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Abstract

The Río de la Plata is the world’s largest estuary, draining the second largest basin of
South America. In spite of its key significance at the local and global scale, this is the first attempt
made so far to characterize the bacterial diversity of this system. We employed a suite of molecular
methods to analyze the bacterial diversity at different phylogenetic levels, both in terms of richness
and evenness, and we related these findings to physicochemical parameters. A clear environmental
gradient was observed with 3 distinctive zones (riverine, frontal, and marine). In turn, the bacterial
communities appeared highly diverse and strongly structured by the environment. A high number
(324) of different operational taxonomic units (OTUs) were identified by automated ribosomal intergenic
spacer analysis (ARISA). The ARISA results suggested that the composition of bacterioplankton
communities was strongly related to physicochemical water characteristics, and that the bacterioplankton
had little overlap with the bacterial assemblage of the sediments. Catalyzed reporter
deposition-fluorescence in situ hybridization (CARD-FISH) indicated that Betaproteobacteria and
Actinobacteria were more abundant in the riverine zone, whereas Gammaproteobacteria and Planctomycetes
preferred the marine waters. The relative abundance of Bacteroidetes peaked in the
frontal zone, and Alphaproteobacteria were numerically dominant throughout the estuary. The
analysis of about 300 almost full-length 16S rRNA gene sequences pointed to the existence of ubiquitous
phylotypes, but also suggested habitat preferences among certain groups. This first look into
the bacterial community of the Río de la Plata demonstrated marked diversity patterns, providing further
insights into the ecology of globally distributed groups of aquatic bacteria.