Nitrogen Fixation and a Nitrogen-Fixing Bacterium From the Roots of Eragrostis Ferruginea

Abstract

An appreciable amount of nitrogen-fixing (acetylene-reducing) activity was observed in the excised roots of graminaceous weed, Eragrostis ferruginea. The highest activity obtained at 5 % oxygen concentration was 26.8 nmol acetylene reduced/g/hr. From the excised root, an aerobic bacterial strain with rapid growth on nitrogen-deficient medium and high acetylene-reducing activity was obtained. The isolate, termed ER201, was Gram-negative nonmotile rod and resembled Azotobacter, although its taxonomic position is uncertain. Glucose, sucrose, ethanol, acetate, lactate, malate, pyruvate, and succinate supported its N2-dependent growth, while lactose, rhamnose, starch, xylose, glycerol, citrate, and p-hydroxybenzoate did not. At the expense of 1 g of glucose, 7.3 to 7.5 mg of nitrogen was fixed. Specific activity of the isolate for acetylene reduction was 37.7 nmol acetylene reduced per mg cell protein per min under 10.6 % oxygen concentration. The isolate failed to grow in nitrogen-deficient media with initial pH below 6.6, while its growth was abundant at pH 6.8 or higher.In a survey of nitrogen-fixing activities in the rhizosphere of herbaceous plants in Tokyo University Forest in Aichi, an appreciable amount of acetylenereducing activity was observed in the excised roots of graminaceous weed, Eragrostis ferruginea. A nitrogen-fixing bacterial strain was isolated from the roots and its characteristics were studied. MATERIALS AND METHODSPreparation of the plant root samples. E. ferruginea was collected several times from the field of the University Forest during May to July, 1977. Usually the root systems of 4 or 5 plants were supplied simultaneously. The root systems were cut off from the whole plant, mixed well, and, after gently shaking once in 261