Relationships among thermal stress, bleaching and oxidative damage in the hermatypic coral, Pocillopora capitata

To examine the response to exposure to a thermal gradient in coral, we assessed the effect of a gradual 10 °C temperature increase (22 to 32 °C over 10 h) on normal (N), partially bleached (P) and control (C) samples collected from different branches of the same coral (Pocillopora capitata). We examined markers of oxidative stress, including lipid peroxidation (MDA) and superoxide dismutase (SOD) activity, indicators of bleaching, including chlorophyll a (Chl a) and carotenoid pigment (PC) levels, as well as zooxanthellae density. Our results revealed that N, P and C coral samples all contained higher levels of PC versus Chl a. The levels of both pigments increased as the temperature increased from 22 to 28 °C only in N and C samples, whereas P samples showed less cellular damage than N and C samples at temperatures between 26 and 28 °C, and had greater antioxidant activities at temperatures between 26 and 30 °C. The rate of zooxanthellar expulsion consistently increased with temperature in all three coral types across the entire temperature range. Collectively, these results indicate that temperature has a direct effect on the antagonistic relationship between temperature-induced damage and protective antioxidant mechanisms in this type of coral.