To perform the a culture-independent characterization of the microbial diversity observed within a water body in the Amazon, we constructed a library of bacterial and archaeal 16S rRNA genes isolated from environmental samples from the Tucuruí Hydroelectric Plant reservoir. The sampling locations included the upstream compartment (MR), which is located 50 km upstream of the dam, and Itupiranga (ITU), which is located upstream on the Tocantins River at the inlet of the reservoir. The bacterial phyla Cyanobacteria (46%), Proteobacteria (12%), Actinobacteria (9%), Bacteroidetes (4%) and Verrucomicrobia (2%) were identified at the MR. Phyla including the Chloroflexi, Acidobacteria, Planctomycetes and TM6 candidate division were observed at a frequency of <2%. The phyla Cyanobacteria (62%), Proteobacteria (5%) and Actinobacteria (7%) were identified at ITU. The archaeal diversity was limited to the Euryarchaeota and Crenarchaeota phyla; the latter was found in greater abundance and mainly consisted of uncultured phylotypes. In both regions, 25% and 95% of the bacteria and archaea, respectively, that were identified were previously unclassified. The phylum Cyanobacteria was almost exclusively represented by the genus Synechococcus sp.; however, a wide variety of microorganisms from the phylum Proteobacteria were identified from the classes alpha, gamma, delta and beta proteobacteria, with the latter being the most abundant. This study demonstrates that high prokaryotic diversity exists in the Amazonian rivers in addition to the previously reported megadiversity of fauna and flora.
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