Nitrification and simultaneous denitrification by Azospirillum brasilense 12S.

Azospirillum brasilense, an associative diazotrophs from sorghum roots grows autotrophically on NH + 4 and CaCO3. NH + 4 a is also oxidized to NO – 2 and then denitrified. Addition of malate to the autotrophic medium enhances both NH + 4 oxidation as well as NO – 2 dissimilation. The incomplete nitrification linked denitrification results in a rapid loss of nitrogen from the growth medium. The bacterium also shows assimilatory NO3− and NO 2 reductases and fixes nitrogen at < 50 μg N/ml of NH + 4 , NO−3 or NO-2.

In vitro and in vivo nitrogenase activity of Rhizobium mutants and their symbiotic effectivity.

A slow growing nitrogen-fixing strain of Vigna radiata var. aureus (mung bean) Rhizobium which expressed nitrogenase activity in a synthetic medium was isolated from its native population. Mutants with decreased and increased nitrogenase activity were derived from this strain by treatment with acridine orange and ethidium bromide. These mutants were tested for symbiotic effectivity in vivo. The effectivity of mutants with decreased nitrogenase activity in the culture medium was lower than the parent strain; however, the effectivity of mutants with higher nitrogenase activity did not increase above that of the parent. This suggests that the plant is perhaps a limiting factor in the full expression of rhizobial nitrogenase in the nodules.