As the world's ice thaws, so will everything trapped in it—including ancient viruses we've never seen before.
As the world's ice caps melt away, polar humans will find all kinds of things that were previously hidden under sheets of ice. New sailing routes, for one, and probably a pair of keys or two. But a new discovery hints at a future find that's a little more unsettling: A French research team revived a 30,000-year-old giant virus that had been lying dormant in Siberian permafrost.
Viruses tend to be small, simple organisms, but the discovery of giant viruses—viruses that can be seen with a light microscope, and which encode more proteins than some bacteria—a decade ago has changed that view. There are two known genera of giant virus, Pandoravirus and Megavirus. But according to research published in Proceedings of the National Academy of Sciences, the newly discovered species, Pithovirus sibericum, is deserving of its own genus because it combines bits of both.
At 1.5μm in length, it's enormous—about half-again as long as representative Pandoravirus species—but with a "surprisingly smaller" 600 kb genome, which is smaller than Megavirus (1.25 Mb) or Pandoravirus (2.8 Mb) genomes.
Pithovirus imaged by an electron microscope inside of an Acanthamoeba cell. The length of the particle is ~1.5 µm with a 0.5 µm diameter. Image: Julia Bartoli and Chantal Abergel
Now, I know we're all envisioning a sequel to The Thing, but the virus in question preys on the Acanthamoeba genus of amoebae, not humans, which made it an ideal research candidate. (When attempting to reawaken ancient viruses, it's best to start on the safe side.) As the team led by Matthieu Legendre of Aix–Marseille Université explains in the paper, they used "Acanthamoeba as bait, [and] isolated a large DNA virus from a 30,000-year-old permafrost layer."
Once it infected Acanthamoeba cultures, Pithovirus was able to replicate successfully, giving researchers a peek into its life cycle. It combines characteristics of both Pandoravirus and Megavirus along with other virus genera, which led researchers to conclude that "pandoravirus-like particles may be associated with a variety of virus families more diverse than previously envisioned."
While that insight into the development of giant viruses is interesting enough on its own, the work gets truly fascinating when you compare Pithovirus to extant species. "Although Pithovirus is the prototype of a new family, its genome structure and replication cycle are similar to that of other large eukaryotic DNA viruses, many of which are human or animal pathogens."
It's stunning that such an old organism could be revived—the researchers say they think it's the most ancient eukaryote-infecting virus yet revived—but as the researchers note, it's probably not unique in that characteristic. So while Pithovirus may be an ancient amoeba-eater, its contemporaries might have more multicellular tastes. And as the world's ice thaws, those relatives might find themselves freed.
"In the 20th century, the permafrost in the Northern Hemisphere has diminished by 7 percent. This no doubt corresponded to a large release of microorganisms from previously frozen soils, an unknown fraction of which was revived upon thawing," write the authors.
The good news is that Pithovirus was found 30 meters deep in the Siberian permafrost, which means it's unlikely that its peers will escape on their own. But as the Arctic thaws and drilling operations increase, especially in Siberia, so does the possibility that a formerly-frozen pathogen gets loosed upon the world.
"The rich mineral resources and oil reserves of the arctic regions are under increasing pressure for their industrial exploitation (involving mining and drilling)," they write. "It thus becomes urgent to examine which viruses are expected to be encountered not only near the surface, but in increasingly deeper and more ancient permafrost layers (down to 3 My)."
As the authors note, studying new viruses as they did is fairly inexpensive and straightforward, and makes for cheap insurance against the viral apocalypse. Still, it's a bit unsettling to be reminded that, as the world's ice thaws, so will everything trapped in it—including ancient viruses we've never seen before.