Transmembrane Ferricyanide Reduction and Membrane Properties in the Euryhaline CharophyteLamprothamnium papulosum

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Abstract
The euryhaline charophyte Lamprothamnium papulosum has the ability to reduce the extracellular electron acceptor ferricyanide (Fe3+Cy). Addition of 0.5 mol m−3 Fe3+Cy stimulated H+-efflux at a rate of 0.8 H+/Fe3+Cy-reduced and increased K+-efflux into a potassium-free medium at a rate of 0.66 K+/Fe3+Cy-reduced. 0.5 mol m−3 Fe3+Cy-induced maximum membrane depolarization for cells with resting potentials more negative than the diffusion potential. The peak value of Fe3+Cy-induced depolarizations was similar to the potential obtained by poisoning the electrogenic pump with DCCD. The value of maximum depolarization was determined by (K+)0. Em tended to more positive values with increasing (K+)0. Depolarizations coincided with a decrease in membrane resistance (Rm) from a resting value of 1.5 Ωm2 to 0.2 Ω m2 in the depolarized state. Depolarization increased the sensitivity of the membrane potential (Em) to (K+)0. The resting potential was only slightly changed when (K+)0 was increased from 3 to 15 mol m−3. The Fe3+ Cy-induced depolarized Em changed in a Nernstian fashion when (K+)0 was increased. It is concluded that Fe3+Cy reduction causes a net depolarization current flow across the plasmalemma. The depolarization shifts the membrane from a hyperpolarized pump dominated state into a depolarized K+ diffusion state.