As if there weren't enough problems with thawing tundra. A virus of unprecedented size has been isolated from Russian permafrost 30,000 years old and reactivated.
Dubbed a pithovirus after the Greek pithos, meaning a large earthenware jar like an amphora, the virus infects amoebas but does not appear to harm human or mouse cells.
Even so, now that this virus has been revived from the permafrost, so too could potentially harmful pathogens, possibly including viruses humans have never encountered before, the researchers say.
"There's good reason to think there could be pathogenic viruses in there too," says Chantal Abergel of Aix-Marseille University in Marseille, France, and co-leader of the team that discovered the virus.
"Thirty per cent of the world's oil reserves are thought to be hidden under the permafrost, along with gold and other key minerals, so exploration is bound to increase," says Jean-Michel Claverie, co-leader of the team. "So we must be careful to take precautions when prospecting – if people become sick with strange symptoms, it might be wise to quarantine and clear them of dangerous new infections before sending them back," he says.
The pithovirus itself is very different from any known virus. At 1.5 micrometres long by 0.5 micrometres wide, it is around 30 per cent bigger than what had been the largest known virus – the pandoravirus, also found by Claverie's team.
Yet despite being physically larger, the pithovirus has only a fifth as many genes as the 2500 in the pandoravirus. The two giant viruses share just five genes.
Reviving the pithovirus needed no sophisticated techniques. Rather, Claverie and Abergel "baited" the peat-like permafrost sample with amoebas. "We used the amoebas to draw out the virus, as we know these giant viruses tend to infect amoebas," says Claverie.
The researchers tracked and filmed the pithovirus's entire life cycle. Once inside an amoeba, it migrates to the wall of a chamber called a vacuole. It does this with the help of an unusual structure at one end that the team discovered, which serves as a kind of cork.
"Its function is to seal and protect the amphora-shaped particle, but as soon as it enters a vacuole, the cork is removed to initiate the infection," says Abergel. "It allows the internal membrane of the virus to fuse with that of the vacuole," she says.
Next, material from the virus spills out into the vacuole and turns it into a factory for making components of daughter virus particles. After a few hours, these assemble and mature into thousands of new pithoviruses which gather at the vacuole's edges before bursting out of a completely emaciated amoeba to find new host cells to infect. "It literally sucks the life out of the cell within 12 to 14 hours," says Abergel.
Unlike most viruses, including the pandoravirus, which hijack genes in the host nucleus to build new virus particles, the pithovirus uses its own genes, proteins and enzymes to replicate. In this respect, it resembles the mimivirus, the first giant virus found by Claverie's team a decade ago.
The team is now hunting for other viruses in the permafrost sample from the Kolyma region of northern Siberia. Claverie received the sample after contacting a team led by Elizaveta Rivkina of Russia's Institute of Physicochemical and Biological Problems of Soil Sciences in Pushchino, near Moscow. A year ago, Rivkina and her colleagues announced that they had revived a plant from the same layer of permafrost that eventually yielded the pithovirus.
Other permafrost researchers said the possibility that it harbours revivable viruses is real, and adds to the fear that reawakened microbes in thawing permafrost may increase global warming by digesting organic matter and releasing greenhouse gases such as methane and carbon dioxide.
"If on top of this we are now also seeing a possible release of potentially viable pathogenic viruses that are otherwise not living today, this will certainly add a whole further and new dimension to the thawing problem," says Torben Christensen of Lund University in Sweden. "It may mean that we are confronted not just with indirect climate warming impacts from thawing permafrost, but also direct human-health-related issues."
"Successful revival of any kind of ancient virus is always newsworthy," says Buford Price of the University of California at Berkeley. "My own group finds bacteria present at all depths in deep ice in the Antarctic and Greenland. The pithovirus is so large that we might be able to see it in ice cores more than 100,000 years old at their bases," he says.
But Janet Jansson of the Lawrence Berkeley Laboratory in California, who studies permafrost biodiversity, is more cautious about describing permafrost as a reservoir of potential pathogens. "Most of the microbes found in permafrost that we have studied are similar at the phylum level to microbes in most soils and some marine environments, so I don't think that permafrost per se should host more pathogens than any other environment," she says.
Claverie says that his team is also planning to hunt for large viruses in much older permafrost samples, from as early as 3 million years ago, to see if any can be revived.
Journal reference: PNAS, DOI: 10.1073/pnas.1320670111
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