[L&O Featured Article] L&O Featured Article, Vol. 50 (4) July 2005

[lo-feature at aslo.org]

The Featured Article in the July 2005 issue of L&O is:

Enríquez, Susana, Eugenio R. Méndez, and Roberto Iglesias-Prieto. 2005.
Multiple scattering on coral skeletons enhances light absorption by
symbiotic algae. Limnol. Oceanogr. 50(4): 1025-1032.

The article is freely available at:

http://aslo.org/lo/toc/vol_50/issue_4/1025.pdf

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Introductory comments by Michael P. Lesser (L&O Associate Editor)
 
In the pages of Limnology and Oceanography and elsewhere, coral reef
biologists worldwide are pre-occupied with the overall decline in coral
reefs as a result of systematic anthropogenic insults, and in particular
studies on thermal stress and the bleaching phenomenon dominate the coral
reef literature these days.  Some coral biologists have even suggested that
this pre-occupation with the bleaching phenomenon has kept us from making
significant advances in our basic understanding of algal-invertebrate
symbioses (Edmunds and Gates 2003).  The stress response that leads to
bleaching in corals has, however, provided the impetus for new areas of
study (e.g., Symbiodinium molecular genetics and physiology), and therefore
new insight into the biology of symbiotic associations.  One area of
historical study in coral symbioses that has not yet benefited from this
renaissance is the relationship between the underwater light field and the
absorption of light by the intact symbiosis.  This is not a trivial task and
most efforts at elucidating the absorption and scattering of light within
corals have used destructive techniques.  Recent advances in microsensors
have begun to reveal the complexity of the light field and the role of the
aragonite skeleton in path length amplification of photons and optical
properties of corals (Kühl et al. 1995).

This month's featured L&O paper by Enríquez et al. (2005) makes an important
contribution to this area of study as it relates to the photobiology of
corals.  Enríquez et al. (2005) provide us with fine resolution measurements
of absorption spectra on intact corals and show the utility of their
technique using the Caribbean coral, Porites branneri, under a range of
symbiont densities, and chlorophyll concentrations.  To do this they created
polished sections or laminae ~3.0 mm in diameter, which maintained the
integrity and orientation of the skeleton, host tissue, and distribution of
the dinoflagellate symbionts known as zooxanthellae.  Using these laminae,
and bleached laminae as a reference, they obtained absorption spectra from
380 to 750 nm, the scattering properties of the skeleton, and chlorophyll a
specific absorption coefficients.  The increased absorption capacity of the
zooxanthellae, when compared to terrestrial plants, was related to the
diffuse light produced by the multiple scattering effects of the coral
skeleton.  The scattering process leads to an enhancement of the local
irradiance within the coral tissue similar to what had been described with
micosensors (Kühl et al. 1995).  

These results have important ramifications for our understanding of
photosynthesis in these symbioses and also the effects of high irradiances
of visible, and also ultraviolet, radiation on the normal and stressed
photobiology of corals.  The theoretical underpinnings of these results also
have implications for the evolution of symbiotic corals and calcification,
and that algal symbionts may have had more to gain from their hosts then the
metabolic waste products of the host by maximizing their light absorption
with low pigment investments.  Studying and unraveling coral symbioses by
understanding the optical properties of both their zooxanthellae and host
associated pigments (e.g., green fluorescent proteins) can provide
significant advances in our understanding of symbioses in general, and the
diversity of algal-cnidarian symbioses specifically.  The general optical
principals discussed in this paper should be of wide interest to the readers
of Limnology and Oceanography, and many other photobiologists.

References:

Edmunds, P. J., and R. D. Gates. 2003.  Has coral bleaching delayed our
understanding of fundamental aspects of coraldinoflagellate symbioses.
BioScience, 53: 976-980. 

Enríquez, S., E. R. Méndez, and R. Iglesias-Prieto. 2005.  Multiple
scattering on coral skeletons enhances light absorption by symbiotic algae.
Limnology and oceanography 50: 1025-1032. 

Kühl, M, Y. Cohen, T. Dalsgaard, B. B. Jørgensen, and N. P. Revsbech. 1995.
Microenvironment and photosynthesis of zooxanthellae in scleractinian corals
studied with microsensors for O2 pH and light.  Marine Ecology progress
Series 117: 159-172.