Direct demonstration of ammonia as an intermediate in nitrogen fixation by Azotobacter.

When Azotobcter vinelandii is grown on molecular nitrogen, it utilizes ammonia without enzymatic adaptation. Furthermore, the distribution of N15 in amino acids derived from cell hydrolysates is the same whether the isotope is supplied to the organism as ammonia or as Nx. These and other findings (1, 2) provide strong evidence that ammonia is the key intermediate in nitrogen fixation by this organism, but direct evidence such as has been recently supplied for the anaerobic system in Clostridium pasteuriunum (3) has been lacking. Through use of short time experiments with suspensions instead of growing cultures, we have now succeeded in obtaining concurrent fixation of Nz and assimilation of added NH,+-N. This has enabled us to provide a more rigorous demonstration of the r81e of ammonia as an intermediate in the fixation pathway of this organism.