Glutamate as a Carbon Source for N2-Fixing Bacteroids Prepared from Soybean Root Nodules

Abstract

Summary: The possibility that glutamate might serve as a source of reducing power, supporting fixation of N2 to NH3 by bacteroids in soybean root nodules, was investigated. Suspensions of bacteroids were retained within a reaction chamber supplied with a flow of solution containing dissolved air, buffer, oxyleghaemoglobin and either disodium glutamate (1 or 10 mM) or no added substrate (2 or 20 mM-NaCl). The solution passing out of the chamber was analysed for dissolved NH3 and proportional oxygenation of leghaemoglobin was determined spectrophotometrically. Rates of production of NH3 and of consumption of O2 and the concentrations of free, dissolved O2 prevailing during steady states were calculated. At 1 mM, glutamate stimulated O2 demand of the bacteroids and enhanced NH3 production. However, increased production of NH3 was shown to arise entirely from deamination of [15N]glutamate and not from N2 fixation. At 10 mM-glutamate, the proportion of NH3 arising from [15N]glutamate increased with increased steady state concentrations of free, dissolved O2 in the chamber, but there was also significant and efficient stimulation of N2 fixation when the concentration of free, dissolved O2 was between 10 and 70 nM. The kinetics of O2 consumption by bacteroids using endogenous substrates or 10 mM-glutamate were similar.