FUNCTIONAL POLARITY OF THE TENTACLE OF THE SEA-ANEMONE ANEMONIA-VIRIDIS - ROLE IN INORGANIC CARBON ACQUISITION

The oral epithelial layers of anthozoans have a polarized morphology: photosynthetic endosymbionts live within endodermal cells facing the c oelenteric cavity and are separated from the external seawater by the ectodermal layer and the mesoglea. To study if this morphology plays a role in the supply of inorganic carbon for symbiont photosynthesis, w e measured the change in pH and the rate of OH- (H+) fluxes induced by each cell layer on a tentacle of the sea anemone Anemonia viridis. Li ght-induced pH increase of the medium bathing the endodermal layers le d to the generation of a transepithelial pH gradient of similar to 0.8 pH units across the tentacle, whereas darkness induced acidification of this medium. The light-induced pH change was associated with an inc rease of total alkalinity. Only the endodermal layer was able to induc e a net OH- secretion (H+ absorption). The light-induced OH- secretion by the endodermal cell layer was dependent on the presence of HCO3- i n the compartment facing the ectoderm and was sensitive to several inh ibitors of ion transport. [C-14] HCO3- incorporation into photosynthat es confirmed the ectodermal supply, the extent of which varied from 25 to >90%, according to HCO3- availability. Our results suggest that th e light-induced OH- secretion by the endodermal cell layer followed th e polarized transport of HCO3- and its subsequent decarboxylation with in the endodermal cell layer. This polarity may play a significant rol e both in inorganic carbon absorption and in the control of light-enha nced calcification in scleractinian corals.