High light alongside elevated alleviates thermal depression of photosynthesis in a hard coral ().

The absorbtion of human-emitted CO by the oceans (elevated ) is projected to alter the physiological performance of coral reef organisms by perturbing seawater chemistry (i.e. ocean acidification). Simultaneously, greenhouse gas emissions are driving ocean warming and changes in irradiance (through turbidity and cloud cover), which have the potential to influence the effects of ocean acidification on coral reefs. Here, we explored whether physiological impacts of elevated  on a coral-algal symbiosis (Symbiodiniaceae) are mediated by light and/or temperature levels. In a 39 day experiment, elevated  (962 versus 431 µatm ) had an interactive effect with midday light availability (400 versus 800 µmol photons m s) and temperature (25 versus 29°C) on areal gross and net photosynthesis, for which a decline at 29°C was ameliorated under simultaneous high-  and high-light conditions. Light-enhanced dark respiration increased under elevated  and/or elevated temperature. Symbiont to host cell ratio and chlorophyll per symbiont increased at elevated temperature, whilst symbiont areal density decreased. The ability of moderately strong light in the presence of elevated  to alleviate the temperature-induced decrease in photosynthesis suggests that higher substrate availability facilitates a greater ability for photochemical quenching, partially offsetting the impacts of high temperature on the photosynthetic apparatus. Future environmental changes that result in moderate increases in light levels could therefore assist the holobiont to cope with the 'one-two punch' of rising temperatures in the presence of an acidifying ocean.