Coral Reef Science:  Development Highlights

Eric Borneman

This month's science review begins with coverage of what will likely be exciting articles for reef-keepers. Light and its effect on coloration are among the more well discussed topics in the hobby, and until recent years, very little was known about coral coloration. However, recent studies have brought to "light" the ecological and biological reasons for most coloration in Pacific corals (see references below). In particular, coral coloration is a factor of various combinations and types of animal-based, non-zooxanthellar, colored fluorescing proteins, some of which act as photoprotectants to high irradiance visible light in shallow water corals, others that act to channel non-photosynthetically active radiation into usable wavelengths like a "color shifting" mirror in deep water corals.

Charles Mazel, one of the authors in three of these papers, is best known for his work with photographing and analyzing fluorescent spectral signatures of corals (see, for example, www.nightsea.com). His contributions to scientific literature have been less prolific in terms of coral photobiology, but these articles mark a departure and introduce findings for Caribbean corals that, while not directly comparable to the work on Pacific species, indicates that the proteins and their functions may be different in Caribbean species. The authors try to extrapolate their findings to controvert earlier works; however, I do not see that their work should be extrapolated, that the evidence supports such suggestions, nor that the materials and methods are replicative of the other works. It is my editorial comment that these works are quite dissimilar and, if anything, indicate that the nature of coloration is even more complex than has been assumed, and that the various pigments involved, their roles in coral photobiology or other metabolic functions, and aspects of their production are far from completely understood.

Zawada, David G., and Jules S. Jaffe. 2003. Changes in the fluorescence of the Caribbean coral Montastraea faveolata during heat-induced bleaching. Limnology and Oceanography 48(1): 412-425.

Summary:

Using orange and green morphs of the Caribbean large-polyped coral, Montastraea faveolata, the authors subjected the corals to thermally induced bleaching conditions for 28 days and then allowed to recover for 53 days. The primary goal was to analyze the use of fluorescing proteins as stress proxies for bleaching. Spectral signatures of reflected and fluoresced pigments were made, and it was found that distribution of fluorescing proteins was primarily centered around the mouth and oral disk, and did not seem to indicate a photoprotective role for the proteins in this species, but perhaps a use in gonad protection. Variability abounded in this work, with some technical and methodological challenges occurring, resulting in analysis of trends and not absolute values. Interesting is that three primary pigments, green fluorescing protein (585nm), orange fluorescing protein (515nm), and chlorophyll (685nm), had varying responses to elevated temperatures across samples, and it was suggested that the "odd mixture" of zooxanthellae within the host coral, and the potential presence of another non-dinoflagellate symbiont, perhaps a cyanobacterium, could account for the variations in thermal sensitivity and ratios of the fluorescing proteins over time and differing thermal stresses. In any case, there was little correlation between the chlorophyll and the fluorescing proteins in terms of their use as a proxy for bleaching, nor was there good evidence suggesting a biologically important role for the fluorescing compounds as was suggested for Pacific corals.

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Mazel, Charles H., and Eran Fuchs. 2003. Contribution of fluorescence to the spectral signature and perceived color of corals. Limnology and Oceanography 48(1): 390-401.

Summary:

Using species of Agaricia, Montastraea, Scolymia, Diploria and Colpophyllia from the Caribbean, the authors note that coral coloration is due to fluorescence and reflectance on pigments located in both zooxanthellae and host tissues. Some fluorescence is much less readily observed, at least visually, in corals. The perceived color is a function of the saturation of the pigment, and the reflectance and fluorescence from the wavelengths striking it. Two terms are defined - overt fluorescence, where fluorescence is detectable under natural illumination; and covert fluorescence, where specific light sources are required to observe it. In the latter case, the fluorescence is simply overwhelmed by reflected light of a perceived different color, such as chlorophyll. The authors found the most visually apparent fluorescence to occur with fluorescing proteins that absorb wavelengths well transmitted in seawater (i.e. blue and UV), that fluoresced efficiently, that emit at wavelengths only slight different from those they absorb, and at wavelengths human eyes are most sensitive to detecting. Orange was particularly high in these characteristics. In the discussion, the author's present suggestions as to the reasons for coloration, but arrive at nothing conclusive; among other things suggested were responses of reef fish. They conclude that more information about habitat and color morphs are required, experiments to determine the expression of color and the relation of pigment structure and functions are desirable to understand more about how and why corals have various colors. Perhaps most interesting to aquarists is that this study illustrates that coral color by humans and the various fluorescence and reflectance produced by various light spectra are largely perceptual and a function of the specific compounds. Thus, it appears that "better coloration" as is so often assigned to various lamps on aquariums is likely "all in the eye of the beholder" and that they are not actually producing "more colorful corals."

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Mazel, Charles H., Michael P. Lesser, Maxim Y. Gorbunov, Thomas M. Barry, Julianne H. Farrell, Kevin D. Wyman, and Paul G. Falkowski. 2003. Green-fluorescent proteins in Caribbean corals. Limnology and Oceanography 48(1): 402-411.

Summary:

In this study, green fluorescing protein was examined in two Caribbean species of Montastraea. They found no significant correlation between depth and concentration of pigment. They also determined that, based on the concentrations and distributions of the pigments, that they have a negligible role in impacting the amount of solar radiation reaching the zooxanthellae and seem to play neither a photoprotective role, nor any role in photosynthesis. This study was a bit disconcerting in that, while the techniques used to measure and quantify fluorescent pigments were sound, the species were limited to a single genus. They mention the presence of GFP in numerous other taxa that were not studied in this manner at all, and seem to randomly perform some work on certain species, while not on others, using the various species they did use to support work on other species not studied. In conclusion, they suggest that while they do not know a function for the fluorescing proteins, it counters works suggesting a function in important hermatypic Pacific corals. They even suggest the presence of fluorescing proteins, while perhaps serving any number of putative functions, may simply be a remnant genetic trait from earlier corallimorpharian ancestors and serve no function at all. However, I would note that the functions might be different across taxa, and different in Caribbean and Pacific species. A lack of depth related or photobiological function in one type of fluorescent protein in an unmentioned number of two congeneric species around one island is not likely to be representative of all corals.

References:

The following is a fairly complete list of references since 1919 on coral coloration patterns, especially the fluorescing proteins.

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