Analysis of a sediment core taken from the Grangent reservoir in 2004 showed the presence of high concentrations of Microcystis aeruginosa Kütz. colonies at the sediment surface (250 colonies AE mL sediment )1 ) and also at depths of 25-35 cm (2300 colonies AE mL sediment )1 ) and 70 cm (600 colonies AE mL sediment )1 ). Measurements of radioactive isotopes ( 7 Be, 137 Cs, and 241 Am) along with photographic analysis of the core were used to date the deep layers: the layer located at )30 cm dates from summer 2003, and that located at )70 cm from 1990 to 1991. The physiological and morphological conditions of those benthic colonies were compared with those of planktonic colonies using several techniques (environmental scanning electron microscopy [ESEM], TEM, DNA markers, cellular esterases, and toxins). The ESEM observations showed that, as these colonies age, peripheral cells disappear, with no cells remaining in the mucilage of the deepest colonies (70 cm), an indication of the survival thresholds of these organisms. In the benthic phase, the physiological conditions (enzyme activity, cell division, and intracellular toxins) and ultrastructure (particularly the gas vesicles) of the cells surviving in the heart of the colony are comparable to those of the planktonic form, with all the potential needed for growth. Maintaining cellular integrity requires a process that can provide sufficient energy and is expressed in the reduced, but still existing, enzymatic activity that we measured, which is equivalent to a quiescent state.

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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.

The spatio-temporal distribution of benthic colonies of Microcystis aeruginosa in Grangent Reservoir (France) in 2000 was not homogeneous and appeared to be controlled by many external factors: lake depth, station morphometry, substratum and hydraulic regime (lacustrine or fluvial). A most important concentration of benthic colonies was found at deep sites with fine sediment or at sites where the sediment was rich in organic matter. In spite of a stable water level and a minimum flow during summer, the number of benthic colonies showed great variation in the lacustrine downstream part of the reservoir. These variations may be explained by the dynamics of planktonic cyanobacteria. To cite this article: D. Latour, H. Giraudet, C. R. Biologies 327 ( 2004).  2004 Published by Elsevier SAS on behalf of Académie des sciences. RésuméFacteurs influençant la distribution spatio-temporelle des colonies benthiques de Microcystis aeruginosa (Cyanobactérie) sur la retenue de Grangent (Loire, France). La distribution spatio-temporelle des colonies benthiques de Microcystis aeruginosa sur la retenue de Grangent (France) en 2000 n'a pas été homogène et semble être contrôlée par de nombreux facteurs externes : profondeur du lac, morphométrie de la station, nature du substrat et régime hydraulique (fluvial ou lacustre). Les plus fortes concentrations de colonies benthiques ont été trouvées dans les sites les plus profonds avec du sédiment fin ou aux sites dont le sédiment est riche en matière organique. Malgré la stabilité du niveau de l'eau et un débit minimum pendant la période estivale, le nombre de colonies benthiques varie de façon importante dans la zone lacustre, à l'aval de la retenue. Ces variations peuvent en partie être expliquées par la dynamique des colonies planctoniques.

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