BioEd Online

In a plot straight out of a B-movie, French scientists have revived a giant virus that was buried in Siberian ice for 30,000 years — and is still infectious. Its targets, fortunately, are amoebae, but the team suggests that melting ice may trigger the return of more old viruses, with potential risks for human health.

The new virus is the biggest yet, with a 1.5-micrometre-long capsule that is comparable in size to a small bacterium. Jean-Michel Claverie and Chantal Abergel, a husband-and-wife team at Aix-Marseille University in France, named it Pithovirus, after the Ancient Greek work ‘pithos’ for a large container used to store wine and food. “We’re French so we had to put wine in the story,” says Claverie.

Their results are published in Proceedings of the National Academy of Sciences.

Claverie and Abergel have helped to discover other giant viruses — including the first, called Mimivirus, in 2003², and the two Pandoraviruses last year (see 'Giant viruses open Pandora's box'). “Once again, this group has opened our eyes to the enormous diversity that exists in giant viruses,” says Curtis Suttle, a virologist at the University of British Columbia.

Two years ago, the team learned that Russian scientists had resurrected an ancient plant from fruits buried in 30,000-year-old Siberian permafrost. “If it was possible to revive a plant, I wondered if it was possible to revive a virus,” says Claverie. Using permafrost samples provided by the Russian team, they fished for giant viruses by using their usual targets — amoebas — as bait. The amoebas started dying, and the team found gigantic virus particles inside them.

Under a microscope, Pithovirus is a thick-walled oval with an opening at one end, much like the Pandoraviruses. But Abergel says, “They have roughly the same shape but these are totally different viruses.”

Pithovirus has a honeycomb ‘cork’ capping its opening. It copies itself by building replication ‘factories’ in its host’s  cytoplasm, rather than taking over the nucleus as most other viruses do. Only a third of its proteins are similar to those of other viruses. And, to the team’s surprise, its genome is far smaller than those of the Pandoraviruses even despite its larger size.

“That huge particle is basically empty,” says Claverie. “We thought it was a property of viruses that they pack DNA extremely tightly into the smallest particle possible, but this guy is 150 times less compacted than any bacteriophage. We don’t understand anything anymore!”

Claverie and Abergel are concerned that rising global temperatures, along with mining and drilling operations in the Arctic, could thaw out ancient viruses that are still infectious. Some of these, they say, could conceivably pose a threat to human health. “People are disturbing very deep layers of permafrost that have not be perturbed for millions of years,” says Claverie. “That’s where the real danger is. 30,000 years ago, the Variola virus [which causes smallpox] wasn’t eradicated.”

Although giant viruses almost always target amoebae, Christelle Desnues, a virologist at the French National Centre for Scientific Research in Marseilles, recently discovered signs that the giant Marseillevirus was infecting an 11-month-old boy. “It is clear that giant viruses cannot be seen as stand-alone freaks of nature,” she says. “They constitute an integral part of the virosphere with implications in diversity, evolution, and even human health.”

But Suttle points out that we already inhale thousands of viruses every day, and swallow billions whenever we swim in the sea. The idea that melting ice would release harmful viruses, and that those viruses would circulate extensively enough to affect human health, “stretches scientific rationality to the breaking point,” he says. “I would be much more concerned about the hundreds of millions who will be displaced by rising sea levels.”