MECHANISMS OF CARBON ACQUISITION FOR ENDOSYMBIONT PHOTOSYNTHESIS IN ANTHOZOA

In contrast to free-living photoautotrophs, endosymbiontic dinoflagell ates 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 s ource of dissolved inorganic carbon supply for endosymbiont photosynth esis and the transport mechanisms involved. Symbiodinium spp., general ly known as zooxanthellae, live within the endodermal cells of their h osts, 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 sym biotic association is therefore faced with the problem of delivering d issolved 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 fl uid) nor paracellular diffusion could supply enough dissolved inorgani c carbon for endosymbiont photosynthesis. A transepithelial active mec hanism must be present in the host tissues to maintain the photosynthe tic rate under saturating irradiance. A pharmacological approach led t o propose a working model of dissolved inorganic carbon transport from seawater to zooxanthellae. This vectorial transport generates a pH gr adient across the epithelium. The role of this gradient as well as the physiological adaptation of Symbiodinium spp. to symbiotic life are d iscussed.