Green-fluorescent proteins in Caribbean corals

Fluorescent pigments in several Indo-Pacific and Caribbean anthozoans have recently been identified as proteins related to the green-fluorescent protein (GFP) of the hydromedusa Aequorea victoria. Here we show that GFP is widely distributed in many Caribbean species. The fluorescence excitation and emission spectra for the pigment are similar to those reported elsewhere for coral and noncoral GFP and the fluorescence quantum yield is estimated to be 35%. Spectral and molecular characterization of the isolated protein clearly show it to be GFP, and laboratory and in situ fluorescence measurements and Western blot analysis show that it is widespread. Bathymetric studies of GFP content using Western blots for the ecologically important congeneric corals Montastraea faveolata and Montastraea cavernosa show that there is no significant correlation between depth and GFP concentration. Nucleotide sequence data of GFP from M. faveolata and M. cavernosa show 88.2% sequence homology with each other and 46.4% homology with A. victoria GFP, whereas the percent homology with A. victoria at the amino acid level was 31.1 and 28.4% for M. cavernosa and M. faveolata, respectively, and 82.7% with each other. Measurements of reflectance and of the excitation spectrum for chlorophyll fluorescence in GFP-containing corals indicate that GFP absorption, emission, and reflection have negligible impact on the level of solar radiation reaching the zooxanthellae and therefore play no role in coral photosynthesis by either addition or removal of photons. Zooxanthellate corals contain a number of fluorescent substances, one of which is chlorophyll in the endosymbiotic dinoflagellates of the genus Symbiodinium (5zooxanthellae). This photosynthetic pigment contributes a red fluorescence, with peaks near 685 and 735 nm, from the gastrodermal cells in which the zooxanthellae reside. Other pigments fluoresce at shorter wavelengths and are usually contained in the epithelial cells of the cnidarian host (Kawaguti 1944; Logan et al. 1990; Mazel 1995; Salih et al. 2000), although they may at times be located in gastrodermal tissue (Schlichter et al.