Screening by coral green fluorescent protein (GFP)-like chromoproteins supports a role in photoprotection of zooxanthellae
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Green fluorescent protein (GFP)-like pigments are responsible for the vivid colouration of many reef-building corals and have been proposed to act as photoprotectants. Their role remains controversial because the functional mechanism has not been elucidated. We provide direct evidence to support a photoprotective role of the non-fluorescent chromoproteins (CPs) that form a biochemically and photophysically distinct group of GFP-like proteins. Based on observations of Acropora nobilis from the Great Barrier Reef, we explored the photoprotective role of CPs by analysing five coral species under controlled conditions. In vitro and in hospite analyses of chlorophyll excitation demonstrate that screening by CPs leads to a reduction in chlorophyll excitation corresponding to the spectral properties of the specific CPs present in the coral tissues. Between 562 and 586 nm, the CPs maximal absorption range, there was an up to 50 % reduction of chlorophyll excitation. The screening was consistent for established and regenerating tissue and amongst symbiont clades A, C and D. Moreover, among two differently pigmented morphs of Acropora valida grown under identical light conditions and hosting subclade type C3 symbionts, high CP expression correlated with reduced photodamage under acute light stress.
KeywordsGreen fluorescent protein (GFP)-like proteins Chromoproteins Function Photoprotection Growth Coral-zooxanthellae symbiosis
The study was funded by NERC (NE/I01683X/1 & NE/I012648/1 to JW; studentship to EGS), DFG (Wi1990/2-1 to JW), DAAD (Heron Island Fieldwork to JW), European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 311179 to JW and ARC (Discovery grant to AS et al.). We acknowledge the assistance of D. Owsianka (University of Southampton, LED array construction) and F. Schmitt (University of Ulm, field sampling) and thank Tropical Marine Centre (London) and Tropic Marine (Wartenberg) for sponsoring of the NOCS Coral Reef Laboratory. We would also like to thank the two anonymous reviewers and A. Banaszak for their helpful comments.
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