The dynamics of thermal stress events determine whether thermotolerant endosymbionts improve or hamper survival of coral reefs
Coral reefs are increasingly threatened by marine heatwaves which can cause devastating coral bleaching events. Despite this, corals can sometimes survive often altering their symbiont composition toward more thermally-tolerant taxa in response to thermal stress. The long-term efficacy of such a strategy remains poorly understood, in particular, the function of bleaching as an acclimation agent. Here, we use a model of coral-symbiont dynamics, which includes novel and empirically intractable aspects of symbiont thermal tolerance, to investigate how changes in coral endosymbiont composition influence reef survival under differing patterns of heat stress. We find that thermal stress-induced bleaching can act as an acclimation mechanism, but its effectiveness is modulated by the interplay between thermal stress intensity and frequency. When thermal stress remains moderate in frequency and intensity, acclimation can occur as longer intervals between stress events promote post-bleaching coral recovery via a beneficial sequence of hosting thermally-tolerant then thermally-sensitive symbionts, interspersing thermal resilience with periods of faster coral growth. In contrast, when thermal stress intensity increases rapidly and occurs at higher frequency, sustained dominance of thermally-tolerant symbionts is favoured, providing a temporary survival strategy that delays reef collapse but with the trade-off of lower coral growth. While in this latter scenario, acclimation may be less effective due to disrupted reef population structure, it does provide a mechanism for corals to "buy time", during which other adaptive processes may occur enabling longer-term coral survival. However, under a real-world low emission IPCC future scenario, even this flexible survival strategy may still lead to an immediate and sustained fall in coral cover. By capturing both, 1) thermal frequency and intensity dynamics, along with, 2) symbiont thermotolerance preferences and linked growth trade-offs we provide a coral survival model which can be used as a tool to inform strategic coral reef management frameworks promoting reef resilience.