Crossing the Freshwater/Saline Barrier: A Phylogenetic Analysis of Bacteria Inhabiting Both Freshwater and Marine Ecosystems

Abstract

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.

Mina Bižić-Ionescu and Danny Ionescu

After months of politely refusing suggestions to meet from a mutual friend, we eventually had a coffee together in January 2009. Mina, who finished her M.Sc. studies in Belgrade, Serbia, in 2005 and has since finished a second M.Sc. on modern science, and the kosher laws in Judaism in Stockholm, Sweden, was at the time a research assistant at the Kinneret Limnological Laboratory in Israel, working with Werner Eckert. Danny was in his last year of his Ph.D. with Aharon Oren at the Hebrew University of Jerusalem. After a short hello, we both confessed that we are heading for Germany for our next career step, Mina for a Ph.D. and Danny for a postdoc. This was already a sign that this coffee is only the first of many. Our following dates were a series of 24 h experiments that made us realize how well we function together as a research team. Our passion for the sea took us to the next level, as in September 2009 at a depth of 36 m in the Red Sea, Danny proposed; a step that culminated a year later with a sunset wedding on the Mediterranean coast. Mina’s Ph.D. was a collaborative research between the Leibniz Institute for Freshwater Ecology and Inland Fisheries and the Max Planck Institute for Marine Microbiology in Bremen, where Danny did his postdoc. This allowed us to collaborate at work and build a home and a family in Bremen. We have been officially and unofficially involved in each other’s projects resulting in several common publications. However, the part of our collaboration we are most proud of is Duan El’azar born in 2012, making Mina’s Ph.D. “slightly” more exciting, and Eva-Mai Leah born in 2015. Today we are both postdocs at the Leibniz Institute for Freshwater Ecology and Inland Fisheries working on several projects, among them methane production in oxygenated water column and particulate organic matter degradation and succession of associated microbial communities.