The Effects of Methyl Viologen on Gloeocapsa sp. LB795 and Their Relationship to the Inhibition of Acetylene Reduction (Nitrogen Fixation) by Oxygen

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Abstract
Methyl viologen (10 .mu.M) markedly inhibited acetylene reduction (N fixation) by old but not young cultures of Gloeocapsa sp. LB795, apparently by causing the alga to produce H2O2. H2O2 inhibited acetylene reduction when added to cultures at concentrations greater than 10 .mu.M. As catalase (EC 1.11.1.6) is not present in Gloeocapsa sp. LB795, H2O2 is probably removed by a non-enzymic reaction with ascorbate and also by an enzyme-catalyzed reaction with glutathione. Enzymes catalyzing the decomposition of H2O2 were most active in young cells which were better able than old cells to metabolize H2O2 produced in the presence of methyl viologen. The maximum activities of these enzymes coincided with maximum nitrogenase activity during the growth of batch cultures and may provide a protective mechanism for nitrogenase.