Phosphate and sulfate uptake in the halotolerant alga Dunaliella are driven by Na+-symport mechanism

Phosphate and sulfate uptake were measured in the halotolerant alga Dunaliella salina under anion-sufficient or anion-deficient conditions. Both phosphate and sulfate uptake were strictly and specifically dependent on Na+ ions. Pi accumulation was inhibited by Na+ ionophores and by inhibitors of Na+ extrusion. Pi uptake was associated with a stoichiometric influx of Na+ ions at a ratio of 1 Na+/1 Pi. Depletion of phosphate or sulfate from growth media induced a 20-fold or a 2.5-fold stimulation of the corresponding rate of uptake, respectively. The enhancements were associated with an increase in Vmax values, but not in the Km for anion substrates (Km(Pi)∼1 μmol/L, Km(SO4)∼15 μmol/L) or for Na+ (Km(Na+)∼35 mmol/L for Pi, ∼170 mmol/L for SO42-). These results suggest that uptake of phosphate and of sulfate in D. salina are mediated by Na+/anion symporters and are driven by δμNa+ across the plasma membrane. The implications of these results on the availability of phosphate and sulfate in hypersaline solutions are discussed.