Around the world, coral is under siege. Battered by a changing climate, destructive fishing, and other threats, reefs are disappearing. Just last year, the Great Barrier Reef suffered the worst die-off ever recorded. If nothing is done, nearly all reefs will be in serious danger by 2050.

Many people depend on reefs for their food and livelihood. They’re home to more than 25 percent of marine creatures, supplying us with fish and sources of new medicines to fight cancer and other diseases. Reefs also form natural seawalls that protect shorelines from storms and waves, and are magnets for tourism.

“We need a new approach to restoring them, and we need to do that quickly," says Peter Harrison, a marine biologist at Southern Cross University in Lismore, Australia, "because the rate of loss of coral reefs at the moment is near catastrophic.”

To save the world’s reefs, they are transplanting healthy coral into damaged landscapes, and pioneering new strategies to breed stronger corals and guide infant corals to settle on and rebuild degraded reefs.

A brown rice coral in the reefs of Waiopae, East Hawaii Island. John Burns

Infusions of Healthy Coral

Each coral we spy on a reef is a colony, packed with many tiny, soft-bodied animals. These fragile polyps secrete stony outer skeletons. But even with this protection, there are many reasons coral can find itself in trouble.

When the water around them becomes too warm to handle, corals bleach and turn bone-white as they evict the colorful, food-making algae that live in their tissues. This leaves them vulnerable to disease and starvation. Climate change is also turning oceans more acidic, which corrodes coral skeletons and makes it harder for them to build new ones. Reefs may be smothered by sediments and pollution from storm runoff, or ripped apart by dynamite fishing or ship groundings.

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But with a little help, many reefs can bounce back. Since the 1970s, scientists have been taking a hands-on approach to repair the destruction. They collect tiny fragments broken off healthy corals or scavenged from reefs shattered by shipwrecks and hurricanes. Today, these nubbins are usually grown in underwater nurseries, and then transplanted onto wild reefs.

Coral nurseries are cobbled together from whatever material people have on hand, be it bamboo, PVC, or wire mesh. Stephanie Schopmeyer, a research associate at the University of Miami, and her team favor many-branched “trees” that can be loaded with 120 chunks of staghorn coral, a threatened species in the Caribbean.

A coral nursery in Florida. S. Schopmeyer

When it’s time to install coral transplants into their new home, divers hammer nails into the reef and fasten the corals in place with cables or epoxy. “Within three to six months, the corals will actually grow over the nail and begin to put healthy tissue onto the reef,” Schopmeyer says.

By peppering the seafloor with fresh colonies, she and her colleagues hope to grow dense thickets that will bring coral cover back to the levels seen decades ago. They also use transplants to bridge isolated populations. This helps ensure that reefs host a varied assortment of corals, which ultimately makes them more likely to survive in the face of adversity.

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“It’s sort of like the Irish potato famine — if you only have one monoculture of anything [and] the right disease comes in, it can wipe it out all at once,” says Jennifer Koss, Director of the National Oceanic and Atmospheric Administration’s Coral Reef Conservation Program, which also transplants corals. “So we’re trying to approximate genetic diversity in making things be as natural as possible.”

Even so, dwindling wild populations and a reliance on captive bred corals could leave reefs without much genetic variation. Luckily, corals can reproduce sexually, which means they can also contribute to sperm banks. When ready to spawn, corals discharge sperm and eggs into the surrounding waters. Scientists are gathering sperm from wild corals and freezing it, to be defrosted and used later.

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“We need to be cryopreserving as much sperm from as many individuals as we can, so we have material to work with in the future,” says Ruth Gates, Director of the Hawaii Institute of Marine Biology, University of Hawaii at Manoa.

Building a Better Coral

Planting new corals by hand is labor-intensive and won’t be enough to replenish the world’s reefs. But scientists are coming up with plenty of new strategies, too.

“No two reefs in the world are the same,” Gates says. “What we do in Hawaii may not work as well in Australia.”

Divers from Ruth Gates' lab check on corals for a transplant experiment. The team moved thousands of coral fragments between two sites to see how differences in water acidity affected them. Shayle Matsuda

Gates wants to tweak corals so they will be better suited for future oceans. She's investigating why some corals tolerate heat and different strains better than others. “I decided that rather than documenting the death of coral reefs that I’m going to focus on survivors,” she says.

Her team is identifying which corals on a reef persevere while their neighbors succumb to bleaching, then breeding the strongest of them. This year, they will give the offspring of these unions a “stress test” to find out if they have an edge at coping with difficult conditions.

Gates says they're "trying to accelerate what nature does to keep up with the rates of change in the environment," and eventually these upgraded corals will be transplanted back onto reefs.

"We need a new approach to restoring them, and we need to do that quickly because the rate of loss of coral reefs at the moment is near catastrophic."

She’s also testing if it's true that what doesn't kill corals can make them stronger.

“We do what I call running them on environmental treadmills,” Gates says. “The goal is to gently bump up the tolerance.” She sticks corals in tanks of water warmer than they’re used to, then gives them a break. When the corals face their next ordeal, they may have a higher threshold for what they can endure.

You can’t bring an entire reef into the lab, but once scientists fine-tune the formula for toughening up corals, they may temporarily heat the waters around wild reefs. Hopefully these brief, controlled workouts will train the corals, leaving them better prepared to handle a shifting climate.

Another target for coral enhancement is the many strains of algae that serve as food factories. Matching vulnerable corals with heat-tolerant algae could give them a leg up. However, “The corals are very fussy about who they associate with, and it’s very difficult to modify that interaction,” Gates says.

She’s also planning to press into service the bacteria that dwell on and inside of corals. These microbes play an important role in reef health, much like the human microbiome. Just as fecal transplants from healthy people are used to treat sick humans, a gift of mucus and microbes from “strong” corals may bolster those that perform poorly when stressed.

Coral fragments in Hawaii before transplanting. Shayle Matsuda

Corals On the Move

Gates’ tricks may make corals hardier, but there are also ways to level up reef restoration without modifying the corals. Instead of rehoming one chunk of stony coral at a time, Harrison and his colleagues send larvae en masse into degraded reefs.

To do this, the team borrows adult corals ripe with eggs and sperm, then lets them get busy in aquariums. They mix offerings from different colonies, and collect and raise the millions of resulting larvae.

The youngsters are then shepherded to new homes. Harrison’s crew pours the larvae into tent-like enclosures attached to damaged reef areas. They remove the mesh after a few days, then monitor how many larvae settle and survive over time.

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Harrison works in an area of the Philippines where reefs have been decimated by blast fishing. The reefs in Southeast Asia are in particularly rough shape, with 95 percent now threatened by human activities.

The rehabilitation work is starting to pay off. The corals he’s seeded have grown at a brisk pace; after three years, they've grown to the size of dinners and have begun to reproduce. You can “see all of these new colonies growing on what was essentially a dead reef,” Harrison says.

He's now setting his sights on much larger patches of reef. In the future, he plans to deploy larger enclosures to rear many more larvae out on the reef until they are ready to put down roots.

“The real challenge now is to scale this up,” he says.

Hanging On

“When you say, 'What is the risk of doing nothing at this point,' the answer to that question is just so profoundly troubling,” Gates says. “People will not have enough to eat, their land will be eroded, their economies will be diminished.”

To ensure there are still reefs to mend, scientists are preserving corals locally within marine protected areas, or by addressing pollution and collaborating with fishermen. “The most important thing is to protect whatever reefs are left from further damage while we start to work out ways that we can repair them,” Harrison says.

This won’t stop the onslaught of climate change, and it’s too early to tell how much of the world’s corals can be rescued. But with scientists’ help, corals may have a shot at surviving warmer seas and reclaiming what were once dead or tattered reefs.

Reefs are “underwater gardens, teeming with life when they’re healthy,” Gates says. If scientists succeed, these gardens will continue to feed us, protect our coasts, and delight swimmers.

By restoring reefs, we buy corals time. They may adapt on their own, or persist long enough to recover if ocean conditions eventually return to “normal.” “The reefs of tomorrow might not look like the reefs of yesterday, but there will still be coral reefs,” Koss says.

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