The genomic content of outer membrane vesicles (OMVs) in freshwater cyanobacteria
Outer membrane vesicles (OMVs) are secreted by many bacteria, where they are involved in diverse extracellular functions. They contain DNA which has been reported to be a source for lateral gene transfer. However, it is only poorly understood what the natural genetic content of OMVs comprise and how it changes with different environmental conditions. Here, I searched for the existence of OMVs in several cyanobacteria and studied their genomic content. Transmission electron microscopy showed OMVs in the unicellular Synechocystis sp. PCC 6803, as well as in the filamentous Anabaena sp. PCC 7120 and Chlorogloeopsis fritschii PCC 6912 as discrete, spherical blebs having a bilayer membrane. Sequencing revealed the presence of most of the genome including the chromosome and plasmids (if existing) in all three species. Additionally, a temperate phage was identified in C. fritschii PCC 6912. For Syn. sp. PCC 6803, I found that the OMV abundance is significantly higher in bacteria cultured under heat– or high salt– stress in comparison to the control conditions. Sequencing revealed that two small cryptic plasmids were highly overrepresented in the vesicle DNA independent of the growth conditions compared to a genomic DNA sample. I additionally identified overrepresented genes dependent on the growth condition in OMVs. I hypothesize that the enrichment for small plasmids in the vesicles is a stochastic process and a result of their high abundance in the cell, their packed topology, high diffusibility, and location. Further, I speculate that various mechanisms involved in cell cycle, transcription, or gene amplification might increase the likelihood of specific sequences to be packed in OMVs. My observations implicate that OMVs are a transfer mechanism of small plasmids by providing a nuclease protected DNA space and may serve as a mechanism to maintain plasmids and/or important genes for the population.
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