For the first time ever, researchers have succeeded in cryopreserving and reviving pieces of adult coral—a breakthrough that could eventually help save reefs struggling from the effects of climate change. 

Using antifreeze and liquid nitrogen, scientists froze coral fragments in a glasslike state, then thawed and returned them to seawater. For 24 hours after the corals were revived, they consumed oxygen at a rate comparable to corals that had never been cryopreserved, the team reports in a new study published last week in Nature Communications.

This new method could one day preserve other organisms—even human organs—for decades, says marine biologist Mary Hagedorn, a research scientist at the Smithsonian’s National Zoo and Conservation Biology Institute and a co-author of the study. The breakthrough comes at a critical moment for coral reefs, which are facing heightened threats from warming oceans.

“Coral reefs are essential to the baseline health of our oceans, and cryo-conservation of endangered coral species can help to ensure that these invaluable and marvelous organisms do not go extinct,” Matthew Powell-Palm, a mechanical engineer at Texas A&M University and the paper’s lead author, says in a statement.

The new cryopreservation process has been years in the making. Hagedorn and her collaborators previously pioneered the cryopreservation of coral sperm, with techniques similar to those used in human sperm banks.

But while sperm—which are single-celled—are generally easier to freeze than a more complex adult coral, they can be incredibly difficult to collect. Corals often dwell in remote and hard-to-reach areas on the seafloor, and they’ll usually release their sperm on only a few days each year—leaving a narrow window for the researchers to act. 

“It can be very, very challenging to get there at the right time,” Hagedorn says. “One year, we missed it by a whole month, because they spawned early because the water was warm.” Another year, the team was hit by a hurricane, and they had to abandon their work.

Corals are extremely sensitive to temperature changes. If it gets too hot, they will expel the algae living in their tissues and turn completely white. This process, known as coral bleaching, stresses the organisms and makes them more susceptible to death and disease.

brown coral next to white coral
White, bleached coral beside healthy coral Claire Lager / Smithsonian's National Zoo and Conservation Biology Institute

Climate change, overfishing and pollution have already contributed to the disappearance of half the planet’s coral reefs since 1950. The Earth lost 14 percent of its reefs—an area larger than all the coral currently living in Australia—in just one decade, between 2009 and 2018. And researchers have predicted the crisis will only get worse.

A 2018 report from the Intergovernmental Panel on Climate Change estimated with high confidence that the world’s coral reefs would decline by 70 to 90 percent with 1.5 degrees Celsius of warming over pre-industrial levels. Currently, the planet is projected to reach that threshold between 2030 and 2052, if temperatures continue to rise at the current rate. And if air temperatures rise by 2 degrees Celsius, reefs will decline by 99 percent.

“Coral reefs are simply too valuable to lose,” Joe Pollock, a senior coral reef resilience scientist with the Nature Conservancy’s Hawaii and Palmyra Programs who was not involved in the research, says in an email. “They support over a quarter of marine life, protect our coastlines during storms and contribute an estimated $375 billion to the global economy annually.” 

This year, temperatures are hitting historic highs. July 2023 was the hottest ever on record, and seawater temperatures above 100 degrees Fahrenheit in Florida caused mass coral death and bleaching. As the ocean continues to warm, bleaching will become more consistent, Hagedorn says. 

With environmental pressure on the reefs rising, the researchers needed a cryopreservation strategy that was more effective than focusing on sperm.

“We felt like we needed to go faster,” Hagedorn says. “We’re going too slow. … It’s important that we get the genetic diversity and biodiversity while it still exists.”

So, in 2019, Hagedorn and her colleagues—including researchers at the University of California, Berkeley, and Texas A&M University—began their work to cryopreserve and revive entire pieces of finger coral (Porites compressa) from Hawaii. Using their new process, called isochoric vitrification, they preserved coral fragments, each consisting of about 20 individual polyps within a calcium carbonate skeleton. These are among the most complex organisms to be cryopreserved and thawed successfully.

“This work progresses the field by expanding cryopreservation of corals beyond sperm and symbionts to include entire coral fragments,” Pollock says. “Barriers still exist to rearing these fragments beyond a day to two post-thaw, but this is certainly a noteworthy advance.”

coral branches like antlers at the top of a small mound under the water
Elkhorn coral has a branching structure that provides habitat for fish and other reef-dwelling animals. Stephani Gordon / Smithsonian's National Zoo and Conservation Biology Institute

The process is relatively simple, Hagedorn says. First, researchers find a healthy adult coral and harvest off a thinly sliced chunk about the size of a human thumbnail. They bleach the coral and place the fragment into a small aluminum cylinder filled with an antifreeze solution. While the goal is to freeze the coral piece, the team must avoid ice formation, which would damage the animal’s tissue. Finally, the cylinder is dunked into liquid nitrogen and cooled to nearly minus 321 degrees Fahrenheit, causing the coral to rapidly freeze. 

To revive the coral, scientists place the cylinder in a warm water bath for two minutes, then remove the coral fragment and put it back into seawater. 

“It’s conceptually complicated because of thermodynamics,” Hagedorn says. “But the actual process itself is really dead easy.” 

Simplicity was part of the team’s goal; they wanted a process that could be quick and economical for coral reef managers to use in the field around the world. Smithsonian’s National Zoo and Conservation Biology Institute is participating in the Coral Biobank Alliance, a network of professionals working to preserve coral biodiversity. So far, the global team has banked sperm cells from 50 coral species and live specimens of 200 species. Eventually, Hagedorn hopes to have an “army of people” trained to collect and preserve corals—and the biobank alliance wants to store genetic material from every known coral species by the end of the decade.  

“This study represents a very exciting breakthrough for our capacity as coral reef scientists and managers to safeguard coral genetic diversity,” Olivia Williamson, a coral researcher at the University of Miami, says in an email. She was not involved in the new paper but has previously collaborated with Hagedorn and co-author Jonathan Daly of the Taronga Conservation Society Australia on coral sperm cryopreservation. 

Williamson says the new technology needs to be tested on more species, but she’s optimistic it will eventually allow for a “dramatic scaling up of coral genetic banking.” 

In the new study, the team only tested the coral’s survival for 24 hours after its revival, but Hagedorn says they’re now working on refining some of their processes to allow for survival up to three weeks. Down the line, she predicts the technique could be adapted to preserve human organs, like ovaries, testes and embryonic kidneys and hearts.

“I think it’s going to have an amazing trajectory in terms of doing … more whole pieces of organisms,” she says. “It’s very, very cool technology, and it is the wave of the future.”

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