DIAZOTROPHIC SYNCHRONOUS GROWTH OF A MARINE UNICELLULAR CYANOBACTERIUM, SYNECHOCOCCUS SP STRAIN MIAMI-BG-043511, UNDER AEROBIC AND MICROAEROBIC ANAEROBIC CONDITIONS/

The growth attributes of an aerobic nitrogen-fixing Synechococcos stra in, Miami PC 043511, under aerobic and microaerobic/anaerobic conditio ns were examined using conventional batch and synchronous culture meth ods. Generation times of this strain, estimated from the increase in c ell density under aerobic and anaerobic batch culture conditions, were 19-23 h and 15-19 h at 30 degrees C, respectively. It seems, therefor e, that atmospheric oxygen did not seriously affect diazotrophic growt h in this strain. Under a periodic light-dark regime, cells grew synch ronously even under microaerobic/anaerobic conditions. When the aerobi c 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 divis ion was observed at or just after the peak of photosynthetic activity. The difference in the timing of the appearance of nitrogenase activit y in microaerobic/anaerobic cultures was ascribed to the inability of cells to generate sufficient ATP under anaerobic dark conditions. Peri odic changes in cellular carbohydrate content, associated with the per iodic appearances of photosynthetic and nitrogen-fixing activities, we re observed under both aerobic and microaerobic/anaerobic conditions. Cellular carbohydrate content increased from 10% to 60% of cell dry we ight during the phase of photosynthesis under aerobic conditions, whil e it reached only 40% under microaerobic/anaerobic conditions. The amo unt of reserve polysaccharides required to support nitrogen fixation w as larger in aerobic cultures than in microaerobic/anaerobic cultures.