The growth attributes of an aerobic nitrogen-fixing strain, Miami BG 043511, under aerobic and microaerobic/anaerobic conditions were examined using conventional batch and synchronous culture methods. Generation times of this strain, estimated from the increase in cell density under aerobic and anaerobic batch culture conditions, were 19-23 h and 15-19 h at 30 °C, respectively. It seems, therefore, that atmospheric oxygen did not seriously affect diazotrophic growth in this strain. Under a periodic light-dark regime, cells grew synchronously even under microaerobic/anaerobic conditions. When the aerobic culture entered the light period, a peak of photosynthetic activity was followed by a peak of nitrogenase activity. In contrast, a peak of nitrogenase activity preceded a peak of photosynthetic activity under microaerobic/anaerobic conditions. In both cases, however, cell division was observed at or just after the peak of photosynthetic activity. The difference in the timing of the appearance of nitrogenase activity in microaerobic/anaerobic cultures was ascribed to the inability of cells to generate sufficient ATP under anaerobic dark conditions. Periodic changes in cellular carbohydrate content, associated with the periodic appearances of photosynthetic and nitrogen-fixing activities, were observed under both aerobic and microaerobic/anaerobic conditions. Cellular carbohydrate content increased from 10% to 60% of cell dry weight during the phase of photosynthesis under aerobic conditions, while it reached only 40% under microaerobic/anaerobic conditions. The amount of reserve polysaccharides required to support nitrogen fixation was larger in aerobic cultures than in microaerobic/anaerobic cultures.


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