The first taxonomic descriptions of zooxanthellae were done on a symbiotic
dinoflagellate, Symbiodinium microadriaticum, in the medusa Cassiopea.
In several taxa zooxanthellae are transferred from the adult to the egg.
In adult aposymbiotic organisms, the latter can re-establish an association
with zooxanthellae. In adults, the zooxanthellae are mainly regrouped
two or three at a time in the oral endodermic cells and maintained in
a vesicle delimited by a membrane of the animal cell, the perisymbiotic
vesicle. The dinoflagellate and this vesicle form the symbiocosm. Thus,
from an ecological point of view, among animal cells, the zooxanthellae
benefit from a stable environment: they are no longer subject to sedimentation,
are protected from browsing, and benefit from the catabolic products of
the host. The coral itself has a source of carbon which allows it to be
autotrophic in certain cases.
Zooxanthella pigments are at the origin of the colour of corals, except
when these have additional pigments which give them brighter and fluorescent
colouration. The zooxanthellae have a spheroid form of 9 to 12 µm
in diameter, for an average volume of 400 to 600 µm3.
The respective biomass of each partner remains difficult to estimate;
the zooxanthellae are counted polyp by polyp or by surface area of animal
tissue and although their abundance depends on the species, it can be
estimated at between 1 to 10 106 zooxanthellae
for cm2 of surface area of the coral.
The symbiotic association and calcification
The principal characteristics of the "coral-dinoflagellate"
association is the capacity of the zooxanthellae to photosynthesize within
the animal cell itself. It is agreed that zooxanthellae are responsible
for coral reef success in their contribution, among other things, to the
metabolic needs of host cells. The relation between the presence of the
zooxanthellae and the capacity of corals to calcify is unclear. Several
authors point out that light stimulates calcification, a stimulation which
originates from the photosynthetic activity of the zooxanthellae.
On the other hand, Cnidaria, such as the red coral, black coral or sciaphile
corals do not house zooxanthellae but do synthesize a skeleton.
Bleaching phenomenon
Bleaching corresponds to the loss of colour in corals,
leaving the skeleton with a white appearance. It occurs either after a loss
of symbiotic zooxanthellae, or from the disappearance of photosynthetic
pigments. The causes and mechanisms of this alteration are unknown. Most
hypotheses suggest that high temperature, an unusual ultraviolet intensity
or the increase of the partial pressure of CO2
are at the origin of bleaching episodes. Bleaching could be an adaptive
reaction rather than a pathological one: an opportunity taken by the coral
to associate itself with other zooxanthellae more capable of adapting to
a modified environment.
Note: this information was adapted from the following
document :
THESIS University of Nice Sophia-Antipolis Doctorat, Life Sciences.
Tambutté Eric (1996)
"Processus de calcification d'un Scléractinaire Hermatypique
Stylophora pistillata - Croissance in situ à Mururoa"
University Thesis, Nice Sophia Antipolis, pp 295.
University of Nice Sophia-Antipolis Doctorat, Life Sciences. European
Oceanology Observatory of the Scientific Centre of Monaco, Atomic Energy
Commissariat, Laboratory of Detection and Geophysics and Mixed Service
of Biological and Radiological Surveillance of Man and the Environment.
Research Formation Contract of the Atomic Energy Commissariat.
Thesis Professors: Professor Jean Jaubert and Doctor Denis Allemand.
