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Just down the road from the world’s tallest tower, in the shadow of monster sand dunes, marine biologists from around the world clamored onboard a boat for a visit to some of the Persian Gulf’s coral reefs.
The waters off the United Arab Emirates (U.A.E.) coast can be murky and only have 10 percent of the coral reef diversity found in the Indian Ocean or on the Great Barrier Reef. But the researchers came looking for something even more precious: clues that could one day help coral reefs around the world survive the onslaught of global warming.
Most coral reefs in temperate climates can withstand temperatures only as high as 29 degrees Celsius before they bleach—a process in which corals expel their symbiotic algae living in their tissue, causing them to turn white and increasing their vulnerability to disease and death. Corals in the Persian Gulf reefs, however, typically tolerate temperatures as high as 36 degrees C during summer and as low as 13 degrees C in winter. “These temperatures exceed what we are expecting anywhere else in the world in the tropics over the next century,” says John Burt, a marine biologist at a branch of New York University in the Emirates, who led the reef tour in 2012. Burt also helped organize an international conference in February on Persian Gulf reefs at NYU, which brought together 250 scientists. The Persian Gulf corals “offer hope,” he says. Some of the genetic mechanisms they use could help others survive these extreme temperatures.
Coral reefs have long suffered due to widespread pollution, overfishing and coastal development, losing up to 50 percent of their cover in many places across the globe. Yet most scientists say climate change poses the biggest threat in the future. Warming waters and the related ocean acidification spark more and larger bleaching events, making it harder for corals to calcify and therefore grow and thrive. Just this month the National Oceanic and Atmospheric Administration warned there could be a global bleaching event in 2015 due to warming waters—the third in the past two decades.
By 2040, scientists believe most coral reefs will be under threat. The conventional thinking has been that genetic changes in corals and their zooxanthellae—the single-celled algae also known as symbionts that live inside the coral tissues—will simply be too slow to keep pace with the rate of warming, which by some accounts could range from 2 degrees F to 11.5 degrees F (1.1 to 6.4 degrees Celsius) by 2100.
Scientists in the Gulf and elsewhere are challenging this idea, however, showing that coral reefs can bounce back from bleaching events faster than believed and may be able to acquire heat-tolerant symbionts in a relatively short time. “It’s not all doom and gloom for corals,” says Andrew Baker, a marine biologist at the University of Miami. His 2004 Nature paper was one of the first to report that the heat-tolerant symbionts known as clade D were prevalent in the Gulf but were also found at much lower numbers in temperate locales hit by climate change, such as Kenya, Mauritius and Panama. Clades are genetic groupings of the various symbionts. Baker and others have spent the last decade or so identifying the mechanism by which corals can deal with warmer temperatures. The corals “have a repertoire of responses,” he says. “The question has been, well, how quickly can those responses arise? I think we are now getting to this idea that actually, in some cases, these mechanism can arise very quickly, within a few years.” (Scientific American is part of Nature Publishing Group.)
Building on Baker’s work, Burt, Jörg Wiedenmann a professor of biological oceanography at the University of Southampton, and several other colleagues have for the past three years been surveying reef sites off the coasts of the U.A.E. and Oman, through the Strait of Hormuz and into the Arabian Sea to develop an understanding of the coral community and the zooxanthellae they host. The researchers have used genetic analysis to suggest that the thermal tolerance in the brain coral Platygyra daedalea and its zooxanthellae appear to be unique to the Persian Gulf. In a paper last week in Nature Scientific Reports, Wiedenmann, Burt and several other researchers described how they used four genetic makers to determine the algae was indeed new to science. They named it Symbiodinium thermophilum for its ability to withstand the unusually high temperatures of the Persian Gulf.
This new organism, which the researchers demonstrated was the most prevalent symbiont found throughout the year in the Persian Gulf, may have acclimated to play a new role in the Gulf’s harsh conditions. The question, Wiedemann says, is whether Symbiodinium thermophilum evolved its heat-tolerant traits in the relatively isolated gulf waters, which are only about 15,000 years old or was brought in by currents from outside the region and survived “a selection process.” Wiedenmann says, “If they haven’t evolved in the gulf, they must be present in other populations in low numbers elsewhere in the world. This would be good news because that would mean corals elsewhere might have these tolerant individuals among them.”
If symbionts exist elsewhere, then corals faced with bleaching could in theory switch them on as temperatures rise. To test this theory Baker repeatedly bleached the great star (Montastraea cavernosa) corals in his Florida lab and allowed them to recover. In a paper published last year in Global Change Biology Baker and his fellow researchers found that the corals, before the bleaching, contained symbionts that were not tolerant to heat. But afterward they were dominated by the heat-tolerant clade D1a. These retooled corals could handle temperatures 2 degrees C higher than before. “That 2 degrees is certainly good news, in a sense that it will compensate or could compensate for the some of the warming we are expecting to see this century,” Baker says. “But I think on the downside, we are predicting more than 2 degrees for many of these reef environments. So it is still an open question whether corals can continue to deal with increasing temperatures or whether this is a kind of a stopgap measure.”
Critics say it is naive to think that any one component of a diverse and complex coral community will provide a silver bullet that saves them. And because the gulf’s diversity is so limited and its seasonality so extreme, some experts think scientists would be better off looking for answers in more temperate regions such as the Mozambique Channel between Tanzania and Madagascar, which have proven resilient to extreme bleaching events. Oceans across much of the world do not look like the Arabian Sea, explained Tim McClanahan, a senior conservation zoologist with the Wildlife Conservation Society who works on reefs in the western Indian Ocean. “Most of the tropics have different oceanography and different meteorological or climatological environments,” he says. The genetic work on heat-tolerant symbionts is interesting “but it’s more of a curiosity rather than an analogue for the future. I don’t think the future will look like the Arabian Sea,” he adds.
Ove Hoegh-Guldberg, one of the world’s top reef experts and the director of the Global Change Institute at the University of Queensland in Australia, says the Persian Gulf work is intriguing because it shows “evolution can produce corals that are adapted to high temperature extremes.” But he notes it was “misplaced” to hope that the thermal tolerance would offer hope for reefs in temperate climates. Evolution is too slow when it comes to “long-lived organisms such as corals.” In an e-mail interview Hoegh-Guldberg wrote, “Other projections such as the swapping of their symbionts for more thermally tolerant varieties have not been borne out by studies.” He added that projections are flawed by the fact that it's not just the symbionts that need to evolve. “It’s the combination of the host and symbiont that needs to adapt rapidly to changing sea temperatures,” he wrote. Such doubts have done little to dampen the enthusiasm for using the Persian Gulf reefs or the genes found there in future conservation strategies.
Bernhard Riegl, a professor at the Halmos College of Natural Sciences and Oceanography of Nova Southeastern University in Florida who has worked on coral reefs in the Persian Gulf for 20 years, has called for moving corals to the Indo-Pacific to “introduce heat-adaptation, via hybridization with resident genetic material, into regions where it soon will be needed.” Others have talked of crossbreeding gulf corals with those in temperate climates. Baker wants to consider inoculating corals in nurseries with heat-tolerant symbionts and then reseeding areas at risk.
No one doubts there are inherent risks with these strategies—they are costly and could introduce invasive species and disease. And the reintroduced corals might not even cope with new environmental conditions. But Madeleine van Oppen at the Australian Institute of Marine Science, along with several other experts, argued in the February 2 Proceedings of the National Academy of Sciences that the time has come to consider the feasibility of human-assisted evolution measures such as selective breeding to enhance stress tolerance or crossbreeding different populations for restoration. “When I first started talking about it, people would say, ‘Oh God, that will never work. You can never do it at the scale required,’” van Oppen wrote. “People still have those concerns, but they can see that it’s important to develop the tools and to assess what is actually possible.”