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

MNHN, USM505/EA4105 Ecosystèmes et interactions toxiques, 57 rue Cuvier, case 39, 75231 Paris Cedex 05, France; SCE, Département Eau, Pôle Fleuves, Rivières et Milieux Humides, Route de Gachet, 44307 Nantes, France; Institut Pasteur-CNRS URA 2172, Unité des Cyanobactéries, 25-28, rue du Dr Roux, 75015 Paris, France; INSERM, U565, Acides nucléiques : dynamique, ciblage et fonctions biologiques, 57 rue, Cuvier, CP26, Paris Cedex 05, F-75231, France; MNHN, USM503, Département de « Régulations, développement et diversité moléculaire », Laboratoire des Régulations et dynamique des génomes, 57 rue Cuvier, CP26, Paris Cedex 5, F-75231, France; CNRS, UMR5153, Acides nucléiques : dynamique, ciblage et fonctions biologiques, 57 rue Cuvier, CP26, Paris Cedex 5, F-75231, France; INRA, UMR CARRTEL, BP 511, 74203 Thonon-les-Bains Cedex, France; Université Paris 7, 2, Place Jussieu, 75005 Paris, France

^* To whom correspondence should be addressed. Email: humbert{at}thonon.inra.fr. quiblier{at}mnhn.fr.

	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 in 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 proportion of these two sub-populations, we performed a two-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 the mcyA genotype varied considerably over time (30 to 80%). The changes in the proportions of the mcyA genotype appeared to be inversely correlated to changes in the cell density of P. agardhii and also, to a lesser extent, to the availability of certain nutrients and the abundance of cladocerans. In toxic cells, the MC cell quota varied throughout the survey. However, a negative correlation was found between the MC cell quota and the mcyA cell number during two short periods characterized by marked changes in the cyanobacterial biomass. Finally, only 54% of the variation in the MC concentrations measured in the lake can be explained by the dynamics of the MC-producing genotype cell density, suggesting that this is not a satisfactory method for use in monitoring programs intended to predict the toxic risk associated with cyanobacterial proliferation.