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Giant virus pulled from permafrost: scientists revive ancient, record-breaking microbe.

After lying dormant in Siberian permafrost for 30,000 years, the largest virus ever discovered is just as deadly as it was when mammoths roamed the Earth.

The virus targets amoebas rather than humans. But thawing, drilling and mining of ancient permafrost could potentially unleash viruses that infect people, say the discoverers of the oversized microbe.

At 1.5 micrometers long, Pithovirus sibericum is 25 to 50 percent longer than the previous record holders and about 15 times as long as a particle of HTV. Though shaped like another type of giant virus, P. sibericum has a relatively tiny genome, scientists report March 3 in the Proceedings of the National Academy of Sciences.

"It's quite different from the giant viruses already known," says Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Md., who was not involved with the research.

The team was led by Jean-Michel Claverie and Chantal Abergel of Aix-Marseille University in France, who helped discover the world's first giant virus about 10 years ago. Dubbed Mimivirus, the microbe was so large that researchers could see it with a light microscope. Before the finding, Claverie says, "we had this silly idea that all viruses were basically very small."

Years later, the discovery of a few viruses resembling Mimivirus led researchers to believe that all giant viruses might belong to a single family. But last summer, Claverie, Abergel and coworkers uncovered a second, completely different family that includes the even larger Pandoravirus, scooped from the mud of a Chilean river and a pond in Australia (SN: 8/10/13, p. 19).

The team has rattled the field once again with the discovery of yet another family of giant viruses.

"Now, with this Pithovirus, we are totally lost," Claverie says. "It adds to the confusion."

After reading about a plant revived from 32,000-year-old Siberian permafrost (SN: 4/7/12, p. 15), Claverie, Abergel and colleagues went hunting for viruses in Siberia's frozen soil.

The team added samples of permafrost to dishes containing amoebas and then waited to see if the one-celled organisms died. They did. When the researchers looked at the dead amoebas under a microscope, they spotted lots of oval-shaped particles of the virus.

"Either we are very good, we are very lucky or there are many of them," Claverie says.

Now scientists don't know just how big viruses can get, Koonin says. "I would be excited but not terribly surprised if something even larger comes up tomorrow."

Because Pithovirus has survived for so long, Claverie says it's not hard to imagine that viruses harmful to humans can too.

But Koonin is not too worried about dangerous viruses escaping from the permafrost and infecting humans."This is a completely far-fetched idea," he says. There's no evidence that long-frozen soil hides greater amounts of unusual viruses than other environments, he adds.

Koonin is more interested in what the find says about giant viruses. Scientists have only just begun to tap into their diversity, he says.

Caption: The largest virus ever discovered, the 1.5-micrometer-long Pithovirussibericum, seen in this false-color electron microscope image, was roused from 30,000-year-old permafrost.


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Title Annotation:GENES & CELLS; Pithovirus sibericum
Author:Rosen, Meghan
Publication:Science News
Geographic Code:1USA
Date:Apr 5, 2014
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