INORGANIC CARBON SUPPLY TO SYMBIONT PHOTOSYNTHESIS OF THE SEA-ANEMONE, ANEMONIA-VIRIDIS - ROLE OF THE ORAL EPITHELIAL LAYERS

The role of the oral epithelial cell layers of the symbiotic sea anemo ne, Anemonia viridis, in dissolved inorganic carbon (DIG) supply for t he photosynthesis of the symbiotic dinoflagellates (Symbiodinium sp.) was investigated. For this purpose, we used sections of isolated tenta cles prepared as small ''bags'' either normally oriented or everted (i nside-out). Preliminary experiments showed that the dark respiration r ate cannot supply enough CO2 for dinoflagellate photosynthesis and tha t DIC must be obtained from seawater. Experiments with bicarbonate-fre e seawater showed that while the major source of DIC is the external m edium, about one third is absorbed from the internal space. both the e ctoderm and the endoderm seem to be able to absorb DIG. Calculation of spontaneous dehydration of HCO3- into CO2 indicates that HCO3- absorp tion by the host cells is involved in DIC supply to dinoflagellate pho tosynthesis. Further, the permeability of the oral epithelial layers t o HCO3- and CO2 appears to be insufficient to maintain the optimal pho tosynthetic rate. The utilization of an anion carrier inhibitor (400 m u M DIDS) showed that HCO3- uptake is likely operative on both ectoder mal and endodermal cells. The other sources of DIC come from the dehyd ration of bicarbonate into carbon dioxide as suggested by the sensitiv ity to inhibitors of carbonic anhydrase (Diamox) and of H+-ATPase (DES ). Our results suggest the involvement of the ectodermal cell layer in the supply of inorganic carbon for photosynthesis of zooxanthella loc ated inside the endodermal cells.