived on the electrolysed structure."
To his surprise, the results were ignored. Then in 1988, Goreau, a Jamaican marine biologist researching coral growth in the Caribbean, read of Hilbertz's work and got in touch. Together they have spent over a decade working together with the Global Coral Reef Alliance (GCRA), an umbrella organisation dedicated to protecting coral reefs, to construct Biorock reefs around the world (see Map).
Typically, each structure is built from a dome-shaped steel frame up to 12 metres across, and costs about $1000 to construct. The largest so far, one of seven in the Maldives, is 40 metres long and 8 metres wide. And at Pemuteran, on the Indonesian island of Bali, they have installed 22 structures, with a total length of 220 metres.
The amount of electrical power required by each Biorock reef is low. A structure of 100 square metres, for example, requires less than 300 watts to grow. Most reefs built to date take their power from arrays of solar cells, either on shore or aboard a raft floating nearby, but Goreau is also considering using power generated by tidal turbines attached to the seabed.
Most importantly, the variety of creatures living on and around these structures after a few years seems to match that of natural reefs. "The corals [on the biorock reefs at Permuteran] are clearly doing well," says Steve Oakley, an environmental scientist at the Tropical Research and Conservation Centre based in Kuching in Malaysia. "There's good fish diversity, and certainly many more fish on that stretch of reef than others nearby."
In fact, the researchers believe that the corals on their reefs are more resistant to environmental pressures such as pollution and global warming than natural systems. Without needing to use as much of their own energy to create a calcareous skeleton, each coral polyp can put more energy into growth and reproduction, and this seems to make them better at withstanding stress. Goreau and Hilbertz estimate that coral growth on Biorock is 3 or 4 times as fast as on ordinary rock, and the ability of these corals to survive events that kill most corals on natural reefs is striking. "In most parts of the Maldives, for example, less than 5 per cent of corals survived the catastrophic bleaching of 1998," says Goreau. "On our Biorock structures, more than 80 per cent of corals not only survived, they flourished."
Spurred on by these results, Goreau and Hilbertz want to grow small coral gardens just tens or hundreds of square metres in size that could help protect coral reef systems from the threat of global warming. And they hope that a network of these "arks" could ultimately be used to recolonise large areas where coral has died. "Coral nurseries are the only way to keep corals from extinction," says Seferino Smith, operations manager at Sapibenega Kuna Lodge, an eco-resort owned by the Ukupseni community of Kuna Yala in Panama. Here, local groups are working with GCRA to build Biorock nurseries to support both fisheries and tourism. "We should multiply these nurseries all over the Kuna reserve and around the world," says Smith.
However, some questions remain. No one yet knows how rising levels of CO2 in seawater will affect the rate of growth of coral on Biorock. An artificial reef is also unlikely to exactly duplicate a natural one, Goreau admits. Small differences will alter the balance of ecological competitiveness between reef organisms, he says, so the final make-up of creatures may differ. And voracious predators - the coral-eating snail Drupella and the crown of thorns starfish Acanthaster, for example - remain a formidable challenge. In the past, these starfish have caused serious damage to Australia's Great Barrier Reef, for example.
Some experts have yet to be convinced that these reefs offer all the advantages that their creators claim. To prove that Hilbertz and Goreau's idea is solid, other reef scientists will have to see the hard data, says Hoegh-Guldberg. "I certainly don't want to dismiss it at this stage. It is a very interesting approach."
Fans of Biorock are the first to acknowledge that more research is needed. Just why the process increases the rate of coral growth and creates healthier animals isn't fully understood, says Cody Shwaiko, founder and head of the Komodo Foundation, an Indonesian conservation and community development charity. "[There needs to be] more work, experimenting with parameters in a controlled environment in order to better understand the process," she says. Goreau agrees, and the GCRA is working with Shwaiko to establish a research programme in Indonesia. Goreau and Hilbertz are also interested in using Biorock as the foundations for seawalls, breakwaters and jetties, estimating that it could be a tenth as expensive as equivalent concrete structures, making it ideal for use by developing nations threatened with rising sea levels.