Extracellular ferricyanide reduction and nitrate reductase activity in the green alga Monoraphidium braunii

The unicellular green alga, Monoraphidium braunii, reduced extracellular ferricyanide at high rates. Impermeability of the membranes to ferriand ferrocyanide was shown for untreated cells as for those permeabilized with 5% I-propanol. Fe starvation stimulated reduction rates by 50%, light or glucose were ineffective. Extracellular acidification largely corresponded to formation of the new anionic valence of ferrocyanide. 1-Propanol used as permeabilizer for NR assays in the same samples apparently inhibited ferricyanide reduction, however addition of catalase showed that this was not due to interference with ferricyanide reduction but to partial reoxidation by H20 2. Between ferricyanide reduction and nitrate reduction no competition was found in an 'in situ' system with endogenous reductant in the presence of l-propanol, neither with 'induced' algae showing high NR activity nor with 'repressed' algae grown in ammonium medium and showing only constitutive NR activity. According to this data the inducible high-activity NR and the inducible nitrate uptake system are not responsible for reduction of ferricyanide. The plasmalemma-bound, low activity, NR might have extracellular reducing activity, but also for this NR lack of competition in reduction of the two substrates indicated independent reactions in Monoraphidium.