Hydrogen oxidation and efficiency of nitrogen fixation in succinate-limited chemostat cultures ofRhizobium ORS 571 |
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Authors: | Hein Stam Henk W van Verseveld Wytske de Vries Adriaan H Stouthamer |
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Institution: | (1) Department of Microbiology, Biological Laboratory, Vrije Universiteit, Postbus 7161, NL-1007 MC Amsterdam, The Netherlands |
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Abstract: | Rhizobium ORS 571, isolated from stem nodules of the tropical legumeSesbania rostrata is able to grow in the chemostat with molecular nitrogen as sole nitrogen source at a specific growth rate of 0.1 h-1. Samples from nitrogenfixing cultures showed high acetylene reduction activities: 1,500 nmol ethylene formed per milligram dry weight per hour. Under nitrogen-fixing conditions an uptake hydrogenase is induced. Ammonia-assimilating cultures, without additional hydrogen, did not induce hydrogenase. The addition of hydrogen to succinate-limited nitrogen-fixing cultures resulted in an increase in the molar growth yield on succinate (Y
succinate) from 27 to 35 and a slight decrease in the molar growth yield on oxygen (
), showing that hydrogen oxidation is less energy-yielding than the oxidation of endogenous substrates. Respiration-driven proton translocation measured with starved cells indicated the functioning of site 1 and 2 of oxidative phosphorylation. Cytochrome spectra showed that cytochromea
600, present at high dissolved oxygen tension (d.o.t.) almost completely disappeared at low d.o.t. In flash-photolysis spectra only thea-type cytochrome could be detected as an oxidase in cells both grown at high and low d.o.t. Growth yields in ammonia-assimilating cultures were higher than those measured in nitrogen-fixing cultures. Assuming two sites of oxidative phosphorylation, a molar growth yield on ATP (Y
ATP) of about 3 and 6 was calculated for respecticely nitrogen-fixing and ammonia-assimilating cultures. TheY
ATP under nitrogen-fixing conditions is dependent on the amount of H2 formed per mol N2 fixed (H2/N2 ratio). A method has been described to calculate the total amount of ATP use by nitrogenase during the fixation of 1 mol N2 (ATP/N2 ratio) and H2/N2 ratios in aerobic nitrogen fixing organisms. This calculation yielded that nitrogen fixation inRhizobium ORS 571 is a high ATP-consuming process. The calculated ATP/N2 and H2/N2 ratios were respectively 42 and 7.5.Abbreviations d.o.t.
dissolved oxygen tension
A preliminary account of this work was presented at the 5th International Symposium on Nitrogen Fixation, September 1983, Noordwijkerhout, The Netherlands |
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Keywords: | Rhizobium Nitrogenase Hydrogenase Growth yields Chemostat Cytochromes Proton translocation ATP/N2 ratio |
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