The effect of simultaneous N # fixation and light limitation on the growth of two strains of Anabaena sp. Bory de St. Vincent and Aphanizomenon flos-aquae (L.) Ralfs was investigated using continuous cultures. Under severely light-limited conditions, Aphanizomenon showed a broader absorption spectrum (due to the presence of phycoerythrin), a higher maximum efficiency of photosynthesis, a higher steady-state N # fixation activity and a higher growth affinity for light than did Anabaena. On the other hand, under light saturation, Anabaena showed a higher maximum rate of O # production and a higher maximum specific growth rate than Aphanizomenon. These monoculture results characterize Anabaena and Aphanizomenon, in relative terms, as a ' sun ' and a ' shade ' species respectively, and are in accordance with field observations. The difference between the two species in their acclimatory response is discussed in terms of a species-specific alteration of the PSI : PSII stoichiometry. Besides the species-specific modulation of the accessory pigments, such an acclimation would provide a biochemical basis for the observed physiological differences. The monoculture results were used to differentiate the niches of the two species and suggested that Aphanizomenon would competitively displace Anabaena under N # -fixing, light-limited conditions. However, when both species were grown together, Anabaena became dominant and seemed to be the superior competitor for light. In order to explain this finding, the possible effects of release of allelopathic compounds, or dynamic aspects of light supply, are discussed.

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.

The effect of simultaneous nitrogen jixation and phosphorus limitation on the physiological adaptation and growth performance of Aphanizomenon flos-aquae (L.) Ralfs PCC 7905 was studied in continuous culture. I n the absence of ammonia, N2jixation occurred and the maximum growth rate (as determined in diluted batch cultures) was lower. However, no distinction could be made between the steady-state N uptake rates (based on cellular N contents) of N,-Jxing cells and cells grown with ammonia. At the higher dilution rates, the residual P concentration increased with increasing dilution rate, more so under N2fixing conditions, compared to the cultures grown in the presence of ammonia. More generally, the yield of biomass per consumed P, as the biomass concentration itseg decreased with increasing dilution rate, and both were lower under N,-jixing conditions. The restricted biomass production under N2-jixing conditions suggests that reduction of N loading may benefit lake restoration projects. The injuence of N2-Jxation on the sevm't~ of P limitation is discussed in terms of metabolic control analysis. From the increase of the residual P concentration on switching from ammonium to N,-jixing conditions, it is deduced that under N,-jixing and P-limited conditions, control of growth is shared by N and P metabolism.Restoration of eutrophic freshwater lakes by reducing the phosphorus (P) load does not always result in a significant reduction of phytoplankton biomass (e.g. Van Liere and Janse 1992). It was suggested that an additional reduction of the nitrogen (N) load might lead to better (i.e. quicker) results (Lijklema 1983). A complicating factor however could be the occurrence of the cyanobacterial, filamentous species of the order Nostocales, potentially I

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The phosphorus-limited growth of cultures of the nitrogen-fixing cyanobacteria Aphanizomenon and Anabaena was investigated. In conditions of nutrient and light excess Anabaena has a competitive advantage. The lower the light intensity conditions at which Aphanizomenon populations dominate are indicated for future study.