Hydrogen Evolution from Alfalfa and Clover Nodules and Hydrogen Uptake by Free-Living Rhizobium meliloti†

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A series of Rhizobium meliloti and Rhizobium trifolii strains were used as inocula for alfalfa and clover, respectively, grown under bacteriologically controlled conditions. Replicate samples of nodules formed by each strain were assayed for rates of H2 evolution in air, rates of H2 evolution under Ar and O2, and rates of C2H2 reduction. Nodules formed by all strains of R. meliloti and R. trifolii on their respective hosts lost at least 17% of the electron flow through nitrogenase as evolved H2. The mean loss from alfalfa nodules formed by 19 R. meliloti strains was 25%, and the mean loss from clover nodules formed by seven R. trifolii strains was 35%. R. meliloti and R. trifolii strains also were cultured under conditions that were previously established for derepression of hydrogenase synthesis. Only strains 102F65 and 102F51 of R. meliloti showed measurable activity under free-living conditions. Bacteroids from nodules formed by the two strains showing hydrogenase activity under free-living conditions also oxidized H2 at low rates. The specific activity of hydrogenase in bacteroids formed by either strain 102F65 or strain 102F51 of R. meliloti was less than 0.1% of the specific activity of the hydrogenase system in bacteroids formed by H2 uptake-positive Rhizobium japonicum USDA 110, which has been investigated previously. R. meliloti and R. trifolii strains tested possessed insufficient hydrogenase to recycle a substantial proportion of the H2 evolved from the nitrogenase reaction in nodules of their hosts. Additional research is needed, therefore, to develop strains of R. meliloti and R. trifolii that possess an adequate H2-recycling system.

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