Colonies of the Caribbean coral Montastraea cavernosa exhibit a solar-stimulated orange-red fluorescence that is spectrally similar to a variety of fluorescent proteins expressed by corals. The source of this fluorescence is phycoerythrin in unicellular, nonheterocystis, symbiotic cyanobacteria within the host cells of the coral. The cyanobacteria coexist with the symbiotic dinoflagellates (zooxanthellae) of the coral and express the nitrogen-fixing enzyme nitrogenase. The presence of this prokaryotic symbiont in a nitrogen-limited zooxanthellate coral suggests that nitrogen fixation may be an important source of this limiting element for the symbiotic association.

The congeneric species Montastraea faveolata and Montastraea cavernosa are important hermatypic corals on reefs throughout the Bahamas, Caribbean, and the Florida reef tract that have overlapping bathymetric distributions. However, these congeners differ in their respective abundance at similar depths. The underlying mechanism for these patterns may partly be because of their relative dependence on photoautotrophy versus heterotrophy. The dependence of these two corals on photoautotrophy was examined quantifying the optical properties and productivity of these two species of corals at two different depths in the Dry Tortugas. Maximum surface irradiances in the Dry Tortugas during this study varied from 1,900 to 2,100 mol quanta m Ϫ2 s Ϫ1. Spectral attenuation coefficients calculated from the 1995 and 1996 irradiance data differed by as little as 10% within the visible wavelengths (photosynthetically active radiation [PAR], 400-700 nm), suggesting year-to-year similarities in the optical properties of the overlying water column. Underwater irradiances of PAR were ϳ400 mol quanta m Ϫ2 s Ϫ1 and 25 mol quanta m Ϫ2 s Ϫ1 at 10 m and 18 m, respectively. Significantly lower rates of maximum photosynthesis were observed for samples of M. cavernosa compared with M. faveolata at 10 m and 18 m. For samples of M. faveolata from both depths, the mean chlorophyll-specific absorption (a*) across all PAR wavelengths was greater than that of M. cavernosa. When spectrally corrected for the underwater light field and used to calculate the minimum quantum requirements (1/ m ) of these corals at each depth, we observed that M. faveolata always had higher 1/ m than M. cavernosa (50 versus 18 quanta O 2 Ϫ1 and 39 versus 15 quanta O 2 Ϫ1 at 10 m and 18 m, respectively). M. cavernosa, with its greater pigment concentrations and lower a*, exhibits a significant package effect that results in a smaller functional optical cross section and lower maximum photosynthetic capacities, whereas M. faveolata at the same depths, despite the greater minimum quantum requirements, has a larger functional optical cross section and enhanced absorption of available visible radiation, resulting in a greater maximum photosynthetic capacity. Based on polyp size, corallite structure, and surface area considerations, M. faveolata appears to depend on photoautotrophy versus heterotrophy to a greater extent than its congener, M. cavernosa. Recent data suggest, however, that polyp size alone may not be a good indicator for differences in trophic strategies and that coordinated studies on feeding and productivity in corals are needed to better understand their ecological distributions.

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.

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Solar-induced fluorescence from pigments in the host tissues of reef corals can make a significant contribution to their spectral signature and can affect their apparent color as perceived by a human observer. The relative magnitudes of the reflectance and fluorescence contributions to the spectrum can vary as a function of illumination conditions. We have combined measured coral reflectance and fluorescence spectra with modeled downwelling spectral irradiances to investigate the contribution of fluorescence to coral spectra and color. Variations in depth, fluorescence efficiency, and solar zenith angle were modeled. Fluorescence enhancement is greatest when the coral absorbs light at wavelengths that are transmitted well by seawater and emits light efficiently at wavelengths that are moderately attenuated. The methodology provides a means of predicting the combined reflectance and fluorescence spectral signatures and perceived colors of fluorescent corals under arbitrary illumination conditions.

A self-contained diver-operated underwater instrument is developed to make in situ measurements of spectral fluorescence and reflectance of benthic marine organisms and substrates. The instrument, designated the benthic spectrofluorometer (BSF), is built around a single-board spectrometer with a 1024-element CCD array. Light input to the spectrometer is via a fiber optic cable. In fluorescence mode, the unit provides manual user-selectable excitation wavelength and measures fluorescence emission over the full visible spectrum. The BSF can also measure the spectrum of downwelling ambient light or of light reflected from surfaces of interest, enabling calculation of spectral reflectance. The instrument has been used successfully at several field locations, primarily for investigation of optical properties of coral reef organisms and in collecting baseline data in support of remote sensing systems. The instrument is described and representative data are presented. © 1997 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(97)