As if we didn’t already have enough reasons to fear global warming, we can now add ancient herculean viruses to the mix. If that ice melts, these monstrous viruses will be unleashed upon the world.
No, you’re not reading an X-files episode summary. Scientists have discovered a second species of giant virus from a sample of 30,000-year-old Siberian permafrost, and they’ve decided to defrost it.
The newly discovered specimen, dubbed Mollivirus sibericum, measures 600 nanometers across, which is large enough to see under a simple light microscope. For comparison, that’s about 25 times larger than everyday viruses like influenza, which reach a maximum diameter of 120 nm. This makes Mollivirus even larger than its previously discovered sibling, Pithovirus, but both are dwarfed by Pandoravirus, a whopping 1,500 nm virus isolated from a water sample from Chile in 2013. In addition to these three species, scientists know of two others: Mimivirus and Megavirus.
Fortunately, all five types only infect amoebas, the highly mobile single-celled organisms that can be found amongst protozoans, fungi, algae, and animals. But just because they don’t directly infect humans doesn’t make them harmless; before Pandoravirus was identified as a virus, it was discovered inhabiting an amoeba living on the contact lens of a woman with keratitis. To prevent Mollivirus from running rampant on amoebas around the world, the team of French scientists who discovered it thawed out the frozen specimen under highly controlled lab settings in order to examine its structure.
Their gargantuan size aside, these viruses are unique for the amount of DNA they contain. While the simple influenza virus only carries 8 genes, Mollivirus has 523 and the behemoth Pandoravirus bears 2,556! (In comparison, an animal as complex as a human requires around 20,000 to 100,000 genes.) It’s clear that giant viruses are much more complex than their puny brethren, and this latest discovery proves they are also highly diverse. The proteins expressed by Mollivirus bear little resemblance to those of Pithovirus, the first prehistoric specimen found from the same sample. Mollivirus also expresses its viral genes in a different manner than Pithovirus; it replicates within the host’s nucleus where the amoeba keeps its own DNA. Pithovirus on the other hand, can multiply just by using the host’s general machinery.
As we discover more and more of these giant viruses, we are forced to reconsider our classifications of organisms. Taxonomists (scientists who piece together the tree of life) have traditionally excluded viruses from the category of living organisms because they lack ribosomal RNA, a type of genetic material necessary for gene expression in everything from bacteria to multicellular organisms like us. But clearly, viruses have gotten around that shortcoming just fine and figured out their own way to propagate. As argued by Didier Raoult in a 2014 editorial for The Scientist, the genetic complexity of giant viruses suggests that they may deserve a branch on the evolutionary tree of life.
Regardless of their branding, giant viruses should be handled with care and thoroughly studied. Although all known species can only infect amoebas, there’s nothing to rule out the possibility that giant viruses capable of infecting humans remain frozen in the Siberian tundra. Considering the vast differences between the two frozen viruses, there could be anything hidden in that prehistoric ice — perhaps a virus as dangerous as smallpox, except completely unfamiliar to modern medicine. These discoveries only reinforce the need for caution as we deal with melting polar ice.