Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals: Evidence for photoinhibition and photoprotection

Diurnal patterns of photoinhibition have been identified in seven species of shallow water reef corals from the Andaman Sea, off the west coast of Thailand, using pulse amplitude fluorometry. Photochemical efficiency (F(v)/F(m)) and quantum yield (ΔF/F(m)) of symbiotic dinoflagellates within the corals declined after dawn to reach a minimum between midday and early afternoon, recovering to former dawn levels by early evening. Parallel studies on the xanthophylls diadinoxanthin (Dn) and diatoxanthin (Dt), and their inter-conversion, also revealed a strong diurnal pattern as well as inverse correlations between the xanthophyll ratio Dt/(Dn + Dt) and F(v)/F(m) and ΔF/F(m). These findings suggest a photoprotective function for these pigments.