Methyl viologen (10 ,UM) markedly inhibited acetylene reduction (nitrogen fixation) by old but not young cultures of Gloeocapsa sp. LB795, apparently by causing the alga to produce H202. H,02 inhibited acetylene reduction when added to cultures at concentrations greater than 10 ,UM. As catalase (EC 1.11.1.6) is not present in Gloeocapsa sp. LB795, H202 is probably removed by a non-enzymic reaction with ascorbate and also by an enzymecatalysed reaction with glutathione. Enzymes catalysing the decomposition of H,O, were most active in young cells which were therefore better able than old cells to metabolize H202 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.
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