Effects of temperature and UV radiation increases on the photosynthetic efficiency in four scleractinian coral species

Experiments were performed on coral species containing clade A (Stylophora pistillata, Montipora aequituberculata) or clade C (Acropora sp., Pavona cactus) zooxanthellae. The photosynthetic efficiency (Fv /Fm) of the corals was first assessed during a short-term increase in temperature (from 27 °C to 29 °C, 32 °C, and 34 °C) and acute exposure to UV radiation (20.5 W m2 UVA and 1.2 W m2 UVB) alone or in combination. Increasing temperature to 34 °C significantly decreased the Fv /Fm in S. pistillata and M. aequituberculata. Increased UV radiation alone significantly decreased the Fv /Fm of all coral species, even at 27 °C. There was a combined effect of temperature and UV radiation, which reduced Fv /Fm in all corals by 25% to 40%. During a long-term exposure to UV radiation (17 days) the Fv /Fm was significantly reduced after 3 days' exposure in all species, which did not recover their initial values, even after 17 days. By this time, all corals had synthesized mycosporine-like amino acids (MAAs). The concentration and diversity of MAAs differed among species, being higher for corals containing clade A zooxanthellae. Prolonged exposure to UV radiation at the nonstressful temperature of 27 °C conferred protection against independent, thermally induced photoinhibition in all four species.