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

Symbiotic cnidarians absorb inorganic carbon from seawater to supply intracellular dinoflagellates with CO2 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. H14CO3/- uptake in the presence of an outward NaCl gradient or inward H+ gradient, showed no evidence for a Cl- or H+- driven HCO3/- transport. H14CO3/- and 36Cl- uptakes were stimulated by a positive inside-membrane diffusion potential, suggesting the presence of HCO3/- 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 (IC50 = 20 nM) and ethoxyzolamide (IC50 = 2.5 nM). A strong DIDS-sensitive H+ATPase activity was observed (IC50 = 14 μM). This activity was also highly sensitive to vanadate and allyl isothiocyanate, two inhibitors of P-type H+-ATPases. Present data suggest that HCO3/- 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 CO2 by a membrane-bound CA. CO2 then diffuses passively into the cell where it is hydrated in HCO3/- by a cytosolic CA.