Little sacs can bud from cells, and they are known as extracellular vesicles. These little sacs can contain many molecules from the cell, like proteins, RNA, and bits of DNA. Scientists have now found that extracellular vesicles (EVs) in the ocean can facilitate the transfer of molecules between cells. These EVs serve as important modes of microbial communications, and are involved in horizontal gene transfer (HGT), in which mobile genetic elements can be shared among cells. This study has shown that direct contact between cells is not necessary for HGT. The findings have been reported in ISME Communications.
Samples of seawater contain huge numbers of microbes, including viruses, and material like EVs and gene transfer agents (GTAs). GTAs are tiny particles that only contain DNA. In this work, the researchers performed sophisticated experiments to show that DNA in samples of seawater is not using viruses to move. The researchers utilized DNA sequences that were known to originate from certain host cells, and determined how those sequences were moving - whether it was through viruses, EVs, or GTAs. Specific sequences were assigned to certain modes of transport.
"The result was surprising: Apparently, a large proportion of the DNA was not transported via classical routes, but via extracellular vesicles," said study leader Susanne Erdmann, head of the Max Planck Research Group Archaea Virology at the Max Planck Institute in Bremen.
At one time, EVs were thought of as waste, or waste disposal units. But in the past fifteen years, research has revealed that they serve a variety of functions. "Our study clearly highlights the fundamental role that EVs play for the exchange of genetic material between cells," noted first study author Dominik Lücking, a graduate student in the Erdmann lab.
Now, a change in terminology could be warranted, the researchers suggested. When seawater is passed through tiny filters as small as 0.2 microns, the genetic material that is isolated on the filter is usually thought of as viral. But that may not tell the full story because, "we are missing out on the variety of the other, non-virus-like particles" that can be isolated with these filters, such as EVs. Therefore, this portion of the sample might instead be called "protected extracellular DNA (peDNA)."
Now it's time for more research on peDNA in the many ecosystems on Earth, including soil and freshwater habitats. "The new nomenclature will enable us to talk more clearly about the mechanisms and processes not covered by the term virome. In view of the significance of horizontal gene transfer in many ecosystems, we are very sure that there are quite a few more surprises on the way ahead of us," added Erdmann.
Sources: Max Planck Society, ISME Communications