Photoinhibition, bleaching susceptibility and mortality in two scleractinian corals, Platygyra ryukyuensis and Stylophora pistillata, in response to thermal and light stresses

In the present study, we examined the effect of thermal stress on the photoinhibitory light threshold in a bleaching susceptible ( Stylophora pistillata) and a bleaching resistant ( Platygyra ryukyuensis) coral. Four light (0, 110, 520, 1015 μmol quanta m −2 s −1) and three temperature (26, 32 and 34 °C) conditions were used over a 3-h period, followed by 24- and 48-h recovery periods at ∼21 °C under dim light. Dynamic photoinhibition could be detected in both P. ryukyuensis and S. pistillata under 520 and 1015 μmol quanta m −2 s −1 at 26 °C and under 110 μmol quanta m −2 s −1 at 32 °C only in S. pistillata. Chronic photoinhibition was recorded under 520 and 1015 μmol quanta m −2 s −1 at 34 °C in P. ryukyuensis, and under 1015 μmol quanta m −2 s −1 at 32 °C and under all light levels at 34 °C in S. pistillata. These results show that high temperature reduced the threshold light intensity for photoinhibition differently in two corals with different bleaching susceptibilities under thermal stress. No visual paling and mortality in P. ryukyuensis was observed at any treatment, even in chronically photoinhibited specimens, while paling and high mortality of S. pistillata was noted in all treatments, apart from samples at 26 °C. These observations suggest a potential role of the host in differential bleaching and mortality determination.