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An ultrathin section of a Pithovirus particle in an infected Acanthameba castellanii cell observed by transmission electron microscopy with enhancement using the artistic filter ‘plastic packaging’ provided by Adobe Photoshop CS5.

French researchers who have revived a 30,000-year-old giant virus from a sample of Siberian permafrost are cautioning that the mining and drilling of northern regions could potentially free dormant pathogens out of the frozen soil.

"The thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might not be exempt from future threats to human or animal health," the researchers said in their paper published in this week's Proceedings of the National Academy of Sciences.

The paper's authors, Chantal Abergel and Jean-Michel Claverie of the Centre national de la recherche scientifique, were able to discover a new type of giant virus that they named Pithovirus, from pithos, the Greek word for the kind of amphora that looks like the micro-organism they have identified.

The Pithovirus infects amebas and is not a threat to humans, Dr. Abergel said in an interview.

Pithovirus does not mutate a lot so there is no risk of a "genomic drift" into a more toxic strain, even if it replicates in a fashion similar to the pox virus, she added.

Nevertheless, the paper by Dr. Abergel and Dr. Claverie warns about what could happen now that climatic changes have thawed permafrost areas and made it easier to exploit Arctic regions for natural resources.

"Our results … substantiate the possibility that infectious viral pathogens might be released from ancient permafrost layers exposed by thawing, mining, or drilling," the paper said.

"What we're trying to say is to be careful when you go into layers that haven't been disturbed in several thousand or even millions of years. We risk digging up things we don't necessarily want to see," Dr. Abergel said by telephone from her office at Aix-Marseille University.

The risks are slim but "are more than theoretical since we have shown that we can reactivate a virus from such an ancient sample," she said.

She noted that other researchers are already studying outbreaks of Siberian anthrax among domestic reindeer to see if they are linked to climate warming and the proximity of burial grounds of cattle that died from that spore a century ago.

Other scientists not involved in the research praised the French team for discovering the new virus, but were divided about the paper's warning words.

Jacqueline Goordial, a permafrost microbiologist at McGill University, said the French team had found a simple, clever way to extract the Pithovirus.

"As for the pathogenicity, it is a definite possibility to be considered. … We do have evidence that cells can survive these long time scales … so it is a possibility that this could be of concern."

She compared permafrost soil to a big freezer that preserves cells and noted that Russian scientists announced in 2012 that they have been able to regenerate 30,000-year-old frozen plant tissue that was found in the same area where the sample with Pithovirus was harvested.

Curtis Suttle, a virologist at the University of British Columbia, praised Dr. Abergel and Dr. Claverie for revealing the biological and genetic diversity that exists in giant viruses.

He added, however, that the likelihood that viruses encased in glacial ice could be released and harm humans "stretches scientific rationality to the breaking point."

In an e-mail, Dr. Suttle said: "I would be much more concerned about the hundreds of millions of people that will be displaced by rising sea levels than the risk of being exposed to pathogens from melting permafrost."

Pithovirus is the third different type of large viruses to be discovered since virologists started to document their existence a decade ago.

Viruses are usually known to be infection agents that are smaller than bacteria. Instead of reproducing by dividing themselves, as bacteria do, viruses infect a cell and replicate by assembling its genetic material.

In 2003, researchers realized that a micro-organism that was first observed in 1992 and mistaken for a bacterium was, in fact, an unusually large and complex type of virus, which was dubbed Mimivirus.

Last year, another giant virus was isolated, Pandoravirus, which had sharply different features from Mimivirus. The latest discovery, Pithovirus, while shaped like Pandoravirus, has a dissimilar genome profile, suggesting that it comes from yet another different viral family.

"We realize that we have greatly underestimated the diversity of giant viruses," Dr. Abergel said.

The permafrost sample used by the French team came from a frozen outcrop above a meandering riverbank in the Kolyma region, in the far east of Siberia.

Because the river had gouged through the frozen ground, researchers were able to retrieve samples from older layers without having to dig through the upper strata, lessening the danger of contamination, Dr. Abergel said.

The chunk of soil where the giant virus was discovered was collected in 2000 and kept in cold storage. It was carbon-dated to be 34,000 to 37,000 years old, a time when the prehistoric Neanderthal people were being supplanted by modern Homo sapiens.

After the core surface of the permafrost sample was sterilized with alcohol, 400 milligrams of thawed soil were introduced into an ameba culture, which became infected by the revived Pithovirus.

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