Differential impacts of light on coral phenotypic responses to acute heat stress

Reef building corals are in global decline due to climate change and other pressures. Yet, corals show wide variation in physiological responses to heat stress. The influence of light is often dismissed when characterising variation in coral thermal tolerance. Hence, we investigate how light levels affect the photochemical and bleaching responses that typically inform coral thermal tolerance in acute heat-stress experiments using corals from two habitats. Despite natural variations in temperature regimes between sites, both coral populations responded similarly to acute heat-stress across seasons. The decline in the photochemical response (Fv/Fm) after heat stress (winter: +9 °C, summer: + 7.5 °C) was small on average (≤ 6 %) indicating no light stress, but consistently greater in high light (16–24 Daily light integrals, DLI) compared to low light treatments (4–11 DLI). Fv/Fm responses varied per genotype (+3 % to −50 % relative change) and despite small declines in Fv/Fm, all genotypes lost pigment in summer (10 % to 45 % relative change in tissue colour) and paled significantly irrespective of light treatment. These results highlight the complexity of the bleaching response to the primary drivers of temperature and light stress. Further research is needed to understand how light levels affect acute heat-stress responses across studies particularly when thermal tolerance is based on single phenotypes. Incorporating light as a contributing factor to thermal stress indicators will advance our understanding of coral temperature thresholds, to improve coral bleaching predictions based on ubiquitous environmental drivers.