INORGANIC CARBON UPTAKE FOR PHOTOSYNTHESIS BY THE SYMBIOTIC CORAL-DINOFLAGELLATE ASSOCIATION .2. MECHANISMS FOR BICARBONATE UPTAKE

Mechanisms of HCO3- uptake as a source of dissolved inorganic carbon ( DIC) for photosynthesis by the intracellular symbiont, Symbiodinium sp . were studied using microcolonies of the coral Galaxea fascicularis, freshly isolated zooxanthellae (FIZ) and cultured zooxanthellae (CZ). For this purpose we used specific inhibitors of anion transport cetami do-4'-isothiocyanatostilbene-2,2'-disulfonic acid - SITS -, 4,4'-diiso thiocyanato-stilbene-2,2'-disulfonic acid - DIDS -, carbonic anhydrase (acetazolamide, ethoxyzolamide), H+-ATPase (N,N'-dicyclohexylcarbodii mide - DCCD -, diethylstilbestrol - DES -, vanadate) or Ca2+ channels (verapamil). We also tested the effect of ions known to play a role in HCO3- transport, like Na+ and Ca2+. Chloride uptake experiments were also performed to determine whether Cl- and HCO3- fluxes were coupled in CZ. Furthermore, the presence of carbonic anhydrase was tested usin g indirect immunoflurescence. Our results suggest that bicarbonate upt ake by the animal symbiont is likely to be achieved by two types of DI DS-sensitive HCO; carriers, each sharing 50% of the total uptake. The first is Na+-dependent, while the second is Na+-independent. We sugges t the presence of a Na+-independent Cl-/HCO3- exchange and either a Na +-dependent Cl-HCO3- exchange or a Na+/HCO3- symport. Pharmacological data suggest that the enzyme carbonic anhydrase plays an important rol e in maintaining the photosynthetic rate. In the intact symbiosis, the major fraction of carbonic anhydrase activity is located in the zooxa nthellae. Striking differences in DIC absorption mechanisms were found for FIZ and CZ. In FIZ, H+-ATPase and carbonic anhydrase participate in the carbon supply while in CZ the mechanism of HCO3- uptake appears to be strictly Na+-dependent and could be the result of Na+/HCO3- sym port activity. We hypothesize that stimulation of HCO3- uptake by the animal host is a consequence of intracellular pH alkalization by zooxa nthellae photosynthesis. These results were summarized in a synthetic scheme of DIC absorption by both host cell and isolated zooxanthellae.