Giant zombie virus pulled from permafrost

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 scientists who discovered the giant virus.

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

“It’s quite different from the giant viruses already known,” says evolutionary biologist 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 — the even larger Pandoraviruses, scooped from the mud of a Chilean river and a pond in Australia (SN: 8/10/13, p. 19).

This week the group 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.”

To find new viruses, the team systematically searches environments that haven’t been explored by virologists. 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 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.

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

But he’s not worried about dangerous viruses escaping from the permafrost and infecting humans.

“This is a completely far-fetched idea.” There’s no evidence that long-frozen soil hides greater amounts of unusual viruses than other environments, he says.

Abergel acknowledges that amoeba-infecting viruses are easier to find than ones that hit humans.

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

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

Claverie adds: “Either we are very good, we are very lucky or there are many of them.”