Marine microbes shed packets of DNA, nutrients

Superabundant bacteria in the ocean routinely sacrifice parts of themselves, scientists have discovered. This sacrifice, in which bacteria pinch off minuscule spheres called vesicles, may influence climate change by affecting how much carbon dioxide the ocean can absorb.

Photosynthetic bacteria in the genus Prochlorococcus shed two to five vesicles a day, researchers led by biological oceanographer Sallie Chisholm of MIT report in the Jan. 10 Science. Each vesicle is a membrane-wrapped bubble with about one-sixth the diameter of the bacteria and is packed with lipids, proteins, RNA and even DNA. Together, the hordes of bacteria may cast off 10,000 to 100,000 metric tons of organic carbon into the ocean each day in these parcels, Chisholm’s team calculates.

Prochlorococcus, which Chisholm and her colleagues first described in 1988, is the world’s most abundant marine microorganism with an estimated population of an octillion, or 10²⁷ organisms. It and other marine phytoplankton carry out about half of the world’s photosynthesis.

At just 600 nanometers across, Prochlorococcus cells look like specks under a light microscope, Chisholm says. But in electron micrographs, she and her colleagues noticed “these pimples — we call them ‘blebs’ — on the surface.” Microbiologist Steven Biller recognized them as vesicles. The researchers then found vesicles in water samples drawn from Vineyard Sound in Massachusetts and from the Sargasso Sea near Bermuda.

Prochlorococcus lives in the most nutrient-impoverished parts of the ocean, says David Scanlan, a marine microbiologist at the University of Warwick in Coventry, England. The bacteria have to convert carbon dioxide and sunlight into biologically useful carbon; they also scavenge for important nutrients such as phosphorus and nitrogen.

So why would organisms that work so hard to build and scavenge nutrients dump their hard-won resources?  The researchers propose several possibilities that may also help explain how carbon moves through the atmosphere and the ocean.

Most exciting to marine microbiologist Julie Huber of the Marine Biological Laboratory in Woods Hole, Mass., is that ocean bacteria might use vesicles to trade DNA. “It’s a new way of thinking about genetic exchange,” she says.

Prochlorococcus may constantly send out packets of genetic information that other organisms could take up and incorporate into their genomes. Such trading may blur lines between species, Huber says.

Supporting the idea, the researchers discovered DNA from 33 different phyla from all three domains of life inside vesicles, although bacterial DNA was the most abundant. The finding suggests that lots of marine microbes ship off genetic messages in the tiny bottles.

Another hypothesis is that Prochlorococcus may scatter vesicles to distract bacteria-killing viruses known as bacteriophage. Biller, Chisholm and their colleagues found that  bacteriophage can latch onto and inject genetic material into vesicles. If the vesicles serve as a distractor, that could allow the bacteria to thrive and convert carbon dioxide, a greenhouse gas, from the atmosphere into organic carbon. That carbon would eventually drift down to the bottom of the ocean, where it could remain for hundreds to thousands of years — or perhaps even longer, says Suzanne DeLorenzo, a biological oceanographer and microbial ecologist at the Oregon Health & Science University in Portland.

Or the bacteria may feed vesicles to other types of bacteria that the genus needs to flourish. Prochlorococcus cannot break down hydrogen peroxide and other molecules that may damage or kill it, so it must rely on other microorganisms to detoxify the chemicals. If such microbes don’t do photosynthesis, Prochlorococcus could provide them with nutrients in return. Two types of nonphotosynthetic bacteria could grow in seawater with Prochlorococcus vesicles as their only source of carbon, the researchers found.

If vesicles are a food source for other microbes, that may help carbon get consumed by larger organisms and eventually get metabolized back to carbon dioxide. Whether they function as a food source or a defense mechanism, the vesicles could affect the concentration of carbon dioxide in the atmosphere, and in turn, the world’s climate.  

Vesicles may perform all of these functions and perhaps more, Biller says: “They may be the Swiss army knife of the cell.”