Lemna gibba L., grown in the presence or absence of Fe, reduced extracellular ferricyanide with a V max of 3.09 μmol · g(-1) fresh weight · h(-1) and a K m of 115 μM. However, Fe(3+)-ethylenediaminetetraacetic acid (EDTA) was reduced only after Fe-starvation. External electron acceptors such as ferricyanide, Fe(3+)-EDTA, 2,6-dichlorophenol indophenol or methylene blue induced a membrane depolarization of up to 100 mV, but electron donors such as ferrocyanide or NADH had no effect. Light or glucose enhanced ferricyanide reduction while the concomitant membrane depolarization was much smaller. Under anaerobic conditions, ferricyanide had no effect on electrical membrane potential difference (Em). Ferricyanide reduction induced H(+) and K(+) release in a ratio of 1.16 H(+)+1 K(+)/2 e(-) (in +Fe plants) and 1.28 H(+)+0.8 K(+)/2 e(-) (in -Fe plants). Anion uptake was inhibited by ferricyanide reduction. It is concluded that the steady-state transfer of electrons and protons proceeds by separate mechanisms, by a redox system and by a H(+)-ATPase.