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The ROLE of the ZOOXANTHELLAE
coral reefs have formed as the result of the special symbiotic
relationship which exists between polyps and the microscopic
unicellular algae inhabiting their cells.
These algae, which are generally known as "Zooxanthellae",
belong to the group of phytoplankton called dinoflagellata (one
of the branches of the flagellata), and inhabit the gastroderm
(a layer of endodermal cells bordering the cnidarian
gastro-vascular cavity) of polyps.
Coral is therefore said to be a hermatypic (or reef-building)
organism. Most species of coral are members of the Cnidarian
branch (a class of Anthozoa in the order of Scyphozoa).
By opposition, coral species which do not harbour zooxanthellae
are called ahermatypic (non-reef-building) species;
since these are not limited by the light factor (see Photosynthesis),
they can survive at all depths.
can be defined as a durable physical association between two
association is usually mutually beneficial.
In the case of the association between polyps and zooxanthellae,
we have endosymbiosis, which is a special type of symbiosis
where one of the partners lives inside the cells of its host.
This endosymbiotic relationship provides the hermatypic coral with
the following two advantages:
to 98% of all the organic nutrients are produced by the
zooxanthella; they are the main source of the polyps’
organic nutrient requirements.
to the photosynthetic activities
carried out by the zooxanthellae, the calcareous skeleton of
hermatypic coral is able to develop 2 to 3 times more
quickly in the light than in darkness.
the growth of the polyps, the algae multiply and confer on the
coral its typical reddish-brown colour.
zooxanthellae inhabiting hermatypic coral complexes belong to the
genus Symbiodinium, and the species vary depending on the
type of polyps involved. The populations of zooxanthellae living
in symbiosis with the polyps have rather slow growth rates in
comparison with those of the populations of algae grown under
laboratory conditions. This difference shows how greatly the polyp
hosts influence the metabolism of these algae:
have a passive influence, since they limit the space available
to the zooxanthellae and their access to nutrients;
they have active effects via their cellular mechanisms
zooxanthellae also contribute importantly to the nutrition and the
calcification of their hosts; in
addition, they contribute to the polyps’ system of defence, by
secreting various protective chemical substances
Symbiosis, an association with poor future prospects:
special relationship between polyps and zooxanthellae is being
undermined by two processes:
process of eutrophisation: this process consists of an increase
in the nutrient contents of the waters (N, P, organic matter,
etc.) resulting from the arrival of agricultural and/or
urban wastes which stimulate the growth of the algae. The
biomass of the phytoplankton increases accordingly, which
reduces the depth to which the light penetrates into the
water column and prevents the zooxanthellae from carrying
out the process of photosynthesis; and the transfer of
nutrients from the algae to the polyps consequently slows
process of sedimentation: when this process increases,
the suspended particles prevent the light from penetrating
deeply into the waters and the growth of the coral is thus
Under these conditions, the zooxanthellae have to produce
larger amounts of chlorophyll a in order to keep up
the rates of photosynthesis. The energy used by the algae
for this purpose is at the expense of the polyps.
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