French researchers reported in the US National Academy of Sciences the discovery of Mollivirus Sibericum, the fourth type of pre-historic virus found since 2003. Giant viruses are considered "giant" if they are longer than half a micron, or 0.0005 millimetres and are visible by a light microscope. The first discovery of a Giant Virus occurred in started in "2003 with the discovery of Mimivirus. Two additional types of giant viruses infecting Acanthamoeba have been discovered since: the Pandoraviruses (2013) and Pithovirus sibericum(2014), the latter one revived from 30,000-y-old Siberian permafrost. We now describe Mollivirus sibericum, a fourth type of giant virus isolated from the same permafrost sample. These four types of giant virus exhibit different virion structures, sizes (0.6–1.5 µm), genome length (0.6–2.8 Mb), and replication cycles. Their origin and mode of evolution are the subject of conflicting hypotheses. The fact that two different viruses could be easily revived from prehistoric permafrost should be of concern in a context of global warming."
The Siberian Times summarizes:
Before waking it up in safe laboratory conditions, the French team will verify that the bug cannot cause animal or human disease, reported AFP. The prehistoric virus was found in the melting Siberian permafrost, said the respected news agency.'A few viral particles that are still infectious may be enough, in the presence of a vulnerable host, to revive potentially pathogenic viruses,' warned researcher Jean-Michel Claverie.
Exploitation of oil or other mineral resources in these regions could unleash ancient viruses long dormant in the frozen permafrost, now melting due to climate change.
'If we are not careful, and we industrialise these areas without putting safeguards in place, we run the risk of one day waking up viruses such as smallpox that we thought were eradicated,' he claimed.
The same team discovered another giant virus, which they called Pithovirus Sibericum, at the same location in 2013, then revived it in a petri dish, reported AFP.
To the 'astonishment of scientists', these ancient specimens are 'complex genetically'. 'M. Sibericum has more than 500 genes, while another family of giant virus discovered in 2003, Pandoravirus, has 2,500. The Influenza A virus, by contrast, has eight genes,' revealed the AFP report.
A contrarian view from Australia, Edward Holmes, a professor of infectious diseases and biosecurity at the University of Sydney, was cautious of the findings.From the Abstract:
Acanthamoeba species are infected by the largest known DNA viruses. These include icosahedral Mimiviruses, amphora-shaped Pandoraviruses, and Pithovirus sibericum, the latter one isolated from 30,000-y-old permafrost. Mollivirus sibericum, a fourth type of giant virus, was isolated from the same permafrost sample. Its approximately spherical virion (0.6-µm diameter) encloses a 651-kb GC-rich genome encoding 523 proteins of which 64% are ORFans; 16% have their closest homolog in Pandoraviruses and 10% in Acanthamoeba castellanii probably through horizontal gene transfer. The Mollivirus nucleocytoplasmic replication cycle was analyzed using a combination of “omic” approaches that revealed how the virus highjacks its host machinery to actively replicate. Surprisingly, the host’s ribosomal proteins are packaged in the virion. Metagenomic analysis of the permafrost sample uncovered the presence of both viruses, yet in very low amount. The fact that two different viruses retain their infectivity in prehistorical permafrost layers should be of concern in a context of global warming. Giant viruses’ diversity remains to be fully explored.