Microcystin production in Microcystis aeruginosa: effect of type of strain, environmental factors, nutrient concentrations, and N:P ratio on mcyA gene expression

Blooms affect water quality mainly due to the release of microcystins (MCs) by cyanobacteria. The synthesis of MCs is influenced by factors such as nutrient concentration, temperature, pH, light intensity, salinity, turbidity, and the presence of xenobiotics. In this study, we evaluated the effect of environmental factors (temperature and light intensity), the concentration of three nutrients (NaNO₃, K₂HPO₄, and FeCl₃), and the N:P ratio on the growth of two Microcystis aeruginosa strains (Ch10 and UTEX LB2385), as well as on mcyA gene expression and intracellular MC concentration. Under similar conditions, the population growth and chlorophyll a concentration per cell of both strains were different. The mcyA gene was significantly up-regulated from the early growth phase (5 days) to the stationary phase (15 days) in most cases, whereas intracellular MC concentrations varied depending on the assessed factor. The N:P ratio affected the development of both strains and MCs production differently. High concentration of intracellular MCs was recorded at low nitrogen and iron concentrations, low temperature, and high light intensity. The response in mcyA gene expression, related to the incubation time, of both strains was different, because strain Ch10 responded in most cases starting at 5 days of growth, whereas UTEX LB2385 responded until 10 and 15 days. This difference reflects physiological plasticity that could help to understand the permanence and dominance of Microcystis genus blooms in eutrophic freshwaters. The variability in response to the tested environmental factors confirms that population growth, genetic expression, and microcystin production are not related to a single factor but to an array of conditions that, when combined, stimulate MCs production. These conditions can be both stress-causing and favorable; hence, monitoring of environmental factors aimed at alerting against health risks provoked by cyanotoxins is a very complex task.