Growth and Physiology of Azotobacter chroococcum in Continuous Culture

SUMMARY Azotobacter chroococcum (ncib 8003) organisms, grown in continuous culture without fixed nitrogen, had chemical compositions at various dilution rates characteristic of nitrogen-limited populations. Fast-growing variants were selected for at high dilution rates; the efficiency of nitrogen fixation decreased with decreasing growth rate. In suitable media, carbon- and phosphate-limited populations were obtained and showed different compositions; they were very sensitive to inhibition by oxygen. Carbon-limited populations utilizing NH4 under argon were not oxygen sensitive; they formed nitrogenase when they were N-limited. The chemical compositions of the various populations corresponded to theory for the nutritional state considered. Nitrogen fixation entrained a maintenance coefficient of 1·06 g. substrate/g. organism/hr compared with about 0·40 for ammonia assimilation. Assuming most of this maintenance was directed to respiratory protection of nitrogenase, an extrapolated maximum requirement of 4 moles ATP/mole N2 fixed was observed. Attempts to repeat reports of (1) dependence of cytochrome pattern on nitrogen fixation and (2) increased efficiency of fixation with ultraviolet-irradiated N2 were not successful with the strain of A. chroococcum used.