Scientists are discovering and resurrecting ancient viruses trapped in permafrost and frozen remains. Trapped in frigid Arctic soils and riverbeds, the world is teeming with ancient microbes. Bacteria and viruses that existed thousands of years ago are frozen in time within prehistoric layers of permafrost. Rising temperatures could cause much of the ice to melt, freeing these microbes from their icy prisons. The viruses found are harmless, but other microbes, as yet unknown, could be released and infect humans or other animals.
Permafrost in Russia’s Far East is melting rapidly and could lead to the emergence of unknown, long-dormant viruses. Michael Robinson Chavez/The Washington Post
“The risk will inevitably increase in a warming world, with permafrost melting continuing to accelerate and more people settling in the Arctic,” Jean-Michel Claverie, a computational biologist at Aix-Marseille University in France who studies ancient and exotic viruses, told CNN.
Until now, scientists have only studied permafrost viruses that infect single-celled organisms called amoebae, because these viruses are harmless and serve as a good model for other viruses that may lurk beneath the ice.
Pithovirus sibericum is one of the largest viruses ever discovered. The virus is harmless to humans and other animals. It is about 1.5 micrometers long, about the size of a small bacterium, and belongs to a group known as “giant viruses,” which are double-stranded DNA viruses and, with a few exceptions, are visible under a light microscope. P. sibericum looks like a thick-walled oval with a hole at one end, topped with a corky structure and honeycomb-like mesh.
Scientists hunting for unknown pathogens discovered P. sibericum nestled deep within the core of ancient Siberian permafrost that was excavated in 2000 from Kolyma in Russia’s Far East. They resurrected the 30,000-year-old virus by exposing the permafrost sample to amoebas, which are the only known hosts of P. sibericum.
Pithosvirus sibericum is one of the largest viruses ever discovered, with a cork-like structure at one end. Giulia Bartoli/Chantal Abergel, IGS and CNRS-AMU
“Our protocol is to expose cultures of amoebae (in the lab) to different samples in the hope that they will contain viruses that can infect the amoebae,” Claverie said.
The researchers named the virus after the Greek word “pithos,” which refers to the large containers, or amphorae, used by the ancient Greeks to store wine and food. They published their findings in a 2014 study in the journal PNAS.
Mollivirus sibericum was found frozen in the same 30,000-year-old Siberian permafrost sample as P. sibericum. M. sibericum particles are smaller than P. sibericum (0.6 to 1.5 micrometers in length), but they are also visible under a light microscope and are considered giant viruses. The roughly spherical virus is surrounded by a hairy protective layer and can produce and release 200 to 300 new virus particles from each amoeba it infects.
While M. sibericum poses no danger to humans or other animals, the discovery of two ancient viruses in one sample suggests that permafrost may often harbor dormant pathogens, researchers warned in a 2015 study published in the journal PNAS.
“We cannot exclude that distant viruses from ancient Siberian human (or animal) populations may re-emerge as Arctic permafrost layers melt and/or are destroyed by industrial activity,” they write in their study.
Pithovirus mammoth is the second recorded strain of Pithovirus, which was isolated from a 27,000-year-old lump of fossilized mammoth hair found on the banks of the Yana River in the Russian Far East. P. mammoth has a large, elongated particle measuring 1.8 micrometers in length and has a cork-like structure like P. sibericum. Its only host is amoebae.
Mammoth pithovirus, mammoth pandoravirus and mammoth megavirus were found in a single 27,000-year-old permafrost sample containing mammoth wool
Claverie and his colleagues described P. mammoth in a study published earlier this year. That study found 13 viruses revived from Siberian permafrost, three of which — P. mammoth, Megavirus mammoth, and Pandoravirus mammoth — were found in the same prehistoric sample containing mammoth wool.
P. mammoth (Pandoravirus mammoth) is a strain of viruses of the Pandoraviridae family, which make up the vast majority of viruses revived from permafrost. Pandoraviruses are amoeboinfecting, giant viruses that have large amphora-shaped particles up to 1.2 micrometers long.
The researchers found P. mammoth in a frozen sample of 27,000-year-old mammoth hair from the banks of the Yana River and in the fossilized stomach contents of a 28,600-year-old mammoth on the Lyakhovsky Islands off the coast of northeastern Russia.
The team exposed the new strain of Pandora virus to cultures of amoebas, as well as human and mouse cells, which is a standard protocol for testing that viruses cannot infect mammalian cells.
Pandoravirus edoma is the oldest virus to be resurrected from permafrost to date. Researchers discovered the 48,500-year-old pathogen, which infects amoebas, in ice deposits beneath a lake in Yukechi Alas in Russia’s Far East. P. edoma is one of 13 “zombie” viruses described in a study published Feb. 18 in the journal Viruses and has a large, egg-shaped particle measuring 1 micrometer in length.
Pandoravirus edoma found in ice deposits beneath lake in frigid Russian Far East
Researchers date viruses trapped in permafrost using radiocarbon, which is a radioactive type of carbon that decays at a known rate and can help determine the age of organic materials. However, in samples more than 50,000 years old, the amount of radioactive carbon remaining is so small that current methods cannot accurately date the material.
Mammoth megavirus is the first virus found in permafrost and belongs to the Mimiviridae family. Mimiviruses were the first viruses that researchers classified as giant viruses after they were discovered in cooling tower water in Bradford, England, in 1992. Mimiviruses infect amoebae and have particles 0.5 micrometers in diameter enclosed in a capsule with 20 identical triangular faces. Megaviruses such as M. mammoth are members of the Mimiviridae subfamily and have the same characteristics.
The researchers isolated the new strain from a 27,000-year-old piece of ice and mammoth wool found on the banks of the Yana River, along with P. mammoth and P. mammoth.
Pacmanvirus lupus. Pacmanviruses are a recently discovered group of viruses that infect amoebas, which are distantly related to the African swine fever virus of the Asfarviridae family. Scientists named them after the video game Pac-Man because when the protein shell breaks down, it looks like a gaping mouth.
Pacmanvirus lupus is the third member of the group to be recorded and the first strain isolated from permafrost, specifically from the frozen gut remains of a 27,000-year-old Siberian wolf (Canis lupus). Scientists described the recently thawed virus they found at the Yana site in a study published earlier this year.
The Pacman lupus virus has been revived after scientists discovered it in the frozen intestines of a Siberian wolf.
Pacman viruses are classified as giant viruses, but the new strain is only 0.2 micrometers long and is invisible under a light microscope.
Cedratviruses are giant viruses that infect amoebas and belong to a subgroup of the Pithovirus family, which includes P. sibericum and P mammoth. Scientists have isolated three previously unknown Cedratvirus strains from different locations in the Russian Far East and described them in a study published earlier this year.
The researchers extracted Cedratvirus lena from permafrost on the muddy banks of the Lena River in Russia’s Far East. The new strain has an elongated particle, 1.5 micrometers long, that resembles P. sibericum, but it has two cork-like structures at each end instead of one.
The team collected two other strains of Cedrat virus in the Russian Far East: C. kamchatka from frozen soil on the Kamchatka Peninsula and C. duvanny from mud flowing into the Kolyma River as permafrost of varying ages melted.