Ocean warming is driving an increase in the frequency and severity of marine heatwaves, causing untold damage to coral reefs. Tropical corals, which live in symbiosis with tiny single celled algae, are sensitive to high temperatures, and exhibit a stress response called bleaching when the ocean gets too hot. In the last 4 decades, marine heatwaves have caused widespread bleaching, and killed millions of corals. Because of this, a global search is underway for reefs that can withstand the heat stress, survive future warming, and act as sources of heat-tolerant coral larvae to replenish affected areas both naturally and through restoration.
Now, scientists studying reefs in Palau, an archipelago in the western tropical Pacific, have identified genetic subgroups of a common coral species that exhibit remarkable tolerance to the extreme heat associated with marine heatwaves. Further, the scientists found evidence that larvae from these corals are traveling from their birthing grounds deep in Palau’s lagoons, to the outer reef, where they survive and grow, and maintain their heat tolerance.
Understanding both the underlying mechanisms that facilitate heat tolerance of these corals, as well as the dispersal capabilities of their larvae will go a long way toward enhancing coral reef conservation and restoration efforts in the 21st century ocean, according to scientists at the Woods Hole Oceanographic Institution (WHOI) who led the research.
In Palau’s main lagoon, a network of very ancient, fossilized reefs has been uplifted to form a series of mountains known as the Rock Islands. These formations slow water flow in and around them, creating localized environments in which the water temperatures are consistently higher than other areas of Palau’s reefs.
Scientists sampled the keystone coral species Porites lobata (lobe coral) across Palau, including the Rock Islands. They took skeletal biopsies and examined the cores for stress bands, which are telltale signs of bleaching, a stress response corals have to high temperatures. They found corals from the Rock Islands bleached less during the intense 1998 heatwave than corals from other areas of the reef, indicating enhanced thermal tolerance.
Scientists then investigated the genetics of the corals and discovered four distinct lineages within the same species. Within the warmer Rock Islands, certain lineages, designated as “LB” and “RD” lineages, were much more common. The scientists were able to match the genetics of each coral with its own bleaching history and found that fewer individuals from the “LB” and “RD” lineages bleached during 1998, indicating enhanced thermal tolerance.
Remarkably, the scientists found the LB lineage was not restricted to the Rock Islands. They found some LB colonies also living on the cooler outer reefs. An examination of the bleaching histories of these colonies again revealed fewer stress bands, indicating that they maintained the thermal tolerance characteristic of their relatives in the Rock Islands.
“This suggests that the Rock Islands provide naturally tolerant larvae to neighboring areas,” the scientists write in the paper titled “Palau’s warmest reefs harbor thermally tolerant corals that thrive across different habitats,” published in Communications Biology, a journal published by Nature. “Finding and protecting such sources of thermally-tolerant corals is key to reef survival under 21st century climate change.”
“As oceans worldwide continue to warm, corals derived from extreme habitats will be at a competitive advantage and may enable the survival of otherwise vulnerable reefs,” the authors continue. “Identifying and safeguarding natural breeding grounds of environmentally tolerant corals that can thrive under future climate conditions will be fundamental to the persistence of coral reef ecosystems worldwide in the coming decades.”
“We found that some of Palau’s reefs with the highest temperatures have corals that are more tolerant than one would expect,” said the paper’s lead author Hanny Rivera, a graduate of the MIT-WHOI Joint Program. Rivera, who conducted this work as part of her Ph.D. and postdoctoral research, is currently an associate director of business development at Ginko Bioworks. “In addition, they are genetically distinct from the same corals found in other parts of Palau, which suggests that there has been natural selection for hardier corals in these regions..”
Paper co-author Michael Fox added that the study is particularly exciting because it combines coral genetics with historical records of bleaching preserved in their skeletons to shed light on how corals from extreme habitats with high temperature tolerance can be dispersed across a reefscape. “This integrated perspective is essential for improving projections of coral communities in a warming ocean,” said Fox, who was a postdoctoral scholar at WHOI during the research for this paper. He currently is an assistant research professor in the Red Sea Research Center at King Abdullah University of Science and Technology in Thuwal, Saudi Arabia.
The Palau research is directly related to the Super Reefs initiative WHOI launched with The Nature Conservancy and Stanford University to locate coral communities that can withstand marine heat waves, and work with local communities and governments to protect them.
“This work is the scientific basis for the Super Reefs initiative,” said paper co-author Anne Cohen, a scientist at WHOI and Rivera’s advisor on the study. “The Palau research demonstrates that Super Reefs exist and also provides actionable science knowledge that can be used to support their protection.”
Cohen noted that there are other coral reefs, not just in Palau, where coral communities have not bleached as severely as scientists predicted based on the levels of thermal stress. “When we find the coral communities that are heat-tolerant or bleaching-resistant, and we protect them from other stresses that can kill them — like dynamiting, overfishing, or coastal development — they will produce millions of larvae that will travel on the currents, outside of their places of origin as we see on Palau, and they will repopulate reefs that have been devastated by heatwaves,” she said. “Nature is amazing. Our job with the Super Reefs initiative is to protect these thermally resilient reefs and let nature do the rest.”
Rivera added she is in awe of the immense appreciation, respect, and stewardship that the Palauan people have for their environment.
“They have been one of the pioneering countries in promoting marine conservation and ecological protection. It is wonderful to know that these special reefs are in such good hands,” Rivera said. “It is my greatest hope that our research will further support the Palauan people in their efforts to maintain a healthy marine ecosystem.”
Funding for this research was provided by the National Science Foundation, The Seija Family, The Arthur Vining Davis Foundation, the Atlantic Charter Donor Advised Fund, The Dalio Foundation, Inc., the MIT Sea Grant Office, the Woods Hole Oceanographic Institution Coastal Ocean Institute Grant and Ocean Venture Fund, the National Defense Science and Engineering Graduate Fellowship Program, the Martin Family Fellowship for Sustainability the American Association of University Women Dissertation Fellowship, and an Angell Family Foundation Grant.