Inorganic carbon acquisition by Dunaliella tertiolecta (Chlorophyta) involves external carbonic anhydrase and direct HCO 3 − utilization insensitive to the anion exchange inhibitor DIDS

Abstract

A mechanism of bicarbonate uptake with a high sensitivity to the putative anion-exchange inhibitor 4,4-diisothiocyanostilbene-2,2- disulphonic acid (DIDS) has been previously reported in green algae. In this study, DIDS inhibited net oxygen evolution by Dunaliella tertiolecta by up to 22%, but internal pH regulation, intracellular CO_2 accumulation, carbon fixation and affinity for dissolved inorganic carbon (DIC) in Dunaliella tertiolecta showed low or insignificant sensitivity to DIDS. However, in cells grown and tested at pH 9·5, treatment with DIDS elevated the k 0.5(HCO_3^-), suggesting there may be a minor role for a DIDS-sensitive anion-exchange-type HCO_3^- transporter in DIC acquisition by D. tertiolecta at high pH. In contrast, significant external carbonic anhydrase (CA_ext) activity and up to 70% inhibition of DIC-dependent O_2 evolution by acetozolamide (AZ) suggest that CA_ext has an important role in DIC acquisition in D. tertiolecta, in normal seawater conditions and at elevated pH. Furthermore, the rate of DIC-dependent photosynthesis at high pH, in the presence of AZ, was 12 times higher than the calculated uncatalysed rate of CO_2 supply from HCO_3^-. This requires some system for direct HCO_3^- uptake by D. tertiolecta, which may include a DIDS-insensitive mechanism. The effects of DIDS upon indirect measures of DIC acquisition should be interpreted cautiously as DIDS may have non-specific effects upon whole cell function, and affect ion transport processes not directly related to HCO_3^- uptake.