Reef-building stony corals live in a mutually beneficial partnership with photosynthetic algae that is the core energetic driver of coral reefs. Stressful conditions such as high temperature lead to the disruption of this symbiosis known as coral bleaching, and eventual coral death (Brown, 1997). Climate change-related mass bleaching events have become increasingly common, affecting about 75% of coral reefs in Hawaiʻi and 93% of surveyed corals in the Great Barrier Reef, with over 50% mortality in some regions (Couch et al., 2017;Hoegh-Guldberg et al., 2015). Current climate-change mitigation measures (e.g., the Paris agreement), aiming to limit global warming to 2℃ are unlikely to effectively moderate impacts on coral reefs. As temperature stress events become more frequent and severe, most reefs are projected to face near-annual bleaching by mid-century, leaving these ecosystems under an imminent threat of collapse and

Stony corals heavily rely on their intracellular algal symbionts for energetical supply. Increasing extreme weather driven by climate change often leads to disruption of the symbiosis and to coral death, threatening the sole existence of coral reefs, the key underwater ecosystems. As climate change mitigation outcomes are uncertain, it is important to search for ways to increase coral resilience towards future climate conditions, thermal extremes in particular. It has been shown that corals can withstand stress conditions better after previous exposure, but the mechanism remains unclear. Here we show that after threeday thermal preconditioning, stony coral Pocillopora acuta becomes more resilient to acute heat stress through modulations in cell signaling. In preconditioned corals, the expression of pro-survival gene pBcl-2 increases relatively to pro-death genes pBak and pBax during thermal stress, and the coral bleaching rate significantly decreases. After pBcl-2 activity inhibition, preconditioned corals lose the acquired beneficial phenotype and bleach at the same rate as non-preconditioned corals, which confirms the crucial role of programmed cell death in coral bleaching and acclimatization. The detailed analysis points to the involvement of autophagy/symbiophagy rather than apoptosis in the process. A similar shift in gene expression also occurs in thermally stressed corals that have previously acclimatized to summer temperatures in Kaneohe Bay, Hawai'i, suggesting that corals can naturally increase their resilience to warming events during high-risk periods through alterations in cell signaling.An in-depth understanding of molecular mechanisms underlying coral acclimatization and resilience could open the way for restoration practices such as human-assisted evolution.