With the aim of explaining the variations in microcystin (MC) concentrations during cyanobacterial blooms, we studied several Microcystis aeruginosa populations blooming in different freshwater ecosystems located in the same geographical area. As assessed by real-time PCR, it appeared that the potentially MC-producing cells (mcyB ؉ ) were predominant (70 to 100%) in all of these M. aeruginosa populations, with the exception of one population in which non-MC-producing cells always dominated. Apart from the population in the Grangent Reservoir, we found that the proportions of potentially MC-producing and non-MC-producing cells varied little over time, which was consistent with the fact that according to a previous study of the same populations, the intergenic transcribed spacer (ITS) genotype composition did not change (38). In the Grangent Reservoir, the MC-RR variant was the dominant microcystin variant throughout the bloom season, despite changes in the ITS composition and in the proportions of mcyB Microcystins (MCs) are the most common cyanotoxins and have been involved in several animal and human poisoning episodes (8). These hepatotoxic cyclic heptapeptides are synthesized by a multifunctional enzyme complex (10, 40), and the discovery of the gene cluster encoding this complex has made it possible in recent years to develop molecular tools for studying the relative proportions of MC-producing and non-MCproducing cells in natural cyanobacterial populations. Potentially MC-producing and non-MC-producing cells can coexist in these populations, but the factors and processes governing the dynamics of these subpopulations remain unclear.Some recent papers on the Microcystis genus have shown that the proportions of potentially MC-producing cells can differ considerably from lake to lake. For example, in Lake Wannsee, Germany, this proportion was between 0 and 40% (28), as it was in Lake Oneida, United States (18), and in Lake Mikata, Japan (48). In contrast, large variations over time (6 to 93%) of potentially MC-producing cells were found in the Grangent Reservoir, France (4). Major variations (30 to 80%) were also found in a natural French population of Planktothrix agardhii (3), and the variations in the proportions of potentially MC-producing cells reflected those of the MC concentrations. However, only 54% of the variation in MC concentrations could be explained by changes in the proportion of MC-producing cells, suggesting that a considerable part of the MC concentrations was also due to variations in MC cell quotas. These findings suggest that the toxic risks during cyanobacterial proliferations are due to variations in both the proportion of MC-producing cells and the production of MC by the toxic cells.

Microcystis aeruginosa is a toxic cyanobacterium, which is able to bloom in a wide range of freshwater ecosystems. By sequencing the Internal Transcribed Spacer (ITS) of the ribosomal operon, we compared the genetic composition of several French bloom-forming M. aeruginosa populations from two reservoirs located on the Loire River, at two sampling points located between these reservoirs, and finally in two ponds closely linked to this river. No significant difference was found in the genetic diversity of the six Microcystis populations but we evidenced a strong genetic differentiation between most of these populations. Indeed, the Microcystis population in the Grangent reservoir was genetically differentiated from the other three populations sampled further downstream, implying that no massive transfer of population occurs from this reservoir to downstream segments. We also found genetic differentiation between the populations from the two ponds, and between these populations and those from the Loire River. On the other hand, the same dominant genotype was found in the populations sampled both in the river and in the Villerest reservoir, suggesting the selection of a distinct genotype adapted to river conditions and also an accumulation of this genotype in the downstream reservoir. Finally, by comparing our ITS sequences with those available in the GenBank, no biogeographical differentiation could be detected at a global scale, suggesting that most of the Microcystis genotypes seem to be ubiquitous.