Evidence for active CO2 uptake by a CO2-ATPase in the acidophilic green alga Eremosphaera viridis

We examined the mechanism(s) responsible for active uptake of dissolved ino rganic carbon (DIC) during photosynthesis in the green alga Eremosphaera vi ridis De Bary. O-2 electrode measurements of algal oxygen evolution and CO2 fluxes as a function of DIC availability indicate that E. viridis actively imports only CO2 during photosynthesis, and does not possess external carb onic anhydrase (CA). The K-0.5[CO2] was 14.2 and 10.1 muM at pH 5.0 and 8.0 , respectively. Both membrane potential and cellular resistance were measur ed under controlled conditions of [CO2] at either 2 or 15 muM. Active CO2 u ptake was electrically silent, suggesting that CO2 uptake might be mediated by a CO2-ATPase. Comparison of ATPase activity in microsomal preparations at low (0 muM) and high (15 muM) [CO2] indicated a 1.25-fold increase in AT P hydrolysis in high [CO2]. The CO2-ATPase activity was inhibited by the br oad-acting inhibitors diethylstilbestrol (DES) and N',N'-dicyclohexylcarbod iimide (DCCD) but unaffected by vanadate, fluoride, and nitrate. The K-0.5[ CO2] of the ATPase activity was 22.5 muM, a value very similar to the K-0.5 [CO2] for CO2 uptake by whole algal cells. These results suggest the existe nce of a CO2-ATPase as the major importer of DIC for photosynthesis in the microalga E. viridis.