Giant virus comes in from the cold

Scientists have resuscitated a massive virus that has lain frozen for 30,000 years in Siberian permafrost.

Using a culture of single-celled organisms called amoebae as "bait", a team of French scientists added samples of permafrost collected from a region of Siberia called Chukota.

Quite quickly, the amoebae showed signs of infection and Aix-Marseille scientist Matthieu Legendre and his colleagues were able to extract samples of what they have subsequently named Pithovirus sibericum.

The virus they recovered is one of the largest ever discovered. Bigger even than many bacteria, it's shaped like a long flask and in fact, it gets its name from the Greek word "pithos", meaning an amphora. It even has a strange cork-like structure at one end of the particle, which pops out to release the viral DNA once a cell has been infected.

Genetic analysis of Pithovirus sibericum shows that, although there are some genes in common with the two other families of similarly-sized giant viruses that have been discovered in recent years, the virus is clearly a unique and novel entity.

But, aside from the remarkable size of the virus, the discovery is also notable because it is currently the oldest viral life-form ever to have been reanimated from a frozen state.

Previously, scientists working on frozen sediments from Greenland have recovered genetic sequences corresponding to a tomato virus dating back up to 140,000 years, but these were not intact organisms capable of infection.

Although this newly-recovered pithovirus is only a threat to amoebae and cannot infect human cells, these results nonetheless show that frozen soils can harbour a host of ancient infectious viral agents.

Climate change, as well as efforts to recover fossil fuels and other resources by extending mining and drilling operations into previously unexploited, inhospital terrains like the Siberian permafrost, could, the researchers caution in their paper in PNAS where the work is published, unlock these agents, some of which may be pathogenic to humans...