Photosynthetic activities of a synchronously grown aerobic N2-fixing unicellular cyanobacterium, Synechococcus sp. Miami BG 043511

Summary: Photosynthetic activities of a synchronously grown aerobic N2-fixing unicellular cyanobacterium, Synechococcus sp. strain Miami BG 043511, were examined. Cells 1 h after the onset of synchronous growth exhibited an O2 evolution rate of 400-500 mUmol (mg Chl aL)-1 h-1 at saturating light intensity, while it decreased to 40-80 mUmol (mg Chl a)-1 h-1 in 12 h cells. Changes in the relative fluorescence intensity (excitation wavelength 430 nm, emission 685 nm) were not as large as the changes in photosynthetic O2 evolution activity. Photosynthetic electron transport activities of both photosystem II and photosystem I in 12 h cells were lower than those in 1 h cells. Photosystem II activity with 2,5-dimethyl-p-benzoquinone, and photosystem I activity with diaminodurene and methyl viologen in 12 h cells were about 64 and 46%, respectively, of those in 1 h cells. However, the reduction in photosynthetic electron transport activities was not sufficient to account for the reduction in photosynthetic O2 evolution activity in 12 h cells, which was about 14% of that in 1 h cells. Based on these observations, the reduction in photosynthetic O2 evolution activity in 12 h cells was ascribed to the combined effect of a reduction in both the photochemical and the biochemical steps of photosynthesis.