Spatiotemporal variations in microcystin concentrations and in the proportions of microcystin-producing cells in several Microcystis aeruginosa populations.

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(+) cells. Finally, the variations in total MC concentrations (0.3 to 15 microg liter(-1)) and in the MC cellular quotas (0.01 to 3.4 pg cell(-1)) were high both between and within sites, and no correlation was found between the MC concentrations and the proportion of mcyB(+) cells. All of these findings demonstrate that very different results can be found for the proportions of potentially MC-producing and non-MC-producing cells and MC concentrations, even in M. aeruginosa populations living in more or less connected ecosystems, demonstrating the importance of the effect of very local environmental conditions on these parameters and also the difficulty of predicting the potential toxicity of Microcystis blooms.

Genetic diversity in Microcystis populations of a French storage reservoir assessed by sequencing of the 16S-23S rRNA intergenic spacer.

We compared the genetic diversity of the 16S-23S spacer of the rRNA gene (ITS1) in benthic and pelagic colonies of the Microcystis genus isolated from two different sampling stations with different depths and at two different sampling times (winter and summer) in the French storage reservoir of Grangent. In all, 66 ITS1 sequences were found in the different clone libraries. The nucleotide diversity of all the sampled isolates were in the same range (average number = 0.022) regardless of their origin, showing that several clones are involved in the summer bloom event and contribute to the high biomass production. Phylogenetic study and analysis of molecular variance (AMOVA) revealed no obvious genetic differentiation between the benthic and pelagic isolates. This finding confirms that the Microcystis genus in this lake is characterized by having both a benthic phase in winter and spring allowing this organism to survive in unfavorable environmental conditions, and a pelagic phase in summer and autumn when environmental conditions allow them to grow in the water column. Finally, comparing these sequences with those available in the GenBank database showed that some highly conserved genotypes are found throughout the world.