Simultaneous Short-term Uptake of Nitrate and Ammonium by Oscillatoria agardhii Grown in Nitrate- or Light-limited Continuous Culture

The cyanobacterium Oscillatoria agardhii Gomont was grown in continuous culture under nitrate- or light-limited conditions. Short-term uptake rates of nitrate and ammonium were studied at various concentrations of nitrate and ammonium, both of which were present during the short-term assay. In nitrate-limited O. agardhii, the short-term ammonium uptake rate was not influenced by the presence of nitrate. Ammonium inhibited the rate of nitrate uptake in a non-competitive manner. An equation was derived for the short-term nitrate uptake rate which was a function of the ambient concentrations of both nitrate and ammonium. For a low ammonium concentration tested, the pattern of the nitrate-ammonium interaction in light-limited O. agardhii did not differ greatly from that found in nitrate-limited O. agardhii.