Effects of salinity on inorganic carbon utilization and carbonic anhydrase activity in the halotolerant alga Dunaliella salina (Chlorophyta)

Gas exchange characteristics of the halotolerant unicellular alga Dunaliella salina (Dunal) Teodoresco (Chlorophyta) were examined over a range of 0.21 m to 2.22 m NaCl. Cells of D. salina demonstrated a high affinity in photosynthesis for CO2 and HCO3−, suggesting that ribulose-1,5-P2 oxygenase activity is inhibited and photorespiration is consequently suppressed as a result of the activity of a CO2-concentrating mechanism (CCM). The affinity in photosynthesis for CO2 was more pronounced in cells grown at higher NaCl concentrations, with a K½(Co2) for photosynthesis of 0.15 μm at the highest salinity employed (2.22 m NaCl). The activity (and percentage total activity) of carbonic anhydrase accessible at the cell surface was observed to increase substantially with increasing NaCl concentration. Increases in external carbonic anhydrase activity correlated closely with increased affinity for CO2 and HCO3− in photosynthesis, reflecting the importance of external carbonic anhydrase for the effective functioning of the CCM in D. salina. It is suggested that CO2 is the species of inorganic carbon that crosses the plasmalemma in a process facilitated by the action of extracellular carbonic anhydrase.