Distinctive Light and CO2-Fixation Requirements of Nitrate and Ammonium Utilization by the Cyanobacterium Anacystis nidulans

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Abstract
The effect of light intensity on the rates of ammonium and nitrate uptake and of CO2 fixation has been determined in intact Anacystis nidulans cells. Ammonium uptake became saturated at photon flux values of about 60 microeinsteins per square meter per second, whereas both nitrate uptake and CO2 fixation reached saturation at about 250 microeinsteins per square meter per second, the rates of the two latter processes being tightly correlated at any light intensity assayed. Inhibition of ammonium assimilation resulted in the loss of correlation between CO2 fixation and nitrate uptake, the latter process exhibiting then a reduced light requirement. The results establish a clear distinction between ammonium utilization and nitrate utilization with regard to their light requirement and to the nature of their dependence upon CO2 fixation.