Effects of concentrations of substrates supplied to N2-fixing soybean bacteroids in flow chamber reactions

A liquid reaction medium containing dissolved air, oxyleghaemoglobin and low concentrations (0-2 mM) of glucose, succinate, malate or ethanol, was supplied to bacteroids prepared from soybean root nodules and confined in a stirred reaction chamber. Measurements of concentrations of O2, CO2 and NH3 from N2 fixation, at various flow rates through the chamber, were used to calculate rates of respiration and N2 fixation. Glucose was utilized poorly under all conditions. Ethanol was required at 0.2-2.0 mM before stimulations of O2 demand and N2 fixation were observed. Demand for O2 was enhanced by 0.05 mM to 0.5 mM succinate and malate at concentrations of O2 from 10 nM to 50 nM. However, the concentrations of C4-dicarboxylate and dissolved O2 determined whether N2 fixation was stimulated or inhibited by these substrates. It is postulated that these effects were associated with the presence in the bacteroids of two malic enzymes and a pyruvate dehydrogenase complex, the resulting acetyl-GoA being channelled through the tricarboxylic acid (TCA) cycle (N2 fixation stimulated) or to poly-β-hydroxybutyrate (PHB) accumulation (N2 fixation inhibited). Contrary to earlier results, the O2 consumption data suggested that there was a single terminal oxidase (Km 20—26 nM and Vmax 20-29 nmol O2 min-1 (mg dry mass)-1) operating in the range of O2 concentration in which N2 fixation occurred.