A new ally may help speed up the race to restore devastated coral reefs in Australia: robots, combined with mass-manufacturing techniques. Around the world, scientists are also working on other methods to help reefs recover faster and on a larger scale than before. But as corals face existential threats, including a steady increase in the intensity and duration of marine heat waves, experts warn that these solutions need to go hand in hand with action on global warming if they are to bring lasting improvements. 

Taryn Foster is a coral scientist based in Western Australia and chief executive of Coral Maker, which produces small coral skeletons onto which nursery grown coral is attached. She was spurred into action by the mass bleaching she witnessed while doing research for her doctoral thesis on climate impacts on coral reefs.

"I was studying these big reefs and saw how quickly a bleaching event devastates a reef system," says Foster. "In the space of a few weeks during one of the bleaching events, we saw around 90% coral mortality. And I was reconsidering whether or not I wanted to continue to write scientific research papers, or whether I wanted to get more involved in practical, more solutions-based work."

The ocean absorbs 90% of the heat caused by human-driven climate change. Warming oceans are a huge problem for coral reefs as they require temperatures to stay within the range of 79-84F (26.1-28.8C) to remain healthy, grow, and reproduce. Current predictions estimate that 99% of coral could succumb to marine heatwaves by the 2030s if global temperatures continue to rise at the current rate.

This would have dramatic consequences not just for corals, but for the wider reef ecosystem. Although coral reefs only occupy 0.1% of the seafloor, 25% of all named marine species live in reef systems and an estimated one billion people benefit directly or indirectly from coral reefs. Coral ecosystems face a host of threats ranging from increasing ocean acidification to a rise in coral diseases but the steady and alarming increase in ocean surface temperatures poses the most ubiquitous and ominous problem.

Coral restoration typically involves transplanting nursery-grown coral onto the damaged reef by hand using individual divers, a very time consuming and arduous process that can be expensive and hard to do on a large scale. Foster decided that a new approach was needed to scale up coral planting, incorporating lessons from her family's manufacturing business, such as automation and mass production.

"I was thinking we can apply some of these technologies to coral reef restoration," says Foster. "There's lots of parts of the process that are just pick and place, repetitive type tasks which are ideally suited to robotic automation."

She partnered with software company Autodesk, which helped her implement automation and artificial intelligence into a process that could manufacture skeletons for coral. Foster says using artificial intelligence to program and run the robotic side of the operation allows for greater flexibily and accuracy.

"Unlike standard pre-programmed robotic systems, artificial intelligence is able to respond to the variability in coral morphology and adjust robotic movements accordingly," says Foster.

To create corals for planting, robots work alongside humans, attaching nursery-grown coral fragments to mass-produced coral skeletons on an assembly line. The material used for the coral skeletons is rock or cement; currently Coral Maker uses recycled construction rubble.

Foster says current restoration programmes can generally restore about 2.5 acres (1 hectare) of coral reef in a year, but that once Coral Maker is fully operational it could restore around 250 acres (100 hectares) a year.

A race is also underway in the US, to save the bleached corals of the Flordia Keys Reef Tract. Florida's coral reef system, which is the third largest in the world, is under severe threat. Now, a novel approach is boosting the survival chances of two of its primary reef-building coral species, both of which are endangered: the pale brown, pointed-antler-like staghorn (Acropora cervicornis) and the flattened-antler-like elkhorn (Acropora palmata). Both the staghorn and the elkhorn have been devastated by disease and climate change, and have seen a 97% decline in their population since the 1970s. The Coral Restoration Foundation (CRF), the world's largest reef restoration organisation, was formed in 2005 in response to this population crash.

CRF operates coral nurseries in the ocean that grow juvenile coral, which are then transplanted to reef restoration sites in an effort to stave off the extinction of coral species and restore balance to the reef ecosystems. The corals are grown on floating, anchored trees made from polyvinyl chloride (PVC) with fibreglass branches onto which coral fragments are attached with thin plastic lines. This structure allows them to grow particularly fast, says Phanor Montoya-Maya, CRF's restoration programme manager. This is because the suspended corals are surrounded by light and waterborne nutrition and are more protected from storms and other disturbances than they would be if they were anchored to the sea floor. According to Montoya-Maya, the corals are "reef-ready" in six to nine months.

CRF's Tavernier Coral Tree Nursery in the Florida Keys covers 1.5 acres (6,070 sq m) of ocean floor, contains more than 500 coral trees, and is capable of producing 40,000 reef-ready corals every year, the organisation says.

Even restored reefs still face the threat of global warming, however. In July 2023, a marine heat wave in Florida caused ocean temperatures to soar: in some areas ocean temperatures exceeded hot tub levels of 100F (37.7C). The bleaching threshold for coral is typically around 87F (30.5C). When water temperatures cross that red line and stay that way for a month or more, coral is stressed to such a degree that it has to expel the algae (zooxanthellae) in its tissues. This algae gives coral both its distinctive color and, in a unique symbiotic relationship, it also feeds vital nutrients to its coral host. Bleached coral can survive and eventually recover, but any recovery requires water temperatures to revert back to a less extreme range and stay that way. This past summer in Florida the heat was so extreme that in some instances the coral tissue began to dissolve, eliminating any prospect of recovery, according to CRF.

In response to the marine heat wave in July, CRF enacted an emergency plan to remove their coral trees from the nurseries and move them to land-based aquariums where the water is kept at cooler temperatures. Even with the rescue operation, CRF lost about 50% of its coral stock in its four coral tree nurseries. Much of what remains is bleached, making its recovery uncertain, CRF says.

Montoya-Maya says that CRF's research shows a complicated picture of how individual corals respond to temperature increases.

"CRF's research has revealed the importance of genetic diversity in coral populations," says Montoya-Maya. "However, we have found that these responses also vary depending on specific locations and environmental conditions – no genotype displays the same responses across different locations."

99% of coral could succumb to marine heatwaves by 2030 if global temperatures continue to rise at the current rate

Researchers at Australia's tropical marine research agency, the Australian Institute of Marine Science (AIMS), are trying to make juvenile coral more heat-resilient by identifying and selectively breeding heat-tolerant adults and inoculating corals with heat-resilient symbiotic microalgae.

"Results so far are encouraging and there is potential to increase the heat tolerance of corals to improve survival in hotter seas," says AIMS researcher Saskia Jurriaans. "But we need to do this at scale and out of the lab in cost-effective ways."

Some multinational corporations have also decided to invest in reef restoration. In 2019 the cat food brand Sheba, a subsidiary of Mars, began work on one of the world's largest coral restoration programmes, called Hope Reef, which is located off the coast of Sulawesi in Indonesia and had been heavily damaged by past blast fishing and other disruptions. Blast fishing, which uses dynamite or other explosives to kill fish, reduces the underwater landscape to shifting rubble, which doesn't allow coral to grow.

The project uses reef stars, hexagonal steel structures, to which corals are attached, explains Jos van Oostrum, senior director of sustainable solutions at Mars. "These 'loaded' reef stars are interconnected underwater and anchored to the deserted rubble fields, to provide a strong stable platform for attached corals to grow. Over time, native corals settle onto the reef stars, corals grow, and the structures become fully integrated into the reef," he says.

According to van Oostrum, coral coverage of Hope Reef has grown from 2% to 70% thanks to the project, and the fish population has increased by 260%.  Sheba is expanding its coral restoration efforts to key sites around the world in Indonesia, Australia, The Maldives, Kenya, Mexico, and Costa Rica, he says. The project aims to restore more than 221,000 sq yards (185,000 sq m) of reef by 2029.

Not everyone in the field agrees with the premise that human-driven coral restoration is the best approach towards saving and protecting coral. Erika Woolsey, chief scientist and chief executive at the environmental non-profit The Hydrous, says that in some instances coral has shown great resilience and the ability to replenish itself without direct assistance. Woolsey points to recent research on Australia's Great Barrier Reef which shows that coral reefs are recovering on their own from a 2022 mass bleaching event.

"When you look at the impacts of reef restoration efforts it accounts for less than 1% of the whole Great Barrier Reef," says Woolsey. "Whereas the natural cycles are replenishing huge areas which makes you really question the efficacy of rehabilitation projects."

Woolsey isn't opposed to novel reef restoration methods but feels that public education and creating empathy for marine environments are the salient issues.

"Reef restoration efforts can be effective in certain scenarios but all of these different approaches have yet to be proven at scale," says Woolsey. "However, we know that to save coral reefs, we really have to combat climate change, remove local stressors, like overfishing, and prevent overgrowth of macroalgae. Those are the proven solutions."

We know that to save coral reefs, we really have to combat climate change – Erika Woolsey

Meanwhile, Foster at Coral Maker, the robot-assisted project, sees another potential function for mass-produced coral skeletons: helping corals move to cooler waters where they may be less at risk, a step known as assisted migration.

"I think if we had the technology to deploy them [manufactured coral skeletons] at scale, we could consider things like assisted migration where we take corals that are in areas that are in danger, and move them further from the equator, to cooler water temperatures," says Foster.

Since coral lacks fins or legs, it has little ability to adapt to climate change by quickly moving to cooler waters. However, assisted migration for coral is controversial in the scientific community and has not been tried at scale. One the major concerns is that the risks associated with moving coral reefs to new areas are unknown and could negatively impact the ecosystem into which it would be introduced. Transplanted coral of certain types might become invasive, or bring with it microbes that negatively affect native sea life.

"I was trained as a conservationist and you do not interfere, you don't move things to new locations," says Foster. "There's always a can of worms that opens up when you do that sort of thing. But we're dealing with an era of rapid climate change and we know that there's a risk of not acting."

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