DIFFUSIONAL PERMEABILITY OF DISSOLVED INORGANIC CARBON THROUGH THE ISOLATED ORAL EPITHELIAL LAYERS OF THE SEA-ANEMONE, ANEMONIA-VIRIDIS

In a recent report, Benazet-Tambutte et al. (1996a) showed that the ma jor source of dissolved inorganic carbon for photosynthesis of the sea anemone Anemonia viridis was external seawater and a minor source was the internal medium of the tentacles. In order to determine the mecha nism of DIC transfer from the external seawater to the endodermal cell s, we explored the different mechanisms potentially involved in this t ransfer: transepithelial passive diffusion of DIC across the oral epit helial layers, renewal of coelenteric fluid by the mouth or transepith elial active transport. In addition, we also studied the role of expan sion/contraction of the tentacle in the transepithelial transport of D IC. Wa investigated diffusion of DIC, using isolated perfused tentacle s, by measuring transepithelial (HCO3)-C-14 flux. We found that the un idirectional transport of DIC was light-sensitive: the transepithelial flux was about three-fold lower in light than in dark conditions. The flux was insensitive to the anion transport inhibitor, DIDS, and to t he carbonic anhydrase inhibitor, Diamox. It was directly proportional to the concentration of bicarbonate in the surrounding seawater. These results suggest that this flux is simply driven by diffusion via a pa racellular pathway. Use of inhibitors and inside-out tentacles showed that the transepithelial flux measured in the dark was depressed in th e light due to symbiont photosynthesis which diverts about 15% of the DIC flux. Consequently, the passive diffusional flux of DIC can accoun t for less than 1/6 of the dissolved inorganic carbon used for symbion t photosynthesis under saturating light conditions. Expansion/contract ion state of the tentacles did not significantly modify the permeabili ty coefficient (P-d = 2.28 x 10(-5) and 2.50 x 10(-5) cm s(-1) respect ively for expanded and contracted tentacles) in spite of a change of t issue thickness of a factor of about 3. This result suggests that the mesoglea could represent a significant barrier to transepithelial diff usion of molecules, due to change in its water content level upon cont raction. Measurement of water intake by whole sea anemones, using C-14 -Dextran, showed that the renewal of the coelenteric fluid which bathe s the dinoflagellate-containing endodermal cells cannot provide DIC to symbiont photosynthesis. Therefore it is concluded that a carrier-med iated DIC transporter located on the ectodermal host cells, supplies t he bulk of dissolved inorganic carbon used for symbiont photosynthesis . (C) 1998 Elsevier Science B.V.