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 26.8nmol 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 1g of glucose, 7.3 to 7.5mg of nitrogen was fixed. Specific activity of the isolate for acetylene reduction was 37.7nmol 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.