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

Abstract

We examined the mechanism(s) responsible for active uptake of dissolved inorganic carbon (DIC) during photosynthesis in the green alga Eremosphaera viridis De Bary. O2 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 carbonic anhydrase (CA). The K0.5[CO2] was 14.2 and 10.1 µM at pH 5.0 and 8.0, respectively. Both membrane potential and cellular resistance were measured under controlled conditions of [CO2] at either 2 or 15 µM. Active CO2 uptake was electrically silent, suggesting that CO2 uptake might be mediated by a CO2-ATPase. Comparison of ATPase activity in microsomal preparations at low (0 µM) and high (15 µM) [CO2] indicated a 1.25-fold increase in ATP hydrolysis in high [CO2]. The CO2-ATPase activity was inhibited by the broad-acting inhibitors diethylstilbestrol (DES) and N',N'-dicyclohexylcarbodiimide (DCCD) but unaffected by vanadate, fluoride, and nitrate. The K0.5[CO2] of the ATPase activity was 22.5 µM, a value very similar to the K0.5[CO2] for CO2 uptake by whole algal cells. These results suggest the existence of a CO2-ATPase as the major importer of DIC for photosynthesis in the microalga E. viridis.Key words: chlorophyte, CO2 transport, CO2-ATPase, photosynthesis, electrical potential, mass spectrometry.