© 1992 Oxford University Press
RESEARCH-ARTICLE |
Cl– Fluxes and Cl– Content of Dunaliella acidophila—An Alga with a Positive Membrane Potential
Julius von Sachs–lnstitut fur Biowissenschaften, Universitat Wurzburg Mittlerer Dallenbergweg 64, D 8700 Wurzburg, Germany
1To whom correspondence should be addressed
The Cl– fluxes across the plasma membrane and the Cl– content of the acid–resistant alga Dunaliella acidophila (optimal growthat pH 1.0, positive membrane potential) were studied in the presence of 0.01–300 mM Cl–. Up to 40 mM Cl– in the medium, theinternal 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 ofother monovalent anions such as Br– or NO–3 The latter implies a specific Cl– requirement in addition to the role of Cl– as apermeant anion during ion translocations. Growth and photosynthesis tolerate an excess of Cl– up to 300 mM (without stepwiseadaptation 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 NO–3 with K1, values not very much different from the KM. value for Cl–. The anion transport inhibitors SITS and DIDS do not affect photosynthesis, but strongly suppressthe uptake of Cl–. The Cl– channel blockers A–9–C and NPPB cause inhibitions of Cl– uptake as well as of photosynthesis andthe ATP pool. FCCP strongly depresses the internal ATP–pool without a marked effect on Cl– uptake. Cl– efflux was inhibitedby DIDS and SITS, but stimulated or inhibited by FCCP, depending on the external Cl– concentration. Results are in agreementwith the hypothesis that Cl– uptake into D. acidophila is due to catalysed diffusion and is primarily independent of the hydrolysisof ATP. Cl– efflux is assumed to be coupled to an active pump. Data suggest tight co–operativity between the systems responsiblefor Cl– uptake and Cl– efflux, with the cytoplasmic pH and the membrane potential being important mediators.
Key words: Acid resistance, chloride carrier, chloride channels, Dunaliella acidophila, membrane potential, plasma membrane