Corals can’t keep up with climate change, and scientists are stepping in

With tanks of liquid nitrogen, jugs of sperm-clouded seawater, and a glowing ping pong ball, scientists are setting out to save corals.

Coral reefs make up less than 1% of the ocean floor, yet they support a quarter of all marine life. Reefs provide more than ocean habitat — they protect wetlands and coasts, produce food for over a billion people, and are used in creating new medicines. They are also some of the planet’s most spectacularly colorful ecosystems; coral reefs generate over $36 billion annually from tourism alone.

But corals are dying at an alarming rate, leaving behind skeletal graveyards where vibrant reefs once thrived. In 2016, the Great Barrier Reef in Australia lost 30% of its coral. Currently, 75% of reefs are threatened by pressures like overfishing and rising water temperatures, and the National Oceanic and Atmospheric Administration predicts that number will increase to 90% by 2030.

Alarmed by these projections, researchers around the world are working on creative ways to save reefs, or at least buy time for corals to adapt to warmer and more acidic ocean waters.

Grasping the multiple problems that corals face and the potential solutions to those problems requires understanding the strangeness of this marine animal — yes, corals are animals.

What is commonly thought of as a single coral are actually hundreds or thousands of squishy organisms called polyps working together in a colony. Their bright hues come from hosting algae in their tissues, which create food for corals through photosynthesis. When water temperatures are too warm, stressed corals expel the algae and undergo a process called “bleaching,” leaving behind a lifeless, white skeleton.

An increase in water temperatures by as few as one to two degrees Celsius, or roughly 2.5 degrees Fahrenheit, can cause bleaching. As ocean temperatures rise due to global climate change, bleaching events are becoming more common and are impacting larger reefs. In certain cases, corals can survive a bleaching event but will die if the conditions are too severe for too long.

In Florida and other parts of the Caribbean, corals also face stony coral tissue loss disease. The disease kills the coral’s colorful tissue, leaving patches of white skeleton exposed and often resulting in death. Scientists are still struggling to determine the disease’s cause but are racing to save corals as it spreads.

In the face of these overwhelming challenges, here’s a look at three big ideas that are holding out hope for Earth’s fragile reefs:

Idea #1: Frozen in Time

It may seem strange for scientists to try to rescue such temperature-sensitive animals by putting them into a deep freeze, but that’s exactly what a Hawaii-based team is doing. Their achievement was not merely to freeze coral larvae, but to successfully thaw and replant those larvae to grow new corals, which they accomplished for the first time in 2018.

Larvae can be “paused” for years in this frozen state. It’s not as easy as tossing larvae in the freezer, though.

“The most important thing to keep cells alive during the freezing and thawing process is to prevent the formation of ice,” says Jonathan Daly, lead author of a paper describing the breakthrough process and a researcher at the Hawaii Institute of Marine Biology. “When water freezes it expands and forms crystals, which can damage or burst cell membranes.”

To prevent the formation of ice crystals, Daly’s team used chemicals called cryoprotectants which minimized ice formation. Cryoprotectants can be toxic to coral cells in high concentrations, so finding the perfect balance took years.

To cryopreserve larvae, the Hawaii team first collect fresh eggs and sperm from a species of coral called mushroom coral. After fertilizing the eggs with the sperm, the team then froze the resulting larvae using liquid nitrogen. After thawing, the larvae spring back to life, searching for a place to plant themselves and begin their life as a coral.

This breakthrough could prove crucial to conservation efforts, because cryopreservation allows scientists to collect coral eggs and sperm from the most vulnerable corals before their genetic biodiversity is wiped out. For corals threatened by warming seas and disease, cryopreservation may be one of the only ways to press pause on impending extinction. When and if conditions are safe, reefs could be restored with frozen larvae that have been stored safely on land.

Still, it’s not a complete solution, Daly says, because these cryopreserved corals will still need a wild environment that they can survive in. He also notes that corals can produce millions of larvae during spawning events in the wild, a scale that’s currently impossible to keep up with in a lab. Restoring coral reefs would require cryopreserving not just mushroom coral, but hundreds of other coral species, something Daly and his team are currently working on.

“Cryopreservation alone cannot save coral reefs,” says Daly, “but it can play a crucial role in future reef restoration efforts.”

Idea #2: Setting the Stage for Spawning

For corals to reproduce, the conditions have to be perfect. Once a year, when lighting, temperature and water salinity align, corals release their eggs and sperm in a dramatic synchronized act. The resulting larvae then settle on the reef and begin to grow.

These perfect spawning conditions have proved challenging to replicate in a lab, but this summer, scientists at the Florida Aquarium artificially triggered coral spawning for the first time.

Creating the right conditions in the lab was no small feat, says Amber Whittle, director of conservation at the Florida Aquarium. Her team used special ultraviolet lights to replicate sunrise and sunset, mimic the temperature, and more. They even created a miniature glowing moon using an LED light inside a ping pong ball.

For their experiment, the Florida researchers picked species that were some of the first to be killed by stony coral tissue loss disease. Those corals were so damaged that they could no longer reproduce sexually in the wild — the surviving corals were too far apart for their sperm and eggs to reach each other. The only option left for these corals, Whittle says, is to reproduce in a lab.

Whittle says her team’s goal is to eventually replant lab-spawned corals onto damaged reefs, but even if they succeed, climate and disease would still pose threats to coral survival. The Florida researchers could spend all the time and money necessary to plant healthy corals onto a reef, only to see them destroyed by disease or warm waters. Whether artificially-triggered spawning is something that can happen on a large enough scale remains to be seen.

For now, Whittle’s coral larvae are staying on land.

When it comes to ‘“putting our nice healthy babies back into a system that’s been crashing for decades,” says Whittle, “Well, that’s a much tougher question.”

Idea #3: Fast-Tracking Evolution

When climates and ecosystems change gradually over time, animals and plants usually adapt. But humans are transforming the planet at a breakneck pace, and organisms often can’t keep up. Corals are struggling to adapt, which is why some scientists are trying to give them an evolutionary boost.

A team of biologists in Australia is cross-breeding corals to create new, heartier “super corals.” Wing Yan Chan is the lead author of a paper published last year describing the work, which she calls “assisted evolution.”

“We know that over time, species can adapt to environmental changes when the rate of change in the environment is happening slow enough,” she says. But climate change is happening too quickly for corals adapt on their own.

By accelerating what she calls the natural evolutionary processes, scientists can construct corals that are more resilient to warmer waters. To create these super corals, Chan and her team crossed eggs from one coral species with sperm from another to create a hybrid offspring.

By hybridizing two related but genetically different corals, explains Chan, “You produce offspring with greater genetic diversity and, through this process, also a higher chance you might have a resilient genotype.”

The corals they choose matter, too. If they’re too similar, the coral offspring won’t have the genetic diversity to make them more resilient than their parents. If they’re too different, they won’t successfully hybridize. Chan’s team used two species of Acropora corals, a large, branching coral often seen on the Great Barrier Reef.

“We find that the hybrid was about 15% to 35% better in terms of survivorship compared to the purebred,” says Chan. For areas impacted by warming waters and bleaching events, any reduction in coral loss could help sustain reefs for longer.

Whittle is now waiting to see if the super corals they planted will successfully reproduce on their own. Because corals take at least four years to sexually mature, the researchers have to be patient. They’re also waiting to see if the hybridized corals will produce a new generation of hybrid offspring or if they will breed with a non-hybrid parent species.

To help coral reefs in the future, she says these hybrid corals need to be self-sustaining in the wild, meaning they can grow and reproduce on their own. She hopes they’ll have an answer in the next year or two.

Though Chan is optimistic about the future of hybridized super corals, she says that “this is really only buying us time.”

The Upshot

Coral scientists agree: if there is no healthy ecosystem to plant these cryopreserved, artificially-spawned or hybridized coral into, these interventions do little to help coral reefs in the long-term.

For Daly’s research in Hawaii to have global impacts, his team will need to be able to cryopreserve many more species of corals. The researchers have banked sperm from more than 30 coral species, but hundreds more exist, and some species may be harder to freeze.

Artificially triggering spawning can help corals that can’t reproduce sexually, but researchers will have to find a way to produce coral larvae in the order of millions to make a significant difference — a costly and time-intensive process that may prove impossible. And like Daly’s team, Whittle’s group will have to prove spawning can be achieved in more species.

“It took decades to get us to this point,” says Whittle, “and it’s going to take us at least decades of producing and out planting large numbers of coral to get back to an even functional reef.”

The only real long-term solution for corals, Daly, Chan and Whittle agree, is to move aggressively to address climate change by cutting emissions of greenhouse gases — now.