The results show that sucrose concentration had a great effect on both cell growth and nitrogenase activity of G. diazotrophicus. Nitrogenase activity was very low, but still measurable at 0.1% sucrose. The lowest nitrogenase activity was found when cells of G. diazotrophicus were grown at a medium pH of 4.5. The optimum pH of the medium for nitrogenase activity was 5.5. Nitrogenase activities decreased as medium KNO3 or (NH4)2 SO4 concentration increased from 0.1 to 10 mM. At 10 mM KNO3, nitrogenase activity was 15 times less than of the control. Nitrogenase activity increased as temperature increased from 15 to 30°C. Nitrogenase activity of the 5-day-old G. diazotrophicus colonies grown at 30°C was almost three times that of the 14-day-old colonies grown at 15°C. Our results also indicate that G. diazotrophicus has Hup capacity, but its Hup activity was relatively lower under the culture conditions (i.e. H2 consumption by the nif mutant was approximately 7% of the rate of H2 evolution by the wild-type G. diazotrophicus).
In conclusion, our study showed that G. diazotrophicus could grow and fix N2 at different concentrations of sucrose, mineral nitrogen, pH and temperature. The flexibility of G. diazotrophicus to fix N2 under a wide range of conditions increases the possibility of adapting G. diazotrophicus to fix N2 in new host species.
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