Defrosting Pandora’s Box: Eukaryotic Viruses Revived as Permafrost Melts

Last year, Jean-Michel Claverie and his research group in Information Genomique and Structurale at Aix Marseille University released ‘An update on Eukaryotic Viruses Revived from Ancient Permafrost’. Published in Viruses, the paper revealed thirteen newly discovered viral isolates from ancient samples of Siberian permafrost. Crucially, the study confirms the ability of these viruses to remain infectious after being frozen for over 48,500 years. 

This research followed on from the Claverie group’s previous findings in 2014 and 2015 from the Claverie group of two fully infectious eukaryotic viruses from a 30,000 year old permafrost sample in 2014 and 2015. As noted in the most recent paper, the lack of new isolates uncovered since 2015 is not necessarily indicative of a lack of infectious viruses. The group reports findings of numerous infectious eukaryotic viruses, however they remain to be genomically categorised. 

The thirteen viral isolates reveal another concern as a result of global warming. Bacterial and eukaryotic viruses up to 48,500 years old have been reactivated by the group, and as the permafrost which contains them melts, there is a risk that these viruses will be revived outside of the lab.

As permafrost melts

Global rising temperatures act on permafrost much like a microwave on defrost setting. As the Earth warms, permafrost thaws and ice becomes liquid water. This change of state triggers the metabolic reactivation of microorganisms in the soil, such as bacteria, archaea and fungi. 

There can be two outcomes from this reactivation. One is that defrosted, active microorganisms are able to decompose organic material into CO2 and methane gas, which then further adds to greenhouse gases and so rising global temperatures. Another, one which poses a more immediate public health threat, is the physical release and reactivation of so-called “zombie” bacteria and archaea which have been trapped in cryptobiosis (the state of metabolic inactivation organisms enter upon extreme climate conditions) in the permafrost. 

Activating ancient viruses

For the defrosted bacteria, this health threat is not a major concern. Antibiotics are effective at targeting a range of bacterial infections as they generally work similarly between bacteria types meaning ancient bacteria are likely to be able to be treated with our modern antibiotics. 

Defrosted ancient viruses, on the other hand, may pose a much greater risk. Within modern viral infections, each different type of virus requires different vaccines or antiviral agents. This is because viruses often work in different ways, targeting different pathways in the body; viruses don’t have universally conserved druggable processes. Ancient viruses, despite being dormant in permafrost for tens of thousands of years, when woken from cryptobiosis could be equally tricky to protect against. 

An example of the havoc wreaked by such defrosted infectious diseases can be seen in the devastated reindeer populations in 2015 and 2016, which have been linked to the release of Bacillus anthracis spores from permafrost after exceptionally hot summers. Such devastation, according to Claverie, is not a small threat but a large hazard which we are quite likely to see as a result of melting permafrost. Knowing more about the potential strains that may be released as a result of permafrost defrosting due to climate change could act as some protection against them.

Acanthamoeba safety

Awakening ancient infectious bacteria and eukaryotes raises obvious safety concerns. Certain labs can facilitate exploration of viruses more safely due to their safety procedures. Another way to activate viruses without creating a risk of infection to humans/plants/animals is to use a species which is genetically very far removed. Acanthamoeba spp. was used by the Claverie group as the safest way to infect a species without risk of infection, due to its evolutionary distinction from the human/plant/animal genus. 

Acanthamoeba is useful not only for its safety but also for its ability to live in a multitude of environments: water taps, flowerpots, dust particles, marine waters and more. Detecting their viruses may be an indicator for other live viruses in a given setting. 

With this safety blanket in place, the Claverie group was able to reactivate thirteen viral isolates from the different samples. Seven of the thirteen isolates were found to be new members of the Pandoraviridae family.  

Each of the isolates are thought to be distinct both from each other and also contemporary viral strains, and using radiocarbon dating techniques, the oldest sample was dated as being more than 48,500 years old. After tens of thousands of years spent dormant in permafrost, large DNA viruses are still infectious to Acanthamoeba. 

Limits of Detection 

These findings, and unpublished findings from the same group, indicate a large population of potential viruses which could be reawakened as permafrost defrosts. However, the detection of positive viral cultures in the study was conducted using light microscopy. This means that smaller, non-lytic viruses are likely to have passed through microscopy studies undetected; there is likely an even greater population of viruses that can survive in ancient permafrost and more still which may exist that do not infect Acanthamoeba.