Utilization of Fixed Nitrogen by Azotobacter and Influence on Nitrogen Fixation.

The influence of fixed nitrogen on Azotobacter both with respect to utilization and inhibition of nitrogen fixation has been studied by a number of investigators. The results have not always been conclusive because of the relatively poor growths of cultures obtained in culture media now known to have been deficient in one or more of the elements essential for nitrogen fixation or growth, particularly calcium, iron or molvbdenum. Inadequate analyses of the fixed nitrogen consumed by the organisms, which, with few exceptions, were cultured in the presence of nitrogen gas, also leads to uncertainty. The results reported in this paper were obtained by testing a variety of nitrogenous compounds on several strains of Azotobacter in a medium capable of giving growth equivalent to 20 to 30 mg. nitrogen per 100 ml. in a few days. The experiments wvere carried out chiefly in the absence of nitrogen gas in atmospheres containing one-fifth of an atmosphere of oxygen only or with hydrogen added to bring the total pressure to one atmosphere. Under these conditions any visible growth is evidence of utilization of the fixed nitrogen added uncomplicated by any simultaneous assimilation of nitrogen gas. Recent investigations in this laboratory, as well as those of Wyss et al. (1941), have shown that whereas hydrogen inhibits nitrogen fixation, it has no significant effect on the growth of Azotobacter supplied with the usual quantities of readily available fixed nitrogen. In addition to these experiments made in the absence of nitrogen gas a few cultures were grown in the presence of air, with several nitrogen compounds added to the medium in order to test the relative utilization of free and fixed nitrogen.