Transmembrane Ferricyanide Reduction and Membrane Properties in the Euryhaline Charophyte Lamprothamnium papulosum

The euryhaline charophyte Lamprothamnium papulosum has the abilityto reduce the extracellular electron acceptor ferricyanide (Fe³⁺Cy).Addition of 0.5 mol m^–3 Fe³⁺Cy stimulated H⁺-efflux ata rate of 0.8 H⁺/Fe³⁺Cy-reduced and increased K⁺-efflux intoa potassium-free medium at a rate of 0.66 K⁺/Fe³⁺Cy-reduced.0.5 mol m^–3 Fe³⁺Cy-induced maximum membrane depolarizationfor cells with resting potentials more negative than the diffusionpotential. The peak value of Fe³⁺Cy-induced depolarizationswas similar to the potential obtained by poisoning the electrogenicpump with DCCD. The value of maximum depolarization was determinedby (K⁺)₀. E_m tended to more positive values with increasing(K⁺)₀. Depolarizations coincided with a decrease in membraneresistance (R_m) from a resting value of 1.5 m² to 0.2 m² inthe depolarized state. Depolarization increased the sensitivityof the membrane potential (E_m) to (K⁺)₀. The resting potentialwas only slightly changed when (K⁺)₀ was increased from 3 to15 mol m^–3. The Fe³⁺ Cy-induced depolarized Em changedin a Nernstian fashion when (K⁺)₀ was increased. It is concludedthat Fe³⁺Cy reduction causes a net depolarization current flowacross the plasmalemma. The depolarization shifts the membranefrom a hyperpolarized pump dominated state into a depolarizedK⁺ diffusion state. Key words: Ferricyanide reduction, membrane potential, Lamprothamnium