Ammonia as an Intermediate in Nitrogen Fixation by Azotobacter

Abstract

An aerated culture of A. vinelandii grown on N2 was supplied NH4+ labeled with N15; the culture immediately stopped fixing N2 and used NH4+ as its exclusive source of N. When N15-NO3- was supplied under the same conditions, no N15 was found in the cells for 30 min. in contrast to its detection in 1 min. when N15-NH4 + was supplied. Cultures of A. vinelandii previously grown either with N2 or with KNO3 assimilated NH4 in preference to NO3 when supplied with NH4NO3. The organism apparently had preformed enzyme systems capable of utilizing NH4 immediately and to the exclusion of other nitrogenous compounds. To use NO3 the cells must first adapt themselves to it. It is concluded that present information favors the view that NH3 is the first stable intermediate formed in N fixation by Azotobacter.