Effects of O2 Concentrations and Various Haemoglobins on Respiration and Nitrogenase Activity of Bacteroids from Stem and Root Nodules of Sesbania rostrata and of the Same Bacteria from Continuous Cultures

Abstract

Summary: Bacteroids from both stem and root nodules of Sesbania rostrata inoculated with Rhizobium sp. strain ORS571 were studied in the presence of the respective leghaemoglobins or soybean root nodule leghaemoglobin. Concentrations of free dissolved O2 and rates of O2 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 O2; at such concentrations the homologous leghaemoglobins were more effective than soybean leghaemoglobin because of their higher affinity for O2. S. rostrata bacteroids were less efficient in nitrogenase activity (consuming 4 to 11 mol O2 per mol of C2H2 reduced) than soybean bacteroids (02/C2H4 = 3.6) in comparable experiments, but efficiency was not affected by the source of leghaemoglobin. Similar experiments with ORS571 grown in N2-fixing continuous cultures showed that these bacteria also had high-affinity terminal oxidase systems coupled to nitrogenase, but those grown at <1 μM dissolved O2 were less efficient (O2/C2H4 >21) than those grown at 7 to 11 μM-O2 (O2/C2H4 <8). Nitrogenase activity of bacteria grown at the higher O2 concentration increased when the O2 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-O2, nitrogenase activity was inhibited (reversibly) by 50% after an increase from 0.1 to 1.4 μM-O2 in the reaction chamber. After changes in rates of supply of dissolved O2 there were oscillations in rates of O2 consumption before establishment of new steady states. These effects were greater in bacteria grown at 7 to 11 μM-O2 than in those grown at < 1 μM-O2, but with the latter, a sixfold increase in O2 flux produced only very small increases in concentration of dissolved O2.