Growth responses of Microcystis ichthyoblabe Kützing and Anabaena aphanizomenoides Forti (cyanobacteria) under different nitrogen and phosphorus conditions

Abstract

Experiments in batch cultures under controlled sub-optimal light and temperature conditions were undertaken to determine the effect of nitrogen, phosphorus and N:P ratios on the growth of Microcystis ichthyoblabe Kütz. 1843 and Anabaena aphanizomenoides Forti 1912, two toxic cyanobacteria forming blooms from Oued Mellah lake. Phosphorus experiments show that densities of M. ichthyoblabe and A. aphanizomenoides increased differently in the various media. Under non-limiting phosphorus conditions (1000–6960 μg P l−1), 5–7 days of exponential growth was observed, while in P-free and in P-deficient media (0–500 μg l−1), the growth was limited. As with phosphorus experiments, cell growth of M. ichthyoblabe was substantially favoured under high nitrate concentrations (50–84 mg l−1), whereas cultures under N-free or N-deficient conditions (0–10 mg l−1) seemed to be limited. Nitrate-nitrogen at all tested concentrations was not limiting for the growth of A. aphanizomenoides cultures, which reached high density during an exponential growth of 8–9 days. Under low nitrate concentrations (0–5 mg l−1), an increased number of heterocysts was observed. There was a markedly diminished growth with the lower N:P ratio experiments (≤5) only for Microcystis and on the highest N:P ratio experiments (≥30) for both Microcystis and Anabaena aphanizomenoides.