The role of host-based color and fluorescent pigments in photoprotection and in reducing bleaching stress in corals

Coral tissue colors result from both the intracellular symbiotic dinoflagellates and the host's cellular pigments. The brownish colors are due to the symbionts' photosynthetic pigments; and the bright purple-blue and fluorescent colors produced by the coral host are proteins closely related to Green Fluorescent Protein (GFP). One of the documented biological functions of GFP-like pigments in corals is light optimisation. When light is excessive, GFPs reduce symbiont photoinhibition and photo-damage to coral tissues. We explored the ecological roles of coral coloration and found that spectral properties of GFP-like pigments of many common, shallow reef corals match the absorption of dinoflagellate pigments and demonstrate strong evidence for their photoprotective function by absorbing high energy, damaging solar radiation, dissipating excess energy via fluorescence and by overall photon removal/deflection. A survey of the depth-related distribution of fluorescent and purple-blue color-morphs further confirmed the role of GFPs in photoprotection. These color-morphs were most abundant at high-light shallow depths and their numbers dropped with increasing depths. However, at deeper sites, numbers of fluorescent color-morphs increased, suggesting a reversal of GFP function from sunscreening to that of light amplification in light-limited habitats. We also examined the degree of bleaching damage in different color-morphs during and following the 2002 mass bleaching event. We found that low-GFPpigmented morphs had significantly higher degrees of damage to their symbionts, especially to photosynthesis and a higher degree of partial colony mortality than in high-GFP-pigmented morphs. These results further substantiate earlier findings that GFP-like pigments reduce photoinhibition and the severity of bleachingrelated physiological damage of corals