From intensifying wildfires to record-breaking floods year on year, the effects of climate change have manifested in devastating outcomes on ecosystems that threaten species all over the world. One such ecosystem in peril is coral reefs, which play a major role in sustaining biodiversity in the planet's oceans but are facing increasingly severe conditions as waters heat up leading to a phenomenon known as marine heat waves.
For nearly a decade, Katie Barott, assistant professor of biology at the University of Pennsylvania, has led a collaborative team of researchers studying two coral species in Hawaii to better understand their adaptability to the effects of climate change.
Their recent paper published in the Proceedings of the National Academy of Sciences sheds light on this issue, revealing the complex and varied ways corals are adapting, or struggling to adapt, to the rapidly changing oceanic environment.
"We tracked more than 40 large coral colonies over 10 years and found that certain species have an improved ability to endure and recover from subsequent marine heat waves after surviving one such event," Barott says.
"It's a bit like working out; the more often you exercise, the easier it is to go through the same exercise stress."
The researchers studied two dominant coral species in Kaneohe Bay in Oahu, Hawaii: rice coral, Montipora capitata, and finger coral, Porites compressa. Over the decade, these corals were subjected to significant marine heat waves in 2014, 2015, and 2019.
These provided a unique opportunity to identify bleaching-resistant and bleaching-susceptible individuals of each species and then observe their responses to repeated heat stress.
Their findings highlight the resilience of some corals while underscoring the vulnerability of others.
"One of our key discoveries is the role of acclimatization," says Kristen Brown, first author of the paper and a postdoctoral researcher in the Barott Lab. "This refers to the ability of some corals to adjust to higher temperatures, thereby reducing their susceptibility to bleaching, a phenomenon wherein corals expel the algae living in their tissues, causing them to turn white and increasing the risk of death."
The researchers found that bleaching-resistant individuals of both coral species remained pigmented throughout the study period, suggesting a persistent form of thermal tolerance; however, pigmentation alone was not a definitive indicator of overall health or resilience.
The researchers reveal contrasting recovery patterns between the bleaching-susceptible individuals of each species following the heat waves.
Montipora capitata, despite some evidence of acclimatization, repeatedly experienced bleaching and showed significant mortality for as long as three years after the last heat wave; conversely, initially sensitive individuals of Porites compressa exhibited a remarkable capacity for recovery and acclimatization, with no bleaching or mortality during the third heat wave and most physiological metrics returning to normal within one year.
This difference underscores a critical aspect of coral resilience: the ability to not only survive heat stress but to effectively recover from it.
The researchers suggest that coral responses to climate change are diverse and complex, influenced by a range of factors, including species-specific characteristics and past exposure to environmental stressors. And, as a next step, the team plans to continue monitoring and exploring aspects like coral growth, calcification, and the impacts of recurring marine heatwaves.
