Unlike a free alga, the symbiotic dinoflagellate is not directly in contact
with seawater, since it is intra-cellular, inside the endodermic cells of
the animal, inside a vesicle. So it is interesting to determine the respective
roles of the animal host and the symbiotic algae in this association. This
study was undertaken in several complementary ways using reef building corals
and sea anemones.
We have shown that the host animal participates actively in the inorganic
carbon absorption for the photosynthesis of its symbiont. The mechanism
needs a membrane H+-ATPase, which it probably
allows for dehydration of HCO3-
present in great quantities in seawater - into CO2,
which then diffuses in the cell. An intercellular carbonic anhydrase then
facilitates the balance between CO2 and the
intercellular HCO3, preventing a back flow.
It seems that coral is one of the rare organisms capable of actively absorbing
CO2 whereas animals usually reject it. Corals
in this way can control the photosynthesis of their symbionts.
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