Thermal preconditioning postpones coral bleaching by inducing an antioxidant response and cellular defense mechanisms in two coral species, Pocillopora damicornis and Stylophora pistillata.

Coral reefs worldwide are declining due to high sea surface temperatures, causing coral bleaching and the endosymbiosis rupture between the coral polyps and the unicellular algae Symbiodinaceae. Bleached corals will likely die because Symbiodinaceae algae provide most of the energy coral colonies need. Among the newly developed strategies to prevent coral bleaching is thermal preconditioning, i.e. applying sub-lethal temperature on corals before heat stress which has shown promising results in slowing coral bleaching. However, the cellular mechanisms that increase heat tolerance by thermal preconditioning are still unknown. This study showed that thermal preconditioning on the two coral species, namely Pocillopora damicornis and Stylophora pistillata, postponed the adverse effects of heat stress-inducing bleaching. Thermal preconditioning was performed by exposing the coral colonies to the sub-lethal temperature of 28°C for one week, after which thermal stress was applied at 31°C for another seven days. We assessed coral bleaching by analyzing the Symbiodinaceae density, taxonomical identification, and chlorophyll concentration (Chl). Moreover, we compared the expression of Heat shock protein 70 (Hsp70), a molecular chaperone involved in cell protein protection, the activity of three antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase and the lipid peroxidation in preconditioned, non-preconditioned and control colonies. Our results showed that Chl concentration and symbiont density were higher in preconditioned corals than non-preconditioned corals in both coral species. In addition, no difference in Symbiodinaceae identity was observed. Then, preconditioned S. pistillata and P. damicornis had higher antioxidant enzymes activity and higher expression of Hsp70 compared to non-preconditioned colonies. Therefore, our results suggest that thermal preconditioning delayed coral bleaching in both S. pistillata and P. damicornis by mitigating oxidative stress and maintaining cellular homeostasis, indicating the possible role of thermal precondition as a valuable technique for enhancing thermal resistance of coral colonies used worldwide in coral restoration projects.