Influence of N/P ratio on competitive abilities for nitrogen and phosphorus by Microcystis aeruginosa and Aulacoseira distans

Microcystis aeruginosa and Aulacoseira distans strains were grown in batch cultures to investigate the consequences of N/P ratio on the growth of these species and on their abilities to take up nitrogen and phosphorus. N/P ratio did not influence the growth rates, which were similar under all the experimental conditions. However, exponential growth lasted longer in Microcystis than in Aulacoseira, especially under low N/P ratio conditions. Distinct patterns of nutrient uptake for Aulacoseira and Microcystis were observed. N-uptake was higher in Microcystis, but not influenced by N/P ratio. However, the amount absorbed was proportional to the concentration in the culture medium for both strains studied. Although Microcystis showed lower uptake of N per biomass unit, a greater yield of Microcystis growth relative to the diatom was observed. This could have resulted from its ability to produce biomass using less nitrogen per unit of biomass. A variation of N/P ratio in the culture medium during the growth of both species was observed. This owed to the uptake of nutrients, with Microcystis showing greater potential than Aulacoseira to influence the N/P ratio. Thus, in contrast to what has been stated in the literature, our results indicated that a low N/P ratio could be a consequence of the capacities and rates of cyanobacterial uptake of nitrogen and phosphorus.