Involvement of H+-ATPase and carbonic anhydrase in inorganic carbon uptake for endosymbiont photosynthesis

1 Observatoire Océanologique Européen, Centre Scientifique de Monaco, MC-98000 Monaco, Principality of Monaco; 2 Laboratoire de Physiologie et Toxicologie Environnementales, Faculté des Sciences, Université de Nice Sophia Antipolis, F-0608 Nice cedex, France; and 3 Dipartimento Fisiologia e Biochimica Generale, I-20100 Milano, Italy Symbiotic cnidarians absorb inorganic carbon from seawater to supply intracellular dinoflagellates with CO 2 for their photosynthesis. To determine the mechanism of inorganic carbon transport by animal cells, we used plasma membrane vesicles prepared from ectodermal cells isolated from tentacles of the sea anemone, Anemonia viridis . H 14 CO 3 uptake in the presence of an outward NaCl gradient or inward H + gradient, showed no evidence for a Cl - or H + - driven HCO 3 transport. H 14 CO 3 and 36 Cl uptakes were stimulated by a positive inside-membrane diffusion potential, suggesting the presence of HCO 3 and Cl conductances. A carbonic anhydrase (CA) activity was measured on plasma membrane (4%) and in the cytoplasm of the ectodermal cells (96%) and was sensitive to acetazolamide (IC 50 = 20 nM) and ethoxyzolamide (IC 50 = 2.5 nM). A strong DIDS-sensitive H + -ATPase activity was observed (IC 50 = 14 µM). This activity was also highly sensitive to vanadate and allyl isothiocyanate, two inhibitors of P-type H + -ATPases. Present data suggest that HCO 3 absorption by ectodermal cells is carried out by H + secretion by H + -ATPase, resulting in the formation of carbonic acid in the surrounding seawater, which is quickly dehydrated into CO 2 by a membrane-bound CA. CO 2 then diffuses passively into the cell where it is hydrated in HCO 3 by a cytosolic CA. symbiosis; anthozoan; sea anemone; HCO 3 transport; carbon-concentrating mechanism