Diffusional permeability of dissolved inorganic carbon through the isolated oral epithelial layers of the sea anemone, Anemonia viridis

Abstract

In a recent report, Bénazet-Tambutté et al. (1996a) showed that the major 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 mechanism of DIC transfer from the external seawater to the endodermal cells, we explored the different mechanisms potentially involved in this transfer: transepithelial passive diffusion of DIC across the oral epithelial layers, renewal of coelenteric fluid by the mouth or transepithelial active transport. In addition, we also studied the role of expansion/contraction of the tentacle in the transepithelial transport of DIC. We investigated diffusion of DIC, using isolated perfused tentacles, by measuring transepithelial H 14CO 3 flux. We found that the unidirectional 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 the 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 paracellular pathway. Use of inhibitors and inside-out tentacles showed that the transepithelial flux measured in the dark was depressed in the light due to symbiont photosynthesis which diverts about 15% of the DIC flux. Consequently, the passive diffusional flux of DIC can account for less than 1/6 of the dissolved inorganic carbon used for symbiont photosynthesis under saturating light conditions. Expansion/contraction state of the tentacles did not significantly modify the permeability coefficient ( P d=2.28×10 −5 and 2.50×10 −5 cm s −1 respectively for expanded and contracted tentacles) in spite of a change of tissue thickness of a factor of about 3. This result suggests that the mesoglea could represent a significant barrier to transepithelial diffusion of molecules, due to change in its water content level upon contraction. Measurement of water intake by whole sea anemones, using 14C-Dextran, showed that the renewal of the coelenteric fluid which bathes the dinoflagellate-containing endodermal cells cannot provide DIC to symbiont photosynthesis. Therefore it is concluded that a carrier-mediated DIC transporter located on the ectodermal host cells, supplies the bulk of dissolved inorganic carbon used for symbiont photosynthesis.