Preconditioning to high CO2 exacerbates the response of the Caribbean branching coral Porites porites to high temperature stress

Climate change stress on coral reefs occurs as a result of increased temperature and ocean acidification. However, these stressors do not act uniformly: acidification is a 'press' disturbance characterized by chronic increases in CO2, whereas thermal stress is a 'pulse' disturbance characterized by acute episodes of anomalously warm temperatures. Therefore future episodes of thermal stress will develop within the context of pre-existing acidification. Many studies have investigated the effect of combined temperature and CO2 on corals, but no study has yet investigated whether pre-exposing corals to elevated CO2 affects their response to high temperature. We investigated this for the first time using replicate fragments of the Caribbean coral Porites porites preconditioned to either 390 ppm or 900 ppm CO2 at 26 degrees C for 3 mo. After this period, half of the corals from each CO2 level were exposed to 31 degrees C (i.e. 31 degrees C/390 ppm or 31 degrees C/900 ppm) for 2 mo, while the other half were maintained in their original treatments (26 degrees C/390 ppm or 26 degrees C/900 ppm). Calcification, feeding rate, and photochemical efficiency were measured. Corals preconditioned to high CO2 before thermal stress (i.e. 31 degrees C/900 ppm) showed 44% lower calcification rates than the control group, but single stress treatment groups did not experience significant growth reductions. Feeding rates increased for corals exposed to either high CO2 or high temperature singularly, but not when thermal stress was applied following CO2 preconditioning. Photochemical efficiency decreased by 25% for all treatment groups compared to the control. Together, these data suggest that preconditioning to elevated CO2 worsens holobiont response to thermal stress, potentially exacerbating the effects of climate change stressors on coral reefs.