Effects of episodic low aragonite saturation and elevated temperature on the physiology of Stylophora pistillata

• first study to consider short term carbonate- and temperature-stress in combination with a low ΩARAG background signal
• acclimation to low ΩARAG, 24 h exposure to elevated temperature and further lower ΩARAG
• brief exposition to very low ΩARAG has no effect on the photosynthesis of Stylophora pistillata
• short term temperature elevation coerced S. pistillata from O2 production to consumption
• overall, stronger influence of temperature on photosynthesis than ΩARAG during short exposition

As global climate change is predicted to gradually alter the oceans' carbonate system and water temperature, knowledge about the effects an altered marine environment has on the physiology of reef building (hermatypic) coral species is more widely established. However, although it is recognized that seawater temperature and the carbonate system of a coral reef can change rapidly and with great amplitude, little is known about how the interaction of these natural fluctuations with long term effects of climate change may affect the metabolism and productivity of hermatypic corals. To investigate this, we acclimated the hermatypic coral Stylophora pistillata to a “worst case” scenario for carbon dioxide emissions (aragonite saturation state [ΩARAG] = 1.6), and tested how exposure to short term (24 h) elevated temperature (+ 3 °C) and further lowered ΩARAG ( −1 unit) affected its photosynthesis and respiration. While episodic exposure to very low ΩARAG had only little effect on S. pistillata's physiology, short term heat stress caused a shift from net oxygen production to consumption and partial coral bleaching. Higher gross coral respiration, and lowered photosynthetic activity under episodically elevated temperature may have been the result of photoinhibition and partial coral bleaching. These findings suggest that fluctuating environmental conditions in combination with a low ΩARAG background signal may impair basic metabolic processes in calcifying corals. In a future high-CO2 world short term stress could be relevant for reef ecosystem processes, and may affect the resilience of coral reefs to other external influences and effects of climate change.