Frost giants: the unfreezing of an ancient virus


One of the great things about working with bacteria and viruses is that they can be put into suspended animation by sticking them in the freezer. This is handy for researchers as it means that samples can be easily preserved between experiments. What works in a freezer also works in natural ice and researchers have recently re-animated a giant 30,000 year old virus from the Siberian permafrost.
The virus was found by using the amoeba Acanthamoeba as bait. The amoeba was incubated with a sample of the permafrost to see if any viruses would grow inside. Not only did viruses grow, but they were spotted using a light microscope! Under the light microscope they appeared as small dotted particles. Further analysis with an electron microscope revealed a structure 0.0015 mm in length surrounded by a thick membrane containing vertical lines.
Close-up of the viral membrane. Image from reference one. Julia Bartoli & Chantal Abergel; Information Génomique et Structurale, CNRS-AMU
The amoeba Acanthamoeba is already the infective host for two other large viruses: Megaviruses and Pandoraviruses. The ancient virus discovered in the permafrost appears to belong to a third viral type which the researchers named Pithovirus. Despite its large size the Pithovirus does not have a massively large genome – only 600 kb which is comparable to much smaller viruses.
The Pithovirus does not affect the host amoeba’s DNA but instead replicates itself inside the cytoplasm. At the end of the viral membrane is a cork-like ‘stopper’ seen as a pale patch in the membrane in the image above. This is lost as soon as the virus infects its host, allowing the host cytoplasm to mingle with the virus DNA. An area of the host cytoplasm is cleared of internal structures to form a virus-making factory. Within this space new viral membranes form and are built up with the replicated viral DNA inside. These gather inside the amoeba until finally the cell breaks open, releasing new giant viruses.
Pithovirus inside the host cell, having lost the cork and merging with the host cytoplasm. Image from reference 1 with thanks to Dr. J.-P. Chauvin, F. Richard, and A. Aouane from Institut de Biologie du Développement de Marseille
This virus is the most ancient eukaryote-infecting DNA ever to be revived, and while it’s quite exciting that it still works it also raises important questions about what else might be hiding under the permafrost. Both viruses and bacteria enter suspended animation upon freezing, and between drilling, mining and global warming there’s a chance that many more might be released, some in a viable state. Although the amoeba-infecting viruses do not infect humans. studying them can give us an idea of what else could be lurking under the ancient ice.

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