Competition for phosphorus between the nitrogen-fixing cyanobacteria Anabaena and Aphanizomenon

The influence of N2 fixation on the P-limited growth of two strains of Anabaena and Aphanizomenon was investigated using continuous cultures. Under N2-fixing conditions Anabaena had a higher maximum growth rate, a greater affinity for P, a higher yield on P and a higher N2 fixation activity than Aphanizomenon. In contrast to Anabaena, Aphanizomenon did not adjust its specific N2 fixation activity and its cells became N depleted at high growth rates. Metabolic control analysis revealed that the growth rate of Aphanizomenon was controlled to a lesser extent by P than Anabaena's growth rate. As predicted on the basis of these monoculture measurements, Anabaena was the superior competitor for P in competition experiments. The results might help to distinguish the niches of these two closely related species.