Cl- fluxes and Cl- content of Dunaliella acidophila--an alga with a positive membrane potential

The Cl- fluxes across the plasma membrane and the Cl- content of the acid-resistant alga Dunaliella acidophila (optimal growth at pH 1-0, positive membrane potential) were studied in the presence of 0.01-300 mM Cl-. Up to 4.0 mM Cl- in the medium. the internal Cl- concentration is higher than that predicted by the electrochemical equilibrium. whereas at higher external Cl- concentrations internal Cl- levels are lower than expected for the electrochemical equilibrium. Growth in the absence of Cl- is significantly lower than in the standard growth medium (2.2 mM Cl-) and this reduction cannot be overcome by the addition of other monovalent anions such as Br- or NO3(-). The latter implies a specific Cl- requirement in addition to the role of Cl- as a permeant anion during ion translocations. Growth and photosynthesis tolerate an excess of Cl- up to 300 mM (without stepwise adaptation to increasing salinity). The uptake of Cl- measured by tracer techniques) exhibits Michaelis-Menten kinetics (Km = 0.75 mM Cl-) and is stimulated by light and high H+ concentrations. Internal acidification by acetic acid causes an inhibition of Cl- uptake. The uptake of Cl- is inhibited by the monovalent anions Br-, I-, and NO3(-) with Ki values not very much different from the Km value for Cl-. The anion transport inhibitors SITS and DIDS do not affect photosynthesis, but strongly suppress the uptake of Cl-. The Cl- channel blockers A-9-C and NPPB cause inhibitions of Cl- uptake as well as of photosynthesis and the ATP pool. FCCP strongly depresses the internal ATP-pool without a marked effect on Cl- uptake. Cl- efflux was inhibited by DIDS and SITS, but stimulated or inhibited by FCCP. depending on the external Cl- concentration. Results are in agreement with the hypothesis that Cl- uptake into D. acidophila is due to catalysed diffusion and is primarily independent of the hydrolysis of ATP. Cl- efflux is assumed to be coupled to an active pump. Data suggest tight co-operativity between the systems responsible for Cl- uptake and Cl- efflux, with the cytoplasmic pH and the membrane potential being important mediators.