K⁺ Fluxes and K⁺ Content in Dunaliella acidophila, an Alga with Positive Electrical Potentials. I. Low Affinity Uptake System

K⁺ content and K⁺ transport of the acid-resistant green alga Dunaliella acidophila (optimal growth and photosynthesis at pH 1.0) were investigated. In spite of its positive zeta potential and its positive membrane potential D. acidophila is capable of accumulating and regulating its K⁺ content (cytosolic K⁺ concentration about 150 mM). Two systems (two modifications of one system, respectively) are responsible for K⁺ uptake (measured as ⁸⁶Rb⁺). System I, as described in this study, does not discriminate Rb⁺ and exhibits a low affinity for K⁺ (KM = 0.3-0.5 mM). V_max values are between 1 and 2 µmol K⁺ mg^-1 chlorophyll h^-1. The K⁺ uptake is stimulated by light and by decreasing H⁺ concentrations. Omeprazole or HgCl₂ inhibit K⁺ uptake, indicating the involvement of SH-groups in the transport reaction. The K⁺ uptake is clearly against the electrochemical gradient. Nigericin does not affect K⁺ uptake, but strongly reduces the ATP-pool. Also low concentrations of FCCP reduce strongly the ATP-pool without effect on K⁺ uptake. This indicates that the uptake system I is not primarily energized by the hydrolysis of ATP. Data are in agreement with the existence of a K⁺,H⁺ cotransport in the plasma membrane of D. acidophila, as described for acidophilic bacteria.