SUMMARY: (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 NH 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 N 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 N were not successful with the strain of used.


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