Furla P., 1999. The mechanisms of inorganic carbon transports in the symbiotic association cnidaria-Dinoflagellate. PhD Sciences, with distinction Bio sciences of environment and health, Aix-Marseille University.

RESUME

The study of interactions between an Anthozoan and a phototroph Dinoflagellate is of interest not only in order to investigate photosynthetic symbiosis, but also calcification mechanisms. Indeed, these invertebrates are the most important marine calcifying organisms. Thus, in this symbiotic association, calcification and photosynthesis have in common the use of inorganic carbon as substrate.
In this work, we investigated the sources and the mechanisms of inorganic carbon transports for algal photosynthesis and for coral calcification. We first performed microscopic studies on the epithelial layers of a sea anemone, Anemonia viridis, in order to further understand the structural differences between an uncalcifying (the anemone) and a calcifying Anthozoan (the coral). Afterwards, different settings of the Mediterranean sea anemone, Anemonia viridis, tentacle (in vivo, in bag, in perfusion or in Ussing chamber), and the purification of the apical plasma membrane of ectodermal cells, allowed to establish a coherent model of inorganic carbon absorption for the algal photosynthesis. Indeed, our data demonstrated that the major inorganic carbon source for the Dinoflagellate photosynthesis is the HCO3- present in the external medium, inducing the transport of the HCO3- across the ectodermal epithelial layer. This transport is carried through a paracellular pathway (15 % of the total inorganic carbon fixed by the algal photosynthesis), and by a transcellular pathway (85%). The inorganic carbon absorption on the apical membrane of the ectodermal cells is performed by a H+-ATPase associated to a carbonic anhydrase. The absorption of HCO3- by the endodermal cells induces its dehydration in CO2 and OH- ions. The OH- ions secretion toward the coelenteric cavity, induces then an alkalinisation of the medium. Finally, these results demonstrated the presence of a morphologic polarity (the phototroph symbionts are present only in the endodermal cells) associated to functional polarity (HCO3- absorption from the external medium and secretion of OH- toward the coelenteron). The second part of this study describes a compartmental analysis of the 45Ca and 14C-HCO3- incorporation in microcolonies of the reef building coral Stylophora pistillata. This work was performed in order to clarify the sources and the transport mechanisms of inorganic carbon for calcification. Our data demonstrated that the major source of inorganic carbon for calcification is metabolic CO2 (70 % of the total inorganic carbon deposited in the skeleton), although only 30 % originates from the external medium. Moreover, our results determined a predominant role of a carbonic anhydrase and an anion exchanger in the secretion, by the skeletogenic cells, of the inorganic carbon toward the calcification site. Finally, the last study performed allowed to deepen the knowledge of the interaction between calcification and photosynthesis in the association between Cnidarian and Dinoflagellates. The modification of the coelenteric pH, total alkalinity and HCO3- concentration in the corals, Stylophora pistillata and Trachyphyllia geoffroyi, allowed to present a new model of light enhanced calcification which is dependent upon the coelenteric pH.



Key words: Photosynthetic symbiosis Anthozoan Dinoflagellate Inorganic carbon Photosynthesis Calcification Transepithelial transport Inorganic carbon concentrating mechanism functional polarity.