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The oral epithelial layers of anthozoans have a polarized morphology: photosynthetic endosymbionts live within endodermal cells facing the coelenteric 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, we 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. Light-induced pH increase of the medium bathing the endodermal layers led to the generation of a transepithelial pH gradient of ∼0.8 pH units across the tentacle, whereas darkness induced acidification of this medium. The light-induced pH change was associated with an increase of total alkalinity. Only the endodermal layer was able to induce a net OH− secretion (H+ absorption). The light-induced OH− secretion by the endodermal cell layer was dependent on the presence of[Formula: see text] in the compartment facing the ectoderm and was sensitive to several inhibitors of ion transport. [14C][Formula: see text]incorporation into photosynthates confirmed the ectodermal supply, the extent of which varied from 25 to >90%, according to[Formula: see text] availability. Our results suggest that the light-induced OH−secretion by the endodermal cell layer followed the polarized transport of [Formula: see text] and its subsequent decarboxylation within the endodermal cell layer. This polarity may play a significant role both in inorganic carbon absorption and in the control of light-enhanced calcification in scleractinian corals.

In contrast to free-living photoautotrophs, endosymbiontic dinoflagellates of the genus Symbiodinium must absorb their inorganic carbon from the cytoplasm of their host anthozoan cell rather then from seawater. The purpose of this paper is to review the present knowledge on the source of dissolved inorganic carbon supply for endosymbiont photosynthesis and the transport mechanisms involved. Symbiodinium spp., generally known as zooxanthellae, live within the endodermal cells of their hosts, corals and sea anemones. They are separated from the surrounding seawater by the host tissues (oral ectodermal cell layer, collagenous basal membrane, endodermal cell, and perisymbiotic vesicles). The symbiotic association is therefore faced with the problem of delivering dissolved inorganic carbon to an endodermal site of consumption from an, essentially, ectodermal site of availability. Studies using original methods demonstrated that neither the internal medium (coelenteric fluid) nor paracellular diffusion could supply enough dissolved inorganic carbon for endosymbiont photosynthesis. A transepithelial active mechanism must be present in the host tissues to maintain the photosynthetic rate under saturating irradiance. A pharmacological approach led to propose a working model of dissolved inorganic carbon transport from seawater to zooxanthellae. This vectorial transport generates a pH gradient across the epithelium. The role of this gradient as well as the physiological adaptation of Symbiodinium spp. to symbiotic life are discussed.Key words: carbon concentrating mechanism, anthozoan, dinoflagellates, anion transport, symbiosis, transepithelial transport.

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