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Aerobic nitrogen-fixing Synechococcus RF-1 and RF-2, and Gloeothece 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 Synechococcus RF-1 and RF-2 fluctuated more than in the Gloeothece strains. Both Synechococcus and Gloeothece required Ca2+ for their nitrogenase activity in vivo. In the Synechococcus strains, inhibition of nitrogenase activity by EGTA was prevented by Ca2+ and was partly overcome by Sr2+, whereas in the Gloeothece strains only Ca2+ prevented the inhibition. O2 was required for nitrogenase activity of all four isolates in the dark. The optimum concentration of O2 was 5% (v/v); higher levels led to decreased nitrogenase activity. Nitrogenase activity of the four isolates was repressed by 0·01% NaNO3 or (NH4)2SO4. No catalase activity could be detected in any of the isolates. Synechococcus RF-1 and RF-2 lacked urease activity, whereas Gloeothece RF-6 and RF-7 exhibited high urease activity.
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