Aerobic nitrogen-fixing RF-1 and RF-2, and RF-6 and RF-7, were isolated from rice fields. In a light/dark regimen, they fixed nitrogen in a regular pattern, with nitrogenase activity occurring mainly in the dark period. When cells were cultivated under continuous illumination, the nitrogenase activity in RF-1 and RF-2 fluctuated more than in the strains. Both and required Ca for their nitrogenase activity In the strains, inhibition of nitrogenase activity by EGTA was prevented by Ca and was partly overcome by Sr, whereas in the strains only Ca prevented the inhibition. O was required for nitrogenase activity of all four isolates in the dark. The optimum concentration of O was 5% (v/v); higher levels led to decreased nitrogenase activity. Nitrogenase activity of the four isolates was repressed by 0.01% NaNO or (NH)SO. No catalase activity could be detected in any of the isolates. RF-1 and RF-2 lacked urease activity, whereas RF-6 and RF-7 exhibited high urease activity.


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