As Ocean Waters Warm, a Race to Breed Heat-Resistant Coral

Coral reefs are some of the most vulnerable ecosystems on the planet, susceptible to pollution, ocean acidification, and overfishing. And as marine heat waves become stronger and more frequent, they are increasingly driving corals to expel their resident microalgae, which provide them with essential nutrients. Without their algae, corals can lose their vibrant colors, a phenomenon called bleaching, and starve to death.

The worldâ€s corals, from the Caribbean to the Indian Ocean, are currently undergoing their fourth mass bleaching since 1998. That event killed about 8 percent of the worldâ€s coral, and between 2009 and 2018 about 14 percent of the worldâ€s remaining corals — about 4,500 square kilometers of them — were wiped out too, predominantly by heat. The Intergovernmental Panel on Climate Change projects that even if global warming is limited to 1.5 degrees Celsius, the Paris Agreement goal, coral reefs will decline by 70 to 90 percent by 2100.

Working off previous studies that established that some corals naturally withstand heat better than others, the Coralassist team began its project by systematically mapping and heat testing 100 Acropora digitifera coral colonies in the highly diverse reefs of Palau, in the western Pacific Ocean. They exposed fragments from each colony to a temperature stress challenge in a laboratory tank that emulated the duration and intensity of marine heat waves. One group spent 10 days in water that gradually warmed by 3.5 degrees C; another group spent a month in water warmed by 2.5 degrees C.

The team then selected the top and bottom performers and started matchmaking. Those with high heat tolerance would mingle their eggs and sperm together. Low-tolerance corals were similarly paired, and some couples were a mix of both. The resulting larvae, after attaching to ceramic tiles, were moved to nursery tanks on a local reef, where they grew for three to four years.

At the end of their study, the team found that the heritability of heat tolerance was between 0.2 and 0.3 on a scale of 0 to 1, indicating that “about a quarter of the variability in offspring heat tolerance was due to genes passed from their parents,†the authors wrote. “The response is not completely genetically driven, the environment also has some influence,†says Adriana Humanes, a marine ecologist in the Coralassist Lab. “But you have a huge component of the genetics that is influencing the response to the heat stress.â€

In their trials, the tolerance of adult offspring of high-heat-tolerant parents was increased through breeding by almost 1 “degree heating week,†a metric that refers to how much heat stress has accumulated in an area over the previous 12 weeks, compared to the corals with low-heat-tolerance parents. The study serves as proof of concept that selective breeding can boost heat tolerance in just one generation and endure into adulthood.

While the experiment showed there is scope for breeding, the improvement in heat resistance was still “pretty modest compared to climate change,†says Liam Lachs, an ecologist from the Coralassist lab who ran the teamâ€s statistical calculations. This year Palauâ€s waters warmed at 10 degree-heating weeks, and the Caribbean hit 20 degree-heating weeks. In addition, the corals that were more heat resistant to short zaps of heat didnâ€t do as well under longer-term exposure to heat, indicating that different genes might be responsible for resistance to different durations of heat.

Coral researchers know that there is no single gene that confers heat resistance on corals: Itâ€s a “very complex trait encoded by many genes,†says Annika Lamb, who runs a similar project with corals from the Great Barrier Reef at the Australian Institute of Marine Science (AIMS). Her lab is also trying to selectively breed heat-resistant corals, but sheâ€s using a faster method — applying a quick heat zap — as well as focusing on breeding corals from different species in the hopes of making sturdier hybrids.

Breeding efforts must also take into consideration tradeoffs. The genes that make a coral more tolerant to heat might also make it less tolerant to disease, less fertile, less resistant to storms or cold, and slower growing, says Lamb.

Given these considerations, her team is also selectively breeding just the microalgae that live inside corals, which are to a large extent responsible for the actual temperature tolerance of a coral colony, says Madeleine van Oppen, head of AIMS†coral assisted-evolution project.

Van Oppen has been selecting an array of microalgae from coral around the Great Barrier Reef, bolstering their heat tolerance by exposing multiple generations, over the course of 10 years, to elevated temperatures in a lab, and then reintroducing them into chemically bleached adult coral fragments.

After initial lab tests, van Oppen started a trial on an inshore reef of the Great Barrier Reef. The results so far have been “really promising,†she says. During last summer’s heat wave the corals inoculated with heat-evolved microalgae paled less and photosynthesized better than corals with closely related microalgae that hadnâ€t been boosted for heat tolerance. But there are still questions about whether the microalgae will spread and stay healthy, and whether there are factors in the wild that havenâ€t yet been taken into account. “This is a very young field,†says van Oppen. “It’s not surprising that there’s a lot of unknown still.â€

For instance, studies show that corals reared in labs often have weakened skeletons. This suggests they could be compromised once planted in ocean waters, says Terry Hughes, director of the Centre of Excellence for Coral Reef Studies in Australia. “Despite long-standing claims that heat-tolerant super-corals can be bred in the laboratory and used to re-populate reefs, we still donâ€t actually know if thatâ€s true,†he says, noting that adding artificially bred corals to the wild gene pool is unlikely to make a significant difference except in extreme circumstances, as natural selection is already changing the mix of coral species on reefs.

“Corals are always complicated. They’re always doing something unexpected,†says Stanfordâ€s Palumbi, whose lab is identifying heat-resistant corals in the wild and then running what he calls “common gardening†experiments on them. “These corals have been [evolving tolerance to changing conditions] for hundreds of thousands of years. Theyâ€re already out there,†says Palumbi, who also works in Palau, among other places. By collecting thousands of coral fragments from different reefs, testing them for resistance to a heat zap, and then moving the more heat-tolerant species and colonies to human-made reefs on metal frames, his team is giving them a chance to breed with a broader gene pool.

Preliminary findings from these tests, says Palumbi, suggest their offspring are similar in heat tolerance to Coralassistâ€s 1 degree-heating week corals. “We’re basically pursuing two pretty different but very parallel and complementary ways of looking at the same question,†says Palumbi. But the new paper by the Coralassist team is the first to quantify an uptick in heat tolerance. “That number is something that really hasn’t appeared in any other paper so far,†he says.

Still, the practical difficulties and costs of coral husbandry shouldnâ€t be overlooked, says Hughes, and coral reef restoration is far more expensive than restoring seagrasses or mangroves. A million newly settled corals might sound like a lot, but “in reality itâ€s a drop in the ocean,†says Hughes, who notes that the total area of reef worldwide currently occupied by laboratory-reared corals is a handful of square meters.

“The most we can do in terms of reef restoration will always be very small in scale relative to the area to be covered,†says Christopher Jury, a reef ecologist at the University of Hawai’i at MÄnoa. But recovering reefs the whole world over was never the goal, he says. Restoration efforts can protect small areas intended to provide seed material which, through normal reproduction, larval dispersal, and settlement on the seafloor, can foster the rejuvenation of reefs elsewhere. Of course, substantial climate change mitigation is a prerequisite if any of these other strategies are going to work to preserve reefs, he says, and breeding efforts are just meant to buy coral communities some extra time.

The fact that coral abundance has plummeted due to climate change doesnâ€t negate the fact that rapid evolution is already happening naturally in some surviving coral populations. When Juryâ€s team semi-enclosed a naturally occurring coral reef community off Oahu and, for two years, subjected it to a combination of 2 degrees C warming and -0.2 pH units of acidification — akin to what oceans will experience with current rates of global warming — the communities shifted and changed while maintaining high rates of biodiversity. The corals recruited a diverse assemblage of algae, invertebrates, and microbes that helped them withstand the heat. So there is hope.

Ultimately, the data gathered from all the experiments underway can help researchers improve their models of how corals will fare under global warming, says James Guest, the researcher who led the Coralassist Lab work. That team recently plugged their data from years of breeding experiments into computer models that will provide guidance on where and when interventions will be necessary and effective under various climate change scenarios. This work is forthcoming in Science.

“There will be some instances where it’s better just to leave the corals to their devices,†says Guest. “It’s just keeping a really open mind and constantly being prepared to update the advice based on new research.â€