Under high light stress two Indo-Pacific coral species display differential photodamage and photorepair dynamics

The in hospite Symbiodinium symbiont of corals on shallow reefs relies on photoprotection and photorepair during periods of exposure to short-term high light and/or temperature stress. A coral’s susceptibility to bleaching is species specific and determined not only by Symbiodinium type, size and physiology, but also by coral host features. Here, photoprotective, photorepair, photochemical and non-photochemical efficiency parameters of Symbiodinium harboured in two morphologically different coral species were examined on Heron Island (23.4420°S, 151.9140°E) in July 2011. The two coral species were exposed to high light stress for 96 h, with or without inhibition of photosystem (PS) II repair by lincomycin. Symbiodinium harboured in Pocillopora damicornis showed an increase in xanthophyll de-epoxidation under high light exposure, whereas algal symbionts in Pavona decussata showed constant levels of xanthophyll de-epoxidation. High light-treated specimens of P. damicornis maintained steady PsbA protein (D1 protein) content throughout the experiment, but P. decussata showed a peak in PsbA protein content after 48 h of exposure. In hospite Symbiodinium in P. damicornis had greater content of PsbA protein fragments, suggesting higher accumulation of photodamaged products, compared to Symbiodinium in P. decussata, where both maintained steady PSII photochemical capacity over 96 h of exposure. Under inhibition of PSII repair, both species lost PsbA protein content and PSII photochemical capacity. Both species showed increased heat dissipation under inhibition of PSII repair, but differed in photoprotective strategies and photorepair activity. Our results suggest that, as well as any differences in the symbiont, characteristics of the coral host can alter important physiological responses in Symbiodinium.