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One way to save coral reefs? Deep freeze them for the future

As oceans get hotter, scientists are looking for ways to safeguard coral species so they can be used to restore reefs in the future.
Donald Miralle
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Getty Images
As oceans get hotter, scientists are looking for ways to safeguard coral species so they can be used to restore reefs in the future.

Ocean temperatures have been extremely hot this summer, wreaking havoc on some of the world's highly vulnerable coral reefs. With marine heat waves only expected to get worse as the climate changes, scientists are increasingly focusing on an emergency plan: collecting coral specimens and safeguarding them onshore.

A library of corals, brought in from the wild, could be an insurance policy in the face of climate change, providing the genetic material to restore the reefs of the future. For long-term storage, some corals could end up in the deep freezer.

Scientists are working on preserving corals by cryogenically freezing them. In a new study, a team from the Smithsonian's National Zoo and Conservation Biology Institute, UC Berkeley and the University of Hawaii at Manoa report they successfully brought a coral fragment back to life after freezing it at -196 degrees Celsius.

The world's coral reefs face an existential threat from climate change, if humans don't reduce emissions from burning fossil fuels. The reefs' vital ecosystems support about a quarter of all marine life and provide coastal protection from waves and storm surge.

Cryopreservation, as the freezing technique is known, could provide a safe haven for the broad range genetic material across coral species, which could be key to adapting to future conditions. In Florida, as water temperatures rose to 100 degrees Fahrenheit this summer, restoration teams raced to collect coral specimens and put them in onshore tanks to preserve the unique individuals of the reef.

"We have to gut through this," says Mary Hagedorn, senior scientist at Smithsonian's National Zoo and Conservation Biology Institute and the University of Hawaii at Manoa. "We have to do whatever is necessary to maintain the ecosystems on our planet."

Deep freezing coral

As a cryo-preservationist, Hagedorn gets questions that border science fiction, where humans seek to preserve themselves for centuries.

"They think of head freezers," she says, laughing. "They think of all this weird stuff."

Instead, her work is much closer to human fertility treatments, where eggs and sperm are frozen to use later. Marine scientists have used the technique on coral sperm and young coral larvae, but freezing a living animal is much more complex.

Collecting coral sperm is tricky, however, since most corals only spawn only one or two days a year. Since they're fastened to the sea floor, they release their genetic material all at once in big underwater clouds. To capture it, scientists have to be in the right place at the right time, often in remote and challenging conditions. Preserving living pieces of coral would allow more opportunities to collect specimens.

"That would allow us to go out almost every day of the year and collect material throughout the world," Hagedorn says. "It would speed up our ability to secure the genetic diversity of corals."

With coral fragments from Hawaii, Hagedorn and her colleagues froze them at extremely cold temperatures in special chambers that prevent damaging ice crystals from forming. They were then able to thaw them and show the corals were still alive.

Making the corals healthy again after thawing is the next challenge, especially given that corals have roommates. Photosynthetic algae live in their tissue, creating food for the coral. But when stressed, the corals lose their algae, turning them white. Hagedorn says goring forward, restoring the algae will be key to reviving coral that can reproduce again.

Genetic library of coral

As marine heat waves become more common and intense, a global effort is underway to preserve the genetic diversity of corals. The Coral Biobank Alliance is seeking to collect specimens from all over the world. Hagedorn says cryopreservation could be a vital strategy for building a library of corals, which hopefully one day could repopulate reefs.

"Noah's ark only brought two," she says. "We're doing way more than that."

Reefs can recover from a bleaching event, but repeated marine heat waves make it less likely for corals to survive or reproduce successfully. The oceans are also acidifying as they absorb carbon dioxide from burning fossil fuels, which can harm corals and make it difficult for them to build their skeletons.

"Corals will continue to die and unless we are replacing them with restoration, there won't be corals available in the future, even if we were to fix all of the threats," says Jennifer Moore, coral recovery coordinator with the National Oceanic and Atmospheric Administration.

As temperatures peaked on Florida's reefs this summer, Moore says some corals died outright from the heat. Research teams raced to preserve specimens from key coral species, collecting 150 elkhorn coral and 300 staghorn coral samples.

"That's less than one percent of their previous population," she says. "So we're already in a diminished state of genetic diversity. We can't afford to lose anything that we have."

Having those specimens could help breed the reefs of the future, since it's unknown what combination of genes could make corals more resistant to the threats they may face, be it heat or disease.

Moore says cryopreservation could be a key piece of the puzzle in bringing reefs back. But the success of restoration depends on curbing the emissions from burning fossil fuels.

"It is not a foregone conclusion," Moore says. "There are still things that can be done to combat climate change so that we are not forced to be on a trajectory to a place where there are no more coral reefs."

Copyright 2023 NPR. To see more, visit https://www.npr.org.

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Lauren Sommer covers climate change for NPR's Science Desk, from the scientists on the front lines of documenting the warming climate to the way those changes are reshaping communities and ecosystems around the world.