Browsing by Author "Johanna Dobereiner"

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    Confirmation of nitrogen fixation in two tropical grasses by 15N2 incorporation
    (1977) Helevecio De-Polli; Elichi Matsui; Johanna Dobereiner; Eneas Salati
    Intact soil cores containing plants of Paspalum notation or Digitaria decumbens were selected with the acetylene reduction method, and then exposed to 15N2 to confirm nitrogen fixation in tropical grass-bacteria associations. In a preliminary experiment with P. notatum 15N2 incorporation was slow but progressive during 24 h in roots but translocation to rhizomes and leaves ceased after 17 h. With improved assay chambers, enrichments of 0.151 and 0.563 15N atom % excess were obtained in roots of D. decumbens cv transvala and P. notatum systems respectively, after 3 days. Enrichments in rhizomes were similar to those of roots; however in the leaves only 8% of root enrichment was observed. The addition of sucrose to the soil doubled N2-fixation in roots in both grass species studied, but did not result in increased incorporation into the leaves of P. notatum.
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    Nitrogenase Activity and Nitrate Respiration in Azospirillum spp.
    (1979) David B. Scott; Christine A. Scott; Johanna Dobereiner
    The interaction between nitrate respiration and nitrogen fixation in Azoxpirillum lipoferum and A. hraxilenxe was studied. All strains examined were capable of nitrogen fixation (acetylene reduction) under conditions of severe oxygen limitation in the presence of nitrate. A lag phase of about 1 h was observed for both nitrate reduction and nitrogenase activity corresponding to the period of induction of the dissimilatory nitrate reductase. Nitrogenase activity ceased when nitrate was exhausted suggesting that the reduction of nitrate to nitrite, rather than denitrifi-cation (the further reduction of nitrite to gas) is coupled to nitrogen fixation. The addition of nitrate to nitrate reductase negative mutants (nr-) of Azoxpirillum did not stimulate nitrogenase activity. Under oxygen-limited conditions A. hrasilcnxe and A. lipoferum were also shown to reduce nitrate to ammonia, which accumulated in the medium. Both species, including strains of A. hraxilenxe which do not possess a dissimilatory nitrite reductase (nir~) were also capable of reducing nitrous oxide to N2.
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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.