What did heat-acclimated corals gain—and give up—over six years?

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Growing evidence suggests that reef-building corals can acclimate to challenging temperatures by increasing their thermal resistance, raising hopes for their persistence in a warming world. But the potential trade-offs accompanying those gains have been largely unexplored.

In a first report on physiological trade-offs in the globally abundant and ecologically relevant coral Pocillopora acuta, researchers compared long-term exposure to 31 °C with corals of the same species cultivated under a cooler control thermal regime. At both temperatures, the corals consistently looked healthy throughout the six-year study.

At 31 °C, respiration and photosynthesis rates increased, resulting in greater biomass accumulation and total energy reserves than in corals under the ambient regime. Host tissue composition also shifted toward lipid build-up, suggesting a change in energy storage. The increased biomass growth came at the cost of declining skeletal growth rates and denser skeletons. In the long term, the researchers expect this trade-off to reduce extension rates, with potentially major consequences for reef building and reef community composition.

Symbionts at 31 °C were physiologically more compromised and had lower overall energy reserves. This may indicate greater exploitation by the host and potentially lower stress resilience. The researchers describe their findings as initial insight into a successful thermal acclimation mechanism that prioritized energy storage over skeletal growth while placing greater demands on symbionts.

The six-year observation differs from short-term studies in which elevated temperatures depleted coral tissue lipids, highlighting the importance of studying acclimation over biologically relevant timescales. The researchers call for further investigation of trade-offs on those scales and under acute heat stress to better understand the future trajectory of coral reefs. They also say these insights could improve interventions intended to increase coral thermal resilience using thermal preconditioning for stress-hardening.

A graphical abstract is noted.