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SUMMARY: Bacteroids isolated from stem nodules of Sesbania were able to reduce C2H2 when glucose or succinate was the energy-yielding substrate. To be efficient, glucose always required low O2 concentrations, both with free O2 and with O2 delivered by leghaemoglobin extracted from French-bean nodules. Lactate detected in the stem nodule cytosol supported C2H2 reduction by bacteroid preparations, but the O2 tensions required for optimal activity were always higher than those defined for glucose, as predicted by the model of Bergersen&Trinchant (J. theor. Biol. 115, 93-102, 1985). In contrast, lactate was only present at very low concentrations in the cytosol of French-bean nodules and was unable to support C2H2 reduction by bacteroids isolated from these nodules. Lactate dehydrogenase (EC 1.1.1.27) extracted from stem nodule bacteroids and free-living cells of Rhizobium sp. (Sesbania) exhibited a higher affinity for lactate than did the enzyme from Rhizobium meliloti, R. leguminosarum biovar phaseoli or Bradyrhizobium and their symbiotic forms. Lactate appears to be a specific energy-yielding substrate supporting N2 fixation by Sesbania bacteroids and could play a major role when fermentative processes take place in the nodules.

Acetylene Reduction by Bacteroids Isolated from Stem Nodules of Sesbania rostrata. Specific Role of Lactate as an Energy-yielding Substrate