Exploring natural mechanisms and active enhancement of coral thermal tolerance

Coral reefs are being impacted by ocean warming and marine heatwaves caused by climate change, leading to the loss and degradation of these ecologically and economically important ecosystems globally. Despite grim predictions for the fate of corals in the future, some optimism can be found in the variability of responses to elevated temperatures among coral species and regions. This thesis examines the influence of reef thermal regime on the natural capacity for resistance and resilience and assesses the ability for active intervention to enhance coral thermal tolerance. First, I monitored the seasonal dynamics of the coral holobiont lipidome and Symbiodiniaceae associations in three species of coral sourced from reefs with distinct daily thermal regimes (variable vs. stable). I showed that energy provision was similar between sites and seasons, but that corals from the thermally variable site hosted more thermally resistant algal symbionts. Next, to compare thermal tolerance thresholds, I exposed the same species of corals from the same reef sites to chronic moderate warming, and then to an acute high temperature that surpassed their mean summer maxima. In general, corals showed adequate to good performance under the chronic warming scenario but experienced substantial bleaching under the higher acute temperature. Last, I examined the effect of thermal conditioning of adult coral colonies on their offspring. I found no clear evidence of transgenerational acclimation that provided a benefit to offspring performance under elevated temperatures. Overall, my thesis shows 1) that in the absence of stressful thermal anomalies, multiple species of corals from reefs with distinct thermal regimes with site- and species-specific Symbiodiniaceae associations exhibit relatively stable energy provision capacity, 2) that corals can show high thermal tolerance under chronic moderate warming, but this resistance is likely not sufficient to provide protection from marine heatwaves, and 3) that adult thermal conditioning may not be able to broadly increase thermal tolerance in coral offspring. Collectively, the finite upper temperature limits of corals and the lack of evidence for enhanced thermal tolerance via transgenerational acclimation highlight the need for urgent action to mitigate climate change and ensure the persistence of healthy coral reefs.