Nitrogen fixation by Azotobacter chroococcum in the presence of Colpoda steini—II. The influence of agitation

Abstract

The effects of agitation on the growth and N 2 fixation of pure cultures of Azotobacter chroococcum Beijerinck strain 7115 and mixed cultures of A. chroococcum strain 7115 with Colpoda steini Maupas are reported. The liquid cultures were incubated at 28°C and an orbital shaking machine was used at 60 rev/min. In the first 14 days of incubation all the shaken cultures fixed significantly more N 2 than the static cultures ( P < 0.01). N 2 fixation in the static cultures in the third and fourth weeks of incubation surpassed the total quantities of N 2 fixed in the shaken cultures. The quantities of N 2 fixed by mixed and pure cultures of Azotobacter sampled on the same day were not significantly different ( P < 0.05) in either the static or shaken cultures. There were fewer Azotobacter cells present in mixed cultures compared with pure Azotobacter cultures sampled at the same time, when trophic ciliates were present. The differences in the size of Azotobacter populations between the mixed and pure cultures were less marked amongst the shaken than in the static cultures. The normal morphological changes of Azotobacter cells associated with increasing population age did not appear to be delayed by ciliate predation. Also, there was no evidence to show that ciliate predation influenced the proportion of viable cells in the total Azotobacter populations.