"Zombie viruses" from permafrost: how and why they were revived

Up to a quarter of the Earth's northern hemisphere is covered by permafrost.It occupies

a significant part of Siberia, the north of Europeanparts of Russia, Canada and Alaska. Due to a warming climate, soil that has remained frozen for millennia is gradually melting and releasing greenhouse gases.

In addition to active emission, the soil containsmicrobes and viruses, which since prehistoric times arrived in a "half-dead", dormant state. An international group of researchers from Russia, France and Germany decided to test whether "thawed" viruses are able to restore activity and infect modern organisms.

Why is an experiment needed?

Climate warming is especially noticeable in the Arctic.The temperature in this region is rising, according to various estimates, two or even four times faster than the global average. Researchers are recording the melting of permafrost at ever greater depths. As a result, the ancient organic layers that froze tens of thousands of years ago are heated and thawed. In the future, this process may affect the permafrost, which formed several million years ago.

The melting of permafrost is associated not only withthe decomposition of ancient organic matter, which leads to emissions of carbon dioxide and methane into the atmosphere, further enhancing the warming effect. At the same time, it records the restoration of individual ancient microorganisms that cause infection of animals. 

For example, in 2016 on the Yamal Peninsularesearchers have documented a massive outbreak of anthrax among reindeer. Then more than 2.5 thousand animals were infected, of which almost every ninth died. As a result of contact with sick deer, 36 people also became infected, one of whom died. The study showed that the infection was caused by strains of the bacterium Bacillus anthracis from an ancient cattle burial ground. Pathogens were activated due to abnormal temperatures, which led to the melting of the soil at a greater than usual depth.

Ancient layers over a million years oldyears may contain completely unknown pathogens. At the same time, although numerous studies have been devoted to the possibility of “reviving” bacteria from permafrost, the possibility of the emergence of “zombie viruses” and its consequences still remains poorly understood, scientists say.

How are viruses resurrected?

The researchers isolated 13 different viruses fromseven samples of permafrost from Siberia, one from Kamchatka, and one from the muddy bank of the Lena River in Yakutia. The genome of the pathogens differed from the known modern analogues, but they all belonged to five different genera that infect single-celled microorganisms from the genus Acanthamoeba (Acanthamoeba). Among them were: pandoravirus, cedratvirus, megavirus and pacmanvirus, as well as a new strain of pitovirus.

Age of the oldest virus foundresearchers estimated at 48.5 thousand years, nine of them spent at least 10 thousand years in a frozen state. At the same time, the samples were obtained not only from the soil: three of them were preserved in the wool of a mammoth frozen more than 27 thousand years ago, and one more in the intestines of the same ancient Siberian wolf.

Samples of various viruses found in permafrost, under a microscope. Images: Jean-Marie Alempic et al., Viruses

All found viruses are large viruses.It is no coincidence that the researchers specifically focused on studying species that are visible under a conventional light microscope and do not require more sophisticated instruments. This makes such pathogens a good model that is easy to study in the laboratory, the authors note in their paper.

In a closed laboratory, scientists thawedancient pathogens and sequenced their genomes. The researchers then infected amoeba cells with these viruses. Since the species found mainly reproduce inside Acanthamoeba, their activation posed a minimal risk for transmission to laboratory technicians. “The ones we revived pose no danger, they only infect amoebas,” Jean-Michel Claverie, a microbiologist at the University of Aix-Marseille in France and co-author of the new study, told Live Science.

What did the study show?

The analysis showed that after defrosting, the virusesretained the ability to infect amoebas. They successfully penetrated into the cells of microorganisms, while infection initiated active replication of the viral genome and the formation of new particles that were released and infected other microorganisms.

The study confirms the ability of largeDNA viruses that infect amoebas remain infectious after more than 48,000 years spent in deep permafrost. For three of the five genera studied, this is the first example of reactivation after freezing. 

We're looking at these viruses that infect amoeba,as surrogates for every other possible virus that might lurk in the permafrost. We see traces of many, many, many other viruses. We know they are there. We don't know for sure if they are still alive. But we assume that if the amoeba viruses are still alive, then there is no reason why other viruses would be dead and unable to infect their hosts.

Jean-Michel Claverie, co-author of the study in an interview with CNN

The researchers note that in addition to the revivedof viruses that infect amoebae, traces of many other species have been found in samples from everlasting, including some associated with known human pathogens such as poxviruses (poxviruses) and herpesviruses. Due to the high potential for pathogenicity and the risk of spreading among humans, biologists did not check whether these viruses retained the ability to infect other organisms.

Many of the viruses that will be released byas the ice melts, will be completely unknown. It is not yet known whether they will be contagious and how, when exposed to light, heat and oxygen from the environment. But the results of the experiment suggest that at least some of them can be transmitted to humans, while people may not have immunity to such pathogens.

Given the inevitability of global warmingscientists call for caution when mining in permafrost areas, and also note the need for further research to minimize the risk of an epidemic caused by unknown viruses.

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