Thermal refugia indicate physiological evolution in Atlantic reef corals

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Abstract Coral reefs are increasingly threatened by climate change, especially by thermally driven bleaching events. Southwestern Atlantic reefs (Brazil) have been proposed as potential climate refugia owing to high turbidity, which attenuates light and mitigates thermal stress. In contrast, Northwestern reefs (Bermuda) experience greater light penetration and serve as a comparative system, given their phylogenetic proximity to Brazilian corals. We hypothesized that Brazilian species would differ physiologically under their selective regimes. Indeed, they had higher chlorophyll a packing, sustaining comparable autotrophy and lower oxidative stress. Heterotrophic capacity, however, was similar between regions, challenging the turbidity-feeding paradigm. We also expected that Brazilian species would have greater tolerance to bleaching under simulated climate change. Although corals from both regions bleached, Brazilian representatives increased chlorophyll a packing 3.1-fold and maintained lower oxidative stress, while trophic behaviour remained stable. Calcification persisted under stress, indicating functional resilience across regions. These findings show that symbiosis and oxidative status, rather than trophic strategy, distinguish corals from Brazil, suggesting a compensatory role of chlorophyll a packing and enhanced redox balance under low-light regimes, the latter possibly reflecting relaxed selection and, ultimately, driving convergent physiological evolution. This work offers ecophysiological insight to guide conservation strategies, highlighting refugia as conservation priorities under local-scale stressors.

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