A couple of months ago I talked about the connection between global warming and the Zika virus. Today I would like to discuss another interesting side effect we might observe in the next decades thanks to global warming. The ice caps will melt. Big deal, we already knew that. But have you ever thought of the stuff trapped in that ice that's going to thaw? What if some of that stuff isn't really dead, just dormant, waiting to come back? Sounds like fiction, but it's not.
Up until a few years ago the general notion was that viruses were small. How small? Let's think in terms of genome units: viruses usually carry a handful of genes, either coded into DNA or RNA, and you can think of these as longs strings of four letters: A, C, T (or U if it's RNA), or G. The letters are called nucleotides, and the genome of most common viruses is typically in the order of tens of thousands of nucleotides long. By comparison, the human genome, with its 3 billion nucleotides, is enormous.
The notion of viruses being "small" compared to living cells was turned upside down with the discovery of megaviruses in 2010 (over one million bases) and, in 2013, of the pandoraviruses, a family of viruses that can reach a staggering 2.5 million bases in genome size.
Before you freak out: so far these gigantic viruses have only been found in unicellular organisms called amoebas, not in humans or any other animals. Amoebas acquire their nutrients through phagocytosis and that's also how the gigantic viruses infect them: the cell membrane forms a vesicle around the viral particle and engulfs it.
Two specimens of pandoraviruses have been found in shallow water sediments, one in Chile and the other one in Australia. They were both so big that they could be seen with just an optical microscope, reaching 1 μm in length and 0.5 μm in diameter. The researchers found over 2,000 genes in these pandoraviruses, of which over 90% looked nothing like any other previously known gene. In fact, they appear to be unrelated to the previously discovered megaviruses. So what are they? A fourth domain of life? A completely isolated niche in the tree of life? Or could they be -- as the sci-fi writer in me wants to think -- the remnants of a completely different form of life, one that existed so long ago that these gigantic particles are all there is left of it?
I thought I was original when I raised that hypothesis, but I wasn't. The researchers who'd first discovered the pandoraviruses posed the exact same question and, in order to find an answer, they went digging through fossils. They took a sample of Siberian permafrost layer (corresponding to late Pleistocene sediments older than 30,000 years) and used it to inoculate a particular culture of amoebas (called Acanthamoeba castellanii). Lo and behold, they indeed observed particles of a prehistoric giant virus multiplying in the amoeba culture, making it the most ancient eukaryote-infecting DNA virus revived to date! The observed viral particles were examined through transmission electron microscopy and found to have many similarities with the pandoraviruses, only they were even bigger.
The researchers named the giant virus pithovirus and hypothesized that there may be more gigantic viruses frozen out there that could be released from the ice as global warming takes over. Unlike pandoraviruses, though, these pithoviruses showed many more similarities to present-day viruses that normally infect humans and animals. This prompted the researchers to raise the alarm: the average temperature of the surface of the Arctic permafrost has increased by 3 °C during the past 100 years, faster than the average global temperature. Could there be more pathogens lurking in there waiting to be released again?
Now, if you'll excuse me, I think I just got an idea for the my next post-apocalyptic thriller.
Philippe, N., Legendre, M., Doutre, G., Coute, Y., Poirot, O., Lescot, M., Arslan, D., Seltzer, V., Bertaux, L., Bruley, C., Garin, J., Claverie, J., & Abergel, C. (2013). Pandoraviruses: Amoeba Viruses with Genomes Up to 2.5 Mb Reaching That of Parasitic Eukaryotes Science, 341 (6143), 281-286 DOI: 10.1126/science.1239181
Legendre, M., Bartoli, J., Shmakova, L., Jeudy, S., Labadie, K., Adrait, A., Lescot, M., Poirot, O., Bertaux, L., Bruley, C., Coute, Y., Rivkina, E., Abergel, C., & Claverie, J. (2014). Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology Proceedings of the National Academy of Sciences, 111 (11), 4274-4279 DOI: 10.1073/pnas.1320670111