Different sensitivity of Mediterranean scleractinian corals to seasonal changes in temperature along a natural CO2 gradient

The predicted CO2-driven ocean acidification (OA) of the coming century has prompted urgent research to assess its potential effects on the marine environment, with strong emphasis on calcifying organisms. Here we assessed the effects of in situ exposure to different pH conditions on the mortality and growth of three Mediterranean scleractinian corals: the solitary zooxanthellate Balanophyllia europaea, the solitary non-zooxanthellate Leptopsammia pruvoti and the colonial non-zooxanthellate Astroides calycularis. The corals were transplanted in proximity to a volcanic CO2 vent where water is naturally acidified to levels matching future IPCC scenarios. In all three species, mortality at reduced pH was exacerbated when seawater temperatures peaked in summer. Growth rates of the zooxanthellate species did not react to reduced pH, while those of the two non-zooxanthellate species were negatively affected, indicating that zooxanthellate species may be more resistant in a high CO2 world. Natural CO2-leaking marine sites can provide essential information on the combined effects of OA and global warming on Mediterranean scleractinian corals, which may help understand to which extent different corals will be threatened by increasing atmospheric CO2 pollution.