As global temperatures spike to new highs and heat waves pass through the oceans, coral is suffering. An estimated half of all coral has died since the 1950s, partially due to extreme temperatures that stress the animals and cause them to expel their life-giving algae. And with 2023 becoming one of the hottest years on record and an El Niño event threatening to raise temperatures even further in the eastern Pacific, corals from that region are especially vulnerable.
Luckily, we now know that at least one type of coral can thrive and grow rapidly in the area despite rising temperatures, thanks to its more heat-resistant algae. In a study published Tuesday in Proceedings of the Royal Society B, researchers found that Durusdinium glynnii algae in antler coral deliver protection against intense heat with no side effects to the animal’s health.
“It’s really important because this particular [coral] is so prevalent in the eastern Pacific,” says Mark Warner, a co-author of the study and a marine ecologist at the University of Delaware.
Scientists have been on the hunt for heat-tolerant coral that can flourish as the planet warms. Coral reefs form the base of some of the most diverse ecosystems in the world, protect coastlines from storms, and contribute billions of dollars to local economies as they spur tourism and ensure healthy fisheries.
Stony corals are soft, stubby animals that cluster together in colonies joined by limestone skeletons. Each coral is topped by a tiny mouth that takes in dissolved minerals and, occasionally, morsels like copepods. But most of the organism’s energy comes from algae that grow within its tissues in a mutually beneficial arrangement. Plant-like algae cells perform photosynthesis and send sugars and protein building blocks to the corals, and the corals in turn provide shelter and nutrients for the algae.
But extreme temperatures can destabilize the coral and algae’s relationship and lead to coral bleaching. The stress from high temperatures can cause coral to expel its algae and slowly starve to death, which leads it to become a ghostly white.
The researchers behind the new study chose to focus on an organism known as antler or cauliflower coral because it can partner with two different algae, one more resistant to bleaching and the other more susceptible. From 2004 to 2009, scientists visited the same Gulf of California reefs off the coast of Baja California, taking hundreds of samples and measurements to check the coral’s health and physiology when it was paired with each of the two algae partners. They discovered that less than 5 percent of the corals paired with both algae, making it easy to compare the effects of the two algae on their host species.
Corals with heat-resilient algae and susceptible algae showed similar rates of growth. Though the heat-resistant pairing grew more slowly in the winter, it grew more quickly during summer and caught up with the heat-susceptible partners. Similar numbers of egg and sperm in the two groups suggested that both had comparable fertility rates.
Next, the researchers tested how a simulated heat wave would affect the two types of coral. Small cuttings of the corals were glued onto grids and exposed to higher water temperatures for seven days in aquarium tanks at the nearby Autonomous University of Baja California Sur. This stressful episode damaged the metabolism of the susceptible algae and coral pairs but did not affect the energy production in heat-tolerant pairs. In fact, the hot conditions spurred even faster growth for them.
“Going back 20 years ago, 15 years ago, we would jokingly call this the ‘selfish alga,’ because we thought it was keeping all of its resources for itself and almost acting like a parasite inside the coral,” says Warner of the heat-tolerant algae. “And now it’s more complicated.”
Many interacting elements like the species of coral, the type of algae and the local environment’s light and temperature affect the coral-algae pairs’ success rate. This specific duo—antler coral and Durusdinium glynnii algae—shares millions of years of history, and it shows. “When you’re co-evolved for a long time, you work things out,” says senior study author Todd LaJeunesse, an ecologist and evolutionary biologist at Penn State University. “You become highly efficient.”
Prior studies of coral paired with this heat-resistant algae suggested that any advantage during hot temperatures came with serious drawbacks for a coral’s growth and survival. That doesn’t seem to be the case here, says Kira Turnham, lead author of the new study and a coral reef biologist who recently completed her PhD at Penn State.
For at least this type of coral in this region, taking up the heat-tolerant algae means “you are a little more protected to face climate change,” says Ana Palacio-Castro, a coral scientist at the Cooperative Institute for Marine & Atmospheric Studies at the University of Miami who was not involved in the study.
The findings are very promising, according to Mariana Rocha de Souza, a coral reef biologist at the Hawaiʻi Institute of Marine Biology, who was also not involved in the study. She says that ongoing research is expanding the search for heat resistance. Coral reefs in the Red Sea have already survived much higher temperature extremes, suggesting there might be lessons to learn from their coral and algae relationships.
To find and engineer new “super corals” that can withstand rising temperatures, researchers are experimenting on not just the coral themselves but their resident algae. They are searching for more heat-tolerant algae in the wild, breeding more heat-resistant algae in the lab and transplanting the resilient algae into coral nurseries in the ocean.
The search for other heat-tolerant partnerships has led LaJeunesse, Warner and Turnham to a parallel project in Palau, an island country east of the Philippines with an abundance of coral and algae species. Research in Palau has already identified other populations of coral that are heat-resistant and flourishing. Learning as much as possible now about the coral enduring in hotter waters could help inform how to help the remaining coral survive the next 50 years.
“Hopefully in the next year or two,” says LaJeunesse, “we’ll have a comprehensive body of work that really shows there are a lot of super corals out there.”
While the findings are good news for this eastern Pacific species in the near future, finding stronger coral is not a silver bullet against continued harm from humans. “Once we get beyond 2070, 2080, unfortunately all bets are off,” Warner says.