A multi-molecular biomarker assessment of thermal preconditioning in two scleractinian coral species

Coral reefs face escalating threats from rising sea temperatures, triggering widespread mass bleaching and mortality events. Thermal preconditioning, a process in which corals exposed to sublethal thermal stress become more tolerant to subsequent thermal stress, is increasingly recognised as a promising strategy for restoration purposes. However, its underlying mechanisms remain unclear. Furthermore, species-specific responses to thermal preconditioning must be understood to ensure its broad applicability. In this study, we conducted a comprehensive multi-molecular biomarker analysis to investigate the physiological and molecular effects of thermal preconditioning on two coral species, Pocillopora damicornis and Stylophora pistillata, by exposing them to a sublethal thermal stress of + 3 °C above ambient temperature, followed by an acute stress of 32 °C (+ 8 °C). In both species, preconditioned corals showed delayed bleaching, maintaining chlorophyll a and c2 concentrations, along with Symbiodiniaceae density, for longer periods compared to non-preconditioned corals. At the molecular level, this increased thermal tolerance could be linked to the higher activity of antioxidant enzymes measured, suggesting enhanced protection against oxidative stress and consequently lower levels of lipid peroxidation (LPO) damage. Preconditioned corals also exhibited a slower downregulation of Hsp70 or sustained Hsp70 expression above basal levels during acute heat stress, possibly reflecting prolonged cellular homeostasis. Additionally, they demonstrated delayed or reduced hsp70 gene expression, indicating a less immediate response to thermal stress. Furthermore, the acquired thermal tolerance lasted longer in P. damicornis, highlighting a species-specific response to thermal preconditioning. Our study provides crucial molecular insights into the complex mechanisms of thermal preconditioning, which will be essential for developing more effective and species-targeted preconditioning protocols for practical application in coral reef restoration efforts.