Nitrogen-fixing Bacterial Strains Research Articles

In vitro inhibition of nonsymbiotic nitrogen-fixing bacteria by rhizosphere actinomycetes associated with grasses

Eighty-five isolates of actinomycetes were obtained from the rhizospheres of 10 indigenous grasses of central Texas. Of these, 10 were classified as members of the genus Nocardia; the remainder were members of the genus Streptomyces. To determine whether antibiotic production might be a significant factor in the establishment of associative symbioses between grasses and diazotrophic bacteria, 12 strains of free-living nitrogen-fixing bactria were cross streaked against each actinomycete isolate. This in vitro assay indicated that 78% of the 85 isolates were antagonistic to at least one of the 12 nitrogen-fixing bacterial strains.

Characterization of a nitrogen-fixing bacterial strain from the roots of Digitaria sanguinalis.

Rates of nitrogen fixation of 3 to 10 g of N2 fixed per hectare per day were associated with root systems of Digitaria sanguinalis. A Gram-negative motile aerobic bacterial strain that was capable of N2 fixation was isolated from a washed root sample of one of these plants. Optimal growth and N2 fixation occurred at a pH of about 6.5, a temperature of 30-37 degrees C, and at a pO2 of about 0.01 atm. Increased rates of N2 fixation resulted when this strain was grown in mixed cultures with aerobic or facultative bacteria. Observations of cellular and cultural morphology and results of biochemical and physiological studies indicate that the isolate may be related to the Azotobacteraceae but that it is not identical with any of the members of this family. The importance of N2 fixation by this isolate in nature is unknown.