Reef Science: Development Highlights
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
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.
Mazel, Charles H., and Eran Fuchs.
2003. Contribution of fluorescence to the spectral signature
and perceived color of corals. Limnology and Oceanography
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
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.
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.
The following is a fairly complete list
of references since 1919 on coral coloration patterns, especially
the fluorescing proteins.
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