Ocean acidification affects productivity but not the severity of thermal bleaching in some tropical corals

Increasing carbon dioxide (CO₂) emissions are raising sea surface temperature (SST) and causing ocean acidification (OA). While higher SST increases the frequency of mass coral bleaching events, it is unclear how OA will interact to affect this process. In this study, we combine in situ bleaching surveys around three tropical CO₂ seeps with a 2-month two-factor (CO₂ and temperature) tank experiment to investigate how OA and SST in combination will affect the bleaching susceptibility of tropical reef corals. Surveys at CO₂ seep and control sites during a minor regional bleaching event gave little indication that elevated pCO₂ influenced the bleaching susceptibility of the wider coral community, the four most common coral families (Acroporidae, Faviidae, Pocilloporidae, or Poritidae), or the thermally sensitive coral species Seriatopora hystrix. In the tank experiment, sublethal bleaching was observed at 31°C after 5 d in S. hystrix and 12 d in Acropora millepora, whereas controls (28°C) did not bleach. None of the measured proxies for coral bleaching was negatively affected by elevated pCO₂ at pHT 7.79 (vs. 7.95 pHT in controls), equivalent to ~780 matm pCO₂ and an aragonite saturation state of 2.5. On the contrary, high pCO₂ benefitted some photophysiological measures (although temperature effects were much stronger than CO₂ effects): maximum photosystem II quantum yields and light-limited electron transport rates increased in both species at high pCO₂ whereas gross photosynthesis and pigment concentrations increased in S. hystrix at high pCO₂.The field and laboratory data in combination suggest that OA levels up to a pHT of 7.8 will have little effect on the sensitivity of tropical corals to thermal bleaching. Indeed, some species appear to be able to utilize the more abundant dissolved inorganic carbon to increase productivity; however, these gains offset only a small proportion of the massive bleaching-related energy losses during thermal stress