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Marine microbes shed packets of DNA, nutrients: bacterial vesicles may offer genetic exchange opportunities, affect carbon cycle.

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 oceans 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 up to about one-sixth the diameter of the bacteria and is packed with lipids, proteins, RNA and DNA. Together, the hordes of bacteria may cast off 10,000 to 100,000 metric tons of organic carbon daily 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 global population of an octillion, or 1027. 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 alight microscope, Chisholm says. But in electron micrographs, she and her colleagues noticed "these pimples--we call them 'blebs'--on the surface." MIT 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 sun light 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.

The researchers also discovered vesicles carrying DNA from other microbes. In its samples, the team found DNA from a total of 33 phyla from all three domains of life, 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. The researchers found that bacteriophage can latch onto and inject genetic material into vesicles. If the vesicles are distracters, they 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 Prochlorococcus may feed vesicles to other types of bacteria that it 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, the scientists propose. Two types of nonphotosynthetic bacteria could grow in seawater when the researchers fed them with Prochlorococcus vesicles as their only source of carbon.

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."

Caption: Photosynthetic bacteria called Prochlorococcus pinch off bits of themselves to make sacs called vesicles (one indicated by arrow). The vesicles, each about 70 to 100 nanometers across, may affect carbon levels in the oceans.

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Title Annotation:GENES & CELLS
Author:Saey, Tina Hesman
Publication:Science News
Date:Feb 8, 2014
Words:718
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