A new study led by researchers at McGill University and the Smithsonian Tropical Research Institute (STRI) in Panama reveals why some corals resist bleaching while others don't: the answer lies in a complex partnership between corals and their microbial allies, shaped by the history of the waters they inhabit.
As climate change warms the world's oceans, coral reefs - ecosystems that support one-quarter of all marine biodiversity - are increasingly at risk. Bleaching occurs when stressed corals expel the algae living within them, causing the corals to lose their characteristic colours and putting them at greater risk of death. Coral reefs' ecosystems sustain various marine species, so the loss of reefs have broader impacts.
Corals living in waters with frequent temperature swings are better at surviving extreme heat than are corals in waters with more stable temperatures, the researchers found. This suggests that their resilience depends not only on their genes, but on their environment. The stability of the algae and bacteria living inside the coral was also found to play a role. These microbial allies include symbiotic algae, which provide energy, and bacteria, which help regulate stress and disease.
"Understanding what makes some corals more resilient to increasing temperatures helps us figure out how reefs might survive climate change and where to focus conservation efforts," said Victoria Glynn, lead author of the study, who conducted the work as a PhD student in McGill's Department of Biology and STRI.
The team studied coral reefs in two regions of Panama's Tropical Eastern Pacific, each with very different ocean conditions. In the Gulf of Panama, strong seasonal upwelling - a process that causes cold water to rise toward the surface - brings rapid fluctuations in temperature and water chemistry. In the Gulf of Chiriquí, conditions are much more stable.
To test how corals from each region respond to extreme heat, the researchers collected samples and used a specialized lab system called the Coral Bleaching Automated Stress System (CBASS), which mimics marine heatwaves, allowing observation of how corals respond to sudden stress.
They also analyzed the coral's genetic background and their microbiomes - algal and bacterial communities - of each coral and tracked physiological changes like antioxidant capacity and protein content.
More heat tolerance in corals from unstable waters
Despite sharing much of the same DNA, corals from the Gulf of Panama were more heat-tolerant than those from the Gulf of Chiriquí.
"Minor genetic differences seem to contribute to thermal tolerance, but the environmental context makes a big difference," said McGill biology professor Rowan Barrett, who supervised Glynn's doctoral research project.
In lab tests, Gulf of Panama corals maintained their protein levels and resisted oxidative damage more effectively than did their counterparts from the more stable Chiriquí. However, the team also found that bacterial microbiomes from both locations corals became more unstable and variable under heat stress, a sign of bleaching vulnerability.
One of the study's most surprising findings challenges conventional wisdom about coral-algae relationships. While many stressed corals in other regions switch to Durusdinium algae - associated with higher heat tolerance some corals often retained Cladocopium at high temperatures, which provides more energy but less heat protection.
"This suggests there's a trade-off between energy supply and heat resistance," said Barrett.
Implications for reef survival
The findings support the idea that corals regularly exposed to variable conditions may be "pre-adapted" to future climate extremes. That could help explain why the Gulf of Panama's reefs were able to recover from the catastrophic 1982 El Niño event.
"Panama's reefs give us a natural laboratory to understand resilience," said STRI's Sean Connolly, a research biologist who co-advised Glynn's doctoral work.". "By studying how coral communities adapt to their environment, we can better predict which reefs are most at risk - and which might bounce back."
About the study:
The role of holobiont composition and environmental history in thermotolerance of Tropical Eastern Pacific corals was published in Current Biology and co-led by Victoria Glynn and Rowan Barrett from McGill University, and Sean Connolly, Matthieu Leray, David Kline, and Laura Marangoni from the Smithsonian Tropical Research Institute.
Funding was provided by the Mark and Rachel Rohr Foundation. Additional support was provided to lead author Victoria Glynn through a Fulbright U.S. Scholar Grant and a Vanier Canada Graduate Scholarship. Further support came from the Smithsonian Tropical Research Institute, NSERC, and others.
