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