Inorganic carbon acquisition by Dunaliella tertiolecta (Chlorophyta) involves external carbonic anhydrase and direct HCO₃^- 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 CO₂ 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(HCO₃^-), suggesting there may be a minor role for a DIDS-sensitive anion-exchange-type HCO₃^- 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 O₂ 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 CO₂ supply from HCO₃^-. This requires some system for direct HCO₃^- 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 HCO₃^- uptake.