bizarre, evolution, genetics, human impact, microbes

Giant Viruses!

In the last year, multiple new “giant viruses” have been discovered that are challenging what it means to be a virus… and what it means to be alive. Giant viruses are so much bigger than other viruses that until the early 2000s some had been miscategorized as bacteria — the upper size limits that defined viruses did not come close to the sizes that these monsters reach. Their enormity is not only making scientists reevaluate how these giants fit into the viral tree, but also what branch they belong to on the tree of life (the very idea of calling viruses “alive” has long been controversial).

Mimivirus giant virus
This is a mimivirus. This viral group includes the first giant viruses identified, which were originally classified as bacteria. (200 nanometers [nm] equals 0.2 micrometers, or microns [µm].) (Image credit: E. Ghigo, et al.)

After having become aware of their existence in the early 2000s, more and more giant viruses seem to be found all of the time, all around the world, and with reported sizes that keep increasing. Last July, two new, record-breaking giant viruses (called Pandoraviruses) were discovered, measuring in at 1 µm in length and 0.5 µm in diameter. One was found off of the coast of Chile and the other in a pond in Australia. Then earlier this week, an even bigger one — measuring 1.5 µm in length and still 0.5 µm in diameter — was discovered in a chunk of Siberian permafrost. For a size comparison, the smallest bacteria are about 0.3 µm long (but are typically 0.4–5.0 µm in length). And the genome sizes of these viruses rival those of some eukaryotes.

Some experts are proposing that the giant viruses may not only need to be reclassified as microbes (since they can be seen with an optical microscope, which is part of the original definition of a “microbe”), but that they should also be given their own branch on the tree of life. This would make the tree contain four main branches: giant viruses, eukaryotes (which includes plants, animals, fungi, and many others), bacteria, and archaea (the latter two groups are primarily microbes). But it might not be this simple — the genetic data typically used to make genetic trees comes from small particles in cells called ribosomes, and viruses do not have ribosomes. Additionally, some experts theorize that giant viruses may actually be the origins of some of the key genetic material in eukaryotes (specifically the nucleus).

There are also peripheral questions about how these viruses might affect human health. The latest record-breaking giant virus, found in the Siberian permafrost, was likely frozen for 30,000 years. But despite being frozen for so long, after being thawed the monstrous virus happily infected an amoeba, raising concerns about what other functional viruses might be released from icy hibernation as global temperatures rise. There have also been multiple reports about giant viruses possibly causing acute illness in people.

So while it’s unclear whether giant viruses pose a risk to humans, it’s important to consider the possibility that viruses known to be deadly and thought to be eradicated (such as smallpox) — or just limited in occurrence — may be lurking (in frozen forms or remote areas). It’d be best to be prepared with vaccinations to combat potential outbreaks.


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