Crossing the Freshwater/Saline Barrier: A Phylogenetic Analysis of Bacteria Inhabiting Both Freshwater and Marine Ecosystems
Excluding the underexplored subsurface environment, aquatic systems are the dominant microbial habitat on earth. Traditionally water on earth has been divided into marine (salty) and limnic (usually referring to inland freshwater) environments, and has been studied accordingly as separate disciplines. All microorganisms can regulate their cytoplasm osmolarity to a certain extent. However due to the higher salinity in their habitat, marine microbes are thought to be genetically and functionally better adapted for constant osmoregulation. Accordingly, most data support the concept that marine and freshwater bacteria are phylogenetically distinct one from the other; however, an in-depth phylogenetic comparative study has never been conducted.
Using the SILVA SSU database of curated 16S rRNA sequences we show that a small, yet significant, number of limnic and marine taxa have highly similar or identical rRNA sequences. These taxa are spread over the entire phylogenetic tree; however, most members belong to the domain Bacteria. To exclude that these sister marine and limnic organisms are a result of environmental connectivity, we show that many of the identical sequences were sampled at great distance one from the other and in different years. Biogeographically this shows the potential for a cross-globe transport of microorganisms that result in successful enough colonization to be picked up by sequence analysis.
While an identical rRNA sequence is not an absolute proof of identical organisms, our results point toward organisms that are able to cross over between saline and freshwater environments. Future studies comparing the vast metagenomics data that is continuously accumulating from both ecosystem types will reveal the true similarity between sister marine and limnic taxa as well as their cellular adaptation toward the opposing environment.
Keywordssaline freshwater barrier marine limnic biogeography osmoregulation
We thank Hans-Peter Grossart, Rudolf Amann, and Pelin Yilmaz for their support and help on this project. M.B.I. thanks the DAAD for sponsoring her Ph.D., in which this work has been commenced.
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