Shading does not lower thermal tolerance in the coral Montipora capitata: implications for conservation intervention

Marine heatwaves trigger severe coral bleaching events that result in dramatic losses of coral reefs worldwide. An increasingly common method used to mitigate coral bleaching is to shade portions of reefs. However, as shaded corals become less exposed to environmental stress, shading has also been hypothesized to lower their thermal tolerance, which would be detrimental for their survival. Here, we investigated how long-term shading modifies the response of Montipora capitata to both light and temperature stressors. After two years of growth, we used the Coral Bleaching Automated Stress System (CBASS) to compare responses of 73% shaded and unshaded corals during 1) temperature stress under ambient light, 2) temperature stress under low light, and 3) ambient temperature with light stress. Results show that shaded corals are less resistant than control ones to light stress and to heat stress under high light, but not to heat stress under low light, where no difference was detected. We further demonstrate that low light intensity during heat stress reduces the decline of photosynthetic efficiency when compared to heat stress at ambient light, regardless of the light history of the coral colonies. Our data support light-mediated stress, both independent of and synergistic with heat-induced stress, and suggest that light and heat stresses trigger different molecular and cellular pathways. We hereby confirm the benefits of coral shading under both short- and long-term temperature stress, and underline the importance of light acclimation in the conservation and restoration of coral reefs.