The Amazon region has several hydroelectric power reservoirs and, among these, is the Tucuruí reservoir, which has the fourth highest power in the world. This reservoir flooded a huge area of native forest, whose organic matter has been decomposed in the water column since then. This microbial process is carried out by several prokaryotic species, which present a great metabolic diversity, including among them methanotrophic bacteria and methanogens. In this study we sought to evaluate the community structure, composition and diversity along a water column of the Tucuruí reservoir located upstream of the dam by using denaturing high performance liquid chromatography (DHPLC) and 16S rDNA gene sequencing. Water samples were taken at intervals of 5 m from the surface down to a depth of 55 m along the water column. Our results show that the structures of communities of both Archaea and Bacteria remained constant down to a depth of 35 m indicating the existence of a stable environment until this depth. Changes in the abundance of some groups could be observed at the 40 m depth and the composition of the communities changed significantly at the depths of 45, 50 and 55 m. The deepest layer (55 m) presented the highest Archaeal diversity, while the highest Bacterial diversity was observed at the 0-40 m layers. Crenarchaeota were predominant in the upper layers, while Euryarchaeota, including methanogenic Archaea, were predominant in the 40 and 50 m layers, indicating the existence of an anoxic zone below 40 m. Actinobacteria, Bacteroidetes and Proteobacteria sequences were recovered from the superficial layers, as well as sequences not assigned to any phylum. Several sequences from the 55 m layer were assigned to Dechloromonas, known for the ability to degrade benzene and other aromatic compounds. We conclude that the structure and diversity of prokaryotic communities in the Tucuruí power reservoir is stable until the 35 m depth, changing in deeper layers.
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