Environmental diversity of bacteria in a warm monomictic tropical freshwater lake

Abstract

The phylogenetic diversity of freshwater bacteria, particularly in tropical lakes, has not been thoroughly investigated. Here, we employed a cloning- and sequencing-based analysis of the 16S rRNA gene to investigate the composition of bacterial communities from the upper (UW, 0 m depth) and bottom (BW, 7 m depth) waters of a tropical unstratified freshwater lake. A diverse bacterial community was found by phylogenetic analysis, with more diversity in BW than UW. Five phyla (OP11, OD1, Deinococcus-Thermus, Elusimicrobia, and Planctomycetes) were specifically found in BW. Chloroflexi was only found in UW, with four phyla (Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes) that were common in both the UW and BW. Unclassified bacteria were also found. LIBSHUFF analysis revealed that the composition of the bacterial community changed markedly between the UW and BW, despite the water circulation. Proteobacteria, especially Alphaproteobacteria, were dominant community members. Although Actinobacteria, Betaproteobacteria, and Bacteroidetes are thought to be the most abundant groups of freshwater bacterioplankton, they were not dominant in our samples. Conversely, poorly characterized groups, such as OP11 and OD1, were enriched in our samples. This study revealed that members of taxa previously described as typical for freshwater had low representation in this tropical freshwater lake. Altogether, the considerable proportion of poorly described phyla together with the high proportion of bacterial sequences from novel species suggests distinct ecological features of these communities from Carioca Lake. These data add significant new information regarding bacteria that inhabit tropical freshwater lakes and may have implications for the typical signature of bacteria in freshwater.