Temporal variations in the dynamics of potentially microcystin-producing strains in a bloom-forming Planktothrix agardhii (Cyanobacterium) population

Abstract

The concentration of microcystins (MCs) produced during blooms depends on variations in both the proportion of strains containing the genes involved in MC production and the MC cell quota (the ratio between the MC concentration and the density of cells with the mcyA genotype) for toxic strains. In order to assess the dynamics of MC-producing and non-MC-producing strains and to identify the impact of environmental factors on the relative proportions of these two subpopulations, we performed a 2-year survey of a perennial bloom of Planktothrix agardhii (cyanobacteria). Applying quantitative real-time PCR to the mcyA and phycocyanin genes, we found that the proportion of cells with the mcyA genotype varied considerably over time (ranging from 30 to 80% of the population). The changes in the proportion of cells with the mcyA genotype appeared to be inversely correlated to changes in the density of P. agardhii cells and also, to a lesser extent, to the availability of certain nutrients and the abundance of cladocerans. Among toxic cells, the MC cell quota varied throughout the survey. However, a negative correlation between the MC cell quota and the mcyA cell number during two short periods characterized by marked changes in the cyanobacterial biomass was found. Finally, only 54% of the variation in the MC concentrations measured in the lake can be explained by the dynamics of the density of cells with the MC producer genotype, suggesting that this measurement is not a satisfactory method for use in monitoring programs intended to predict the toxic risk associated with cyanobacterial proliferation.

FIG. 1.

(A) Example of the standard curves for the mcyA (black circles) and PC (gray squares) genes based on predetermined concentrations of DNA of the MC-producing Planktothrix strain PMC 75.02 amplified by multiplex real-time PCR. The error bars indicate standard deviations (n = 13). (B1) Example of C_T numbers obtained for the mcyA (black circles) and PC (gray squares) genes amplified by multiplex real-time PCR from a mix of MC-producing (PMC 75.02) and non-MC-producing (PMC 87.02) Planktothrix strains. (B2) The ΔC_T was calculated for each mix, and the ΔC_T equation was obtained by relating the calculated ΔC_T to the percentage of cells with the mcyA genotype.

FIG. 2.

Correlations between the Planktothrix cell densities in the BNV from March 2004 to March 2006 as determined by counting under the microscope and as determined by the GCN method for the PC gene (PC gcn) (A) and between the results of the GCN and ΔC_T methods used to evaluate the percentage of cells with the mcyA genotype (B).

FIG. 3.

Phytoplankton, zooplankton, and environmental parameters for the lake from March 2004 to March 2006. (A) Changes in the density of P. agardhii cells and in the proportion of P. agardhii cells with the mcyA genotype. The highest proportions of cells with the mcyA genotype are circled. The error bars indicate standard deviations (n = 4). (B) Densities of P. agardhii cells, chlorophyll a (chla) concentrations, and water temperatures. (C) Total phosphorus (TP) and total nitrogen (TN) concentrations. (D) SRP, ammonium (NH₄⁺), and oxidized nitrogen (NO₃⁻) concentrations. (E) Abundance of the main groups of zooplankton. The four periods are separated by dashed lines.

FIG. 4.

Projection onto the plane defined by the two first axes of the PCA of the data from the 52 samples obtained between March 2004 and March 2006, which are divided into four groups (circled) (A), and the environmental and biological variables (B). The first two axes accounted for 60% of the total variability. TP, total phosphorus.

FIG. 5.

Changes in the density of P. agardhii mcyA genotype cells, the MC concentration, and the MC cell quota from March 2004 to March 2006. eq., equivalents.