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The kinetics of ^14C fixation have been investigated in Anabaena flos-aquae growing on NH^+_4, NO^-_3 and N_2-N in batch cultures. Growth rate was highest with NH^+_4, followed by NO^-_3 and finally N_2. The compensation intensity(I_0) and the half-saturation irradiance(K_1) with N_2 were higher than with other N sources, but the maximum C fixation rate(P_max) was lower. The P_max/K_1 ration, which is analogous to quantum efficiency at low irradiance ranges, was also lower with N_2. All these parameters except K_1 decrease with culture age, or decreasing growth rate. Since ^14C uptake measures net photosynthesis, the higher values of I_0 and K_1, and the low values of P_max and P_max/K_1 ratio with N_2 appear to be related to the high energy demand of N_2 fixation. They may also be related to the low maximum growth rate with N_2-N.
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