1. ¹MNHN, USM505/EA4105 Ecosystèmes et interactions toxiques, Paris Cedex 05, France
2. ²SCE, Département Eau, Pôle Fleuves, Rivières et Milieux Humides, Route de Gachet, Nantes, France
3. ³Unité des Cyanobactéries, Institut Pasteur-CNRS URA 2172, Paris, France
4. ⁴INRA, UMR CARRTEL, Thonon-les-Bains Cedex, France
5. ⁵Laboratoire Microorganismes: génome et environnement, UMR CNRS 6023, Université Blaise Pascal, Clermont-Ferrand II, Aubière Cedex, France
6. ⁶Université Paris Diderot, PRG, Paris, France

Correspondence: J-F Humbert, Unité des Cyanobactéries, Institut Pasteur-CNRS URA 2172, 28 rue du Dr Roux, 75724 Paris Cedex 15, France. E-mail: humbert@pasteur.fr

Received 28 October 2008; Accepted 4 November 2008; Published online 18 December 2008.

Abstract

The variations in microcystin concentrations during cyanobacterial blooms in freshwater ecosystems appear to depend on numerous factors, which have still not been fully identified. To contribute to clarify the situation, we have developed a spatial sampling approach to determine the dynamics and genetic diversity of a bloom-forming population of Microcystis aeruginosa in a large French reservoir, and the variations in the proportions of microcystin-producing genotypes. We demonstrated that marked changes occurred in the internal transcribed spacer (ITS) genotype composition of the M. aeruginosa population during the development of the bloom. These changes led progressively to the selection of one dominant ITS genotype throughout the entire reservoir when the cell number reached its maximum. At the same time, we identified a decrease in the proportion of the mcyB+ genotype, and a significant negative correlation between this proportion and that of the dominant ITS genotype during the bloom. Thus, it appeared that favorable conditions for Microcystis cell growth led to the selection, within the Microcystis population, of a non-microcystin-producing genotype, whereas potentially microcystin-producing genotypes were dominant in this population before and after the bloom, when environmental conditions were less favorable for growth.