We report results obtained during a laboratory study conducted to evaluate the effect of ammonium, iron as well as temperature on the growth rate, protein and microcystin (MC) production in cultures of Microcystis aeruginosa isolated from San Roque reservoir (Córdoba ? Argentina).
Semi-continuous cultures were carried out by triplicate using a modified Z8 culture medium, adapted to achieve different iron and ammonium concentrations. We tested two temperatures (20 and 28°C), two iron concentrations (1 µM and 10 µM) and two ammonium-nitrogen conditions (0 µM and 54 µM).
Growth rates for cultures having 1 µM-Fe were significantly higher than cultures with 10 µM-Fe. Total MC (TMC = MC-LR + MC-RR) per cyanobacteria cell as well as the protein production per cell were significantly higher for cultures containing 10 µM-Fe than the corresponding for 1 µM-Fe. However, the ratio TMC: protein was almost the same for both iron concentrations. Thus, a high level of iron disfavoured cyanobacterial growth rates but enhances both protein and MC production in the same proportion.
Growth rates for cultures having 54-µM ammonium-nitrogen were significantly higher than cultures without addition of ammonium. TMC per cyanobacteria cell as well as the protein production were significantly lowered in presence of ammonium. Nevertheless, the ratio TMC: protein was significantly lower for cultures having ammonium. Thus, in our case, the presence of ammonium favoured cyanobacterial growth rates but reduces the production of MC.
An increase in temperature enhances growth rates but does not affect neither protein production nor the content of TMC per cell. However, we observed that cultures carried out at 28°C maintain the same MC profile during all the studied period, while cultures developed at 20°C show a progressive dominance of MC-RR over MC-LR. Thus, different temperatures produce a significant change in the pattern of MC production but not in the content of TMC per cell.
Results reported here open questions related to the probable inhibitory role of ammonium on MC production as well as on the incidence of the temperature on the production of different variants of MC by Microcystis aeruginosa.
