Marine cyanobacteria — tiny ocean plants that produce oxygen and make organic carbon using sunlight and CO2 — are primary engines of Earth’s biogeochemical and nutrient cycles. They nourish other organisms through the provision of oxygen and with their own body mass, which forms the base of the ocean food chain.
Now scientists at MIT have discovered another dimension of the outsized role played by these tiny cells: The cyanobacteria continually produce and release vesicles, spherical packages containing carbon and other nutrients that can serve as food parcels for marine organisms. The vesicles also contain DNA, likely providing a means of gene transfer within and among communities of similar bacteria, and they may even act as decoys for deflecting viruses.
In a paper published this week in Science, postdoc Steven Biller, Professor Sallie (Penny) Chisholm, and co-authors report the discovery of large numbers of extracellular vesicles associated with the two most abundant types of cyanobacteria, Prochlorococcus and Synechoccocus. The scientists found the vesicles (each about 100 nanometers in diameter) suspended in cultures of the cyanobacteria as well as in seawater samples taken from both the nutrient-rich coastal waters of New England and the nutrient-sparse waters of the Sargasso Sea.
Prochlorococcus is the smallest genome that can make organic carbon from sunlight and carbon dioxide and it’s packaging this carbon and releasing it into the seawater around it,” says Chisholm, the Lee and Geraldine Martin Professor of Environmental Studies in MIT’s Department of Civil and Environmental Engineering and Department of Biology, who is lead investigator of the study. “There must be an evolutionary advantage to doing this. Because the vesicles also contain DNA and RNA, the researchers surmise they could play a role in horizontal gene transfer, a means for developing genetic diversity and sharing ecologically useful genes among the Prochlorococcus metapopulation.