Unprecedented levels of horizontal gene transfer among spatially co-occurring Shewanella bacteria from the Baltic Sea.

High-throughput sequencing studies during the last decade have uncovered that bacterial genomes are very diverse and dynamic, resulting primarily from the frequent and promiscuous horizontal gene exchange that characterizes the bacterial domain of life. However, a robust understanding of the rates of genetic exchange for most bacterial species under natural conditions and the influence of the ecological settings on the rates remain elusive, severely limiting our view of the microbial world. Here, we analyzed the complete genomic sequences and expressed transcriptomes of several Shewanella baltica isolates recovered from different depths in the Baltic Sea and found that isolates from more similar depths had exchanged a larger fraction of their core and auxiliary genome, up to 20% of the total, compared with isolates from more different depths. The exchanged genes seem to be ecologically important and contribute to the successful adaptation of the isolates to the unique physicochemical conditions of the depth. Importantly, the latter genes were exchanged in very recent past, presumably as an effect of isolate's seasonal migration across the water column, and reflected sexual speciation within the same depth. Therefore, our findings reveal that genetic exchange in response to environmental settings may be surprisingly rapid, which has important broader impacts for understanding bacterial speciation and evolution and for modeling bacterial responses to human-induced environmental impacts.