Summary: Bacteroids from both stem and root nodules of inoculated with sp. strain ORS571 were studied in the presence of the respective leghaemoglobins or soybean root nodule leghaemoglobin. Concentrations of free dissolved O and rates of O consumption and nitrogenase activity were measured during steady states in a special reaction chamber. The two types of bacteroids had high-affinity terminal oxidase systems. They were capable of respiration-coupled nitrogenase activity at concentrations below 10 nM free dissolved O; at such concentrations the homologous leghaemoglobins were more effective than soybean leghaemoglobin because of their higher affinity for O. bacteroids were less efficient in nitrogenase activity (consuming 4 to 11 mol O per mol of CH reduced) than soybean bacteroids (0/CH = 3.6) in comparable experiments, but efficiency was not affected by the source of leghaemoglobin. Similar experiments with ORS571 grown in N-fixing continuous cultures showed that these bacteria also had high-affinity terminal oxidase systems coupled to nitrogenase, but those grown at <1 μM dissolved O were less efficient (O/CH >21) than those grown at 7 to 11 μM-O (O/CH <8). Nitrogenase activity of bacteria grown at the higher O concentration increased when the O concentration in the chamber was raised from 0.1 to 3 μM in experiments in which mammalian myoglobin replaced leghaemoglobin in the reaction solution. With bacteria grown at < 1 μM-O, nitrogenase activity was inhibited (reversibly) by 50% after an increase from 0.1 to 1.4 μM-O in the reaction chamber. After changes in rates of supply of dissolved O there were oscillations in rates of O consumption before establishment of new steady states. These effects were greater in bacteria grown at 7 to 11 μM-O than in those grown at < 1 μM-O, but with the latter, a sixfold increase in O flux produced only very small increases in concentration of dissolved O.


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