A team of scientists looking for coral that can better survive global warming have identified a hardier Caribbean coral in the Bahamas.
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The mountainous star coral off Great Harbour caught researchers attention after a searing 2015 bleaching event hit reefs across the islands. As water temperatures on Mermaid Reef soared above 91 degrees, the coral survived. Just a dozen miles away, similar star coral in cooler water died.
The finding, published in the journal Coral Reefs, could help researchers trying to breed more heat resilient coral in labs as they race to save reefs that have lost about 80 percent of their coral since the 1970s.
“This reef is one place that's teaching us about what naturally resilient corals look like,” said co-author and Shedd Aquarium coral researcher Ross Cunning. “We can use corals like these to optimize restoration efforts like growing corals in nurseries...to produce a new generation of corals.”
The reef tract along Florida, the only inshore tract in the continental U.S., has been hit hard in recent years. A disease that appeared off Virginia Key in 2014 has now spread south to the Keys and deep into the Caribbean, reaching as far south as Belize and east as the Dutch Antilles. Researchers suspect the disease may be spread by ballast water, but temperatures may also play a role.
The stony coral disease also coincided with back-to-back bleaching events and the largest and longest bleaching event on record that slammed reefs around the planet.
Coral scientists have been trying to buy time for the reefs by replanting fast-growing staghorn coral grown in nurseries. But a study this month that looked at replanted Keys coral found that although the coral did well initially, they eventually succumbed to the same stresses that killed wild staghorn. Fewer than 10 percent of the corals survived beyond seven years.
That puts more urgency on efforts to raise more heat-resistant coral like the kind being researched at the University of Miami, where researchers at the Rosenstiel School of Marine and Atmospheric Science have been focusing on the algae that live inside the coral. In healthy coral, the algae photosynthesize to provide them with food. When temperatures get too high, the algae can instead start producing toxins so coral expel them.
In the Bahamas, star coral live with just four different kinds of algae that can provide some heat tolerance.
“Corals are an animal and they have this really great partnership with micro-algae,” said Katie Parker, the study’s lead author.
The team found that the star coral on Mermaid Reef not only lived with a specific algae, but an algae that belonged to the same family. They also found the coral themselves were genetically identical to each other, Parker said.
“An easy way to kind of explain this is if you relate it to humans. We're all the same species. We're all humans. But some of us are tall and some of us are short. And that's because of a specific gene that we have,” she said. “It can be the same way with corals, where they're all the same species of corals. But some are more thermally tolerant and some are not because of the genes that they have.”
That combination led them to tolerate the higher temperatures while the nearby reef where coral died had more diverse coral and a greater diversity of algae. It’s likely repeated bleaching events, and evolution, led to only the hardiest surviving on the shallow reef.
“That means that new coral that could have drifted in, that were a different type, didn't have the right genes to fit in there. So they didn't survive,” she said. “So over time, this environment has become this place that only this kind of specific kind of coral genes can fit in and stay alive.”
Identifying the coral and the algae can help lab breeders, but Parker and Ross said much work remains. Getting coral to breed is complicated and coaxing them into living with a specific algae is still a work in progress. Earlier this month, Australian scientists said they had identified 10 different heat-tolerant algae, but researchers still need to crack the code of getting the right coral with the right algae that are also willing to grow in different locations.
Successful reefs also need diversity, Cunning said.
“We want reefs that have lots of different genetic types of coral. Some that are more heat tolerant, some that grow quickly,” he said. “But as an ecosystem, as a coral reef, it will have the highest resilience if it has high genetic diversity. So we want to make sure to preserve that genetic diversity. We can't have the reef entirely composed of one type of coral because then whatever negatively impacts that coral could destroy the whole system.”
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