Browsing by Author "Day, J.M."

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    Physiological Aspects of N2-Fixation by a Spirillum from Digitaria Roots
    (1976) Day, J.M.; Johanna Dobereiner
    Studies of the-physiology of the Spirillum lipoferum recognized as the major organism responsible for Nj-fixation in the roots of Digitaria decumbens cv transvala were performed in order to improve the methods of culture and help to explain the physiology of this N2-fixing grass bacterial association. Methods for isolation, purification and N2-fixation assays are described. Acetylene concentrations used for N2-ase activity measurements should be at least 12%, the Vmax of cultures in the log phase being at a pC2H2 of O12atm and the apparent Km 0-022 arm. Optimal temperatures for N2-dependent growth are between 32 and 40°C, and little N2-fixation is observed below 24°C. At 42°C the N2-ase is inactivated. When cultures grown at 28 or 36°C are transferred to lower temperatures nitrogenase activity declines rapidly. One hour after transfer to 17°C activity is about half that before transfer and is maintained at this level for at least 8 h. After transfer to 10°C activity ceases after 1 h. Growth is very pH dependent, optimal growth on N2 occurring _only between pH 6-8 and 7-8. Nitrogen fixation below pH 5-5 and above 8-0 is less than one-quarter of the optimal. No N2-fixation occurs in the absence of O2 and maximal N2-dependent growth is reached at 1-5% O2 in the gas mixture bubbled through liquid cultures. In contrast to previous reports, several sugars including glucose can be used by the Spirillum for N2-fixation, but only when small amounts of starter nitrogen or organic acids are added to the medium. Efficiencies of N2-fixation on malate and glucose are similar and about 60% of that of cells incorporating NH4-N. Efficiency of NOJ incorporation is 74% of that of NHi"-N grown cultures. High observed efficiencies (52 mg N2 fixed g~ * malate or glucose) are attributed to carbon limited growth at optimum or O2 limited conditions, both facilitated by slow diffusion rates through the semi-solid agar medium used.