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13 pages, 8 figures, 3 tablesThe microbial assemblages of Petit Saut reservoir and Sinnamary river (French Guiana, South America) were analyzed by biochemical analyses (pigments and specific fatty acids), and by polymerase chain reaction (PCR) denaturing gradient gel electrophoresis (DGGE) and sequence analysis of 16S rRNA gene fragments. Samples from a vertical profile in the reservoir and from a longitudinal profile downstream of the dam were analyzed and compared to a reference sample taken in the river, upstream of the reservoir. The distribution of bacterial populations could be explained by the physico-chemical conditions of the water and the hydrodynamics induced by the hydroelectric plant. Two bacterial Œbio-filters¹ were detected by biochemical analyses: methanotrophs at the oxycline level and phototrophic green sulfur bacteria below the oxycline. A statistical analysis of the DGGE fingerprints obtained for each sample showed that the different stations could be related in a dendrogram in good agreement with their geographic location. We detected 53 bands in the bacterial DGGE fingerprints and 3 bands in the archaeal DGGE. The phylogenetic affiliation of the predominant DGGE bands was inferred by a comparative 16S rDNA partial sequence analysis (450 to 500 base pairs). Sequences obtained from the reservoir and the river were related to Proteobacteria (α, β, and γ), Cyanobacteria, green sulfur bacteria, Cytophaga, Methanomicrobiaceae and Thermoplasmales. Thus, methanotrophs and green sulfur bacteria were detected, and bacteria involved in the iron biogeochemical cycle were also shown to be dominant. The 16S rDNA-based analysis indicated that new bacterial and archaeal lineages, not detected upstream of the dam, were present in the reservoir and that the microbial diversity of these assemblages was greater than previously described by biochemical analyses. Our data provide evidence that the former bacterial assemblage was completely modified by the impounding of the reservoir, in agreement with the different biogeochemical cycles established. Minor groups of bacteria became dominant in the reservoir while others disappeared, in adaptation to the new conditions induced by flooding of rain forest.We thank sincerely all the staff of Petit\udSaut Laboratory for help, and technical and human support. We are also grateful to EDF/CNEH for financial support, and particularly to Chantal Sissakian. Parts of this work were also funded by project MIDAS (Microbial Diversity in Aquatic Systems,\udMAS3-CT97-0154) from the European Union. We also thank Professor Louis Labroue for starting this project and Mrs Yvette Durand for managing the administrative part of the project with virtuosity.Peer reviewe

Changes in bacterial and archaeal assemblages in an equatorial river induced by the water eutrophication of Petit Saut dam reservoir (French Guiana)