New science could save dying coral reefs in the Great Barrier Reef and beyond

Thirty miles off the coast of Queensland, Australia, a little piece of history was written last summer: Scientists have transplanted hundreds of coral fragments grown in nurseries in the Great Barrier Reef, in the throes of badaults.

The process itself is not new: coral transplants have been used to help restore damaged reefs for decades. What's new is that it's happening on the world's largest reef, an icon of marine life that has been dubbed one of the seven wonders of nature.

The Australian authority that manages the Great Barrier Reef has always resisted the intervention of reef ecology, preferring to let it regenerate naturally. But the deadly reality of climate change requires a more concrete approach.

A report published November 28 by the US National Academies of Science, Engineering and Medicine (NAS) reaches the same conclusion: human intervention is needed to ensure the sustainability of the planet's coral reefs, which have an invaluable value for -being, national economies and wonders of the future. "

"The coral report is a pragmatic list of tools to help reefs survive," said Stephen Palumbi, Stanford University biologist, who chaired the NAS committee (and is also a member of the executive committee of the committee). the National Geographic Society). "A bit like what always happens when the panic of a crisis subsides and you have to find solutions."

On the Great Barrier Reef, this process has begun.

In 2016 and 2017, the Great Barrier Reef experienced consecutive "sea heat waves" – periods of high sea temperatures that killed nearly one-third of all coral reefs.

For reasons that are not yet entirely clear, coral polyps respond to high heat by expelling the symbiotic photosynthetic algae that feed them; the loss of colored algae "whitens" the corals and can eventually lead to their death. The coral cover in the northern part of the Great Barrier Reef, which stretches 2,300 km, roughly the length of Florida's shoreline, is now at its lowest level ever recorded.

Parts of Opean Reef, a popular dive site and one of more than 2,900 individual coral reefs in the Great Barrier Reef system, suffered catastrophic mortality during the recent bleaching. It's here, late August, that the coral transplant took place.

Marine biologist David Suggett, who heads the Future Reefs program at the University of Technology Sydney, collaborated with a team of researchers and a local reef monitoring company to collect coral fragments that survived bleaching and cultivate them on sandy lagoon adjacent to the reef. Twelve species have been selected, covering a range of coral forms ranging from branches to plaques to globules.

After a few months of growth and stabilization, these fragments were planted with the help of a new type of forceps allowing quick and easy fixation to the reef matrix.

According to Suggett, the research seeks to answer the question of whether the spread and establishment of stress-tolerant corals can accelerate the recovery of reefs, rather than relying on the slower natural process of coral reproduction for coral reefs. replace deceased individuals.

"The success of the project will only be known when we have a new wave of marine heat," Suggett said. It's probably as soon as possible.

Large-scale coral bleaching was conducted every 27 years, notes the independent Australian climate communication organization, the Climate Council, in a report on the reef released in July. The current rate is once every six years. According to the report, if climate change is not halted, by 2030, the Great Barrier Reef could undergo mbad coral bleaching every two years.

According to the report of the National Academies, by 2050, most of the world's reefs will be exposed to bleaching conditions each year.

Corals can recover from bleaching, but not at this frequency. Hence the search for ways to increase the abundance of corals, such as the transplant technique that Suggett is testing. Another Australian team is currently testing a different approach: it sows damaged patches of the Great Barrier Reef with more than one million larvae of laboratory-raised corals.

"Recovery is the key to having reefs in the future," says Suggett.

But resilience is too. The reefs of our day do not only suffer the hammer blows of a catastrophic bleaching. They are also subject to slow compression, similar to a vise, because our carbon emissions regularly increase the background temperature and the acidity of the water surrounding them to levels never reached by most corals.

Around the world, we are looking for corals that to have given such conditions – resilience hotspots where corals have already adapted to the extremes of heat and acidity that may reign over most reefs in the next century. The idea is that these corals – or some of their essential genes, or the symbiotic algae that feed them – can be transplanted into more vulnerable reefs, thus increasing their chances of survival.

In some volcanic sources and submarine springs, for example, where CO2 bubbles naturally from the sea floor, corals form viable calcium carbonate skeletons in water that is acidic enough to be deadly to corals elsewhere. In American Samoa and Palau, Palumbi of Stanford and his colleagues have identified shallow water corals with exceptional heat tolerance. They also identified some of the genes responsible.

For sustained heat extremes, few marine environments correspond to the Persian Gulf, where summer temperatures on the sea surface reach temperatures above 35 ° C (95 ° F). However, more than 55 species of coral live there, with bleaching thresholds several degrees higher than those of most corals. Some of them contain heat resistant symbiotic algae that, if they could be introduced into other corals, could increase their whitening resistance.

There is however a compromise to be taken into account. Research has shown that a thermally tolerant symbiotic algae, while reducing bleaching mortality by 30%, also reduces coral growth rates by more than 50%. Corals on a vulnerable reef that has received an alga such as a transplant may be more likely to survive a bleaching episode – but they would contribute less to the recovery of the reef or its diversity.

Suggett's team is looking for resilient corals in a different extreme environment: near mangroves. 20 km from Opal Reef, off Port Douglas, are the Low Isles, a coral platform with two small islands: one is a sand cay popular with snorkellers and the other is a mangrove swamp.

The shallow, sheltered waters in which mangroves grow are generally warmer than those running over an open reef, and the trees make them more acidic. Yet corals thrive here both in mangroves and off the coast.

Suggett and his team are studying mangrove corals to determine what physiological and behavioral adaptations allow them to survive. They transplanted mangrove corals close to the reef further offshore – and vice versa.

Most coral polyps feed primarily on the photosynthesis of symbiotic algae in their tissues. The polyps usually remain retracted within their skeletons during the day, and only come out at night to supplement their diet using their tentacles to catch plankton and other organic particles in the water. .

"At night, there is more plankton in the water and less risk from visual predators, so it makes sense for the polyps to feed," says Suggett. "And of course, photosynthesis does not happen at night."

In contrast, scientists often see polyps of mangrove corals extend during the day. The metabolic demand of living in this hostile environment can lead to an increase in food activity. "One can badume that the benefits of increased energy intake outweigh the risk of visibility for predators," says Suggett. As the environment on open reefs hardens, active feeding may become a more necessary option for corals.

Climatic pressures intensify and delays are short. Coral reefs are facing not only rising heat and acidity, but also lower oxygen levels, more intense storms, and predators such as the famous star coronet. thorny sea, which remains a threat to the Great Barrier Reef.

Nevertheless, Suggett describes himself as a "pragmatic optimist" of the future. The big map, he pointed out, is the extent to which corals themselves are able to adapt to the changes they are facing.

"Wherever I go in the world, I see corals survive where we do not expect them," says Suggett. "It gives me hope that there are coral communities that can handle the stress of the reefs. Corals may have received less credit than they deserve in terms of their ability to tolerate and adapt to stress. "

One of the surprises at Low Isles, he says, is what happened when his team transplanted offshore coral reefs into mangrove lagoons, that is, relatively harmless conditions to hot areas and acids. "We expected these corals to die," says Suggett. "But after four months in the mangroves, they all did very well."

This is consistent with an observation from the Palumbi team. Stanford researchers have found that the heat tolerance genes identified in American Samoa's corals are also present in cold-water corals of the Cook Islands, 300 km to the southeast. Researcher Rachael Bray, who is now working at the University of California at Davis, has discovered that these genes are rare in the Cook Islands today, but that they could spread as the waters warm up.

The researchers calculated that, according to the researchers, the problem of carbon propagation probably would not occur fast enough to ensure the survival of the reef. A slower rate of emission would help – but transplanting some heat-tolerant corals from warmer climates could also speed up the process.

"What we're trying to do with this work is to understand what would happen in a situation where we had to rely on human intervention to keep the reefs alive," says Suggett. "That's not what we want, of course. Plan A is about reducing emissions, fighting climate change and eliminating threats to the reefs. But we must prepare for the eventuality of a F plan, which is attacking the melting reefs in the world.

"Everyone is wary of the intervention, and rightly so, because the greater the extent of reef restoration, the greater the ecological impact. Let's hope we do not need to go there, but understand the science in case we do. "