by Matthew Cobb
In the early 1670s, pioneer microscopist Antoni Leeuwenhoek peered into the tiny glass ball of his single-lens microscope and looked at a capillary tube that contained water and crushed pepper-corns. Leeuwenhoek was trying to discover why pepper was hot, and although he never found the answer to that question, he made a momentous discovery: microbes. To his amazement, the water was full of bazillions of tiny organisms, and there seemed to be no end to them – life appeared to extend into the infinitely small.
That isn’t quite true, but it is true that our planet is covered with microbial life and above all with viruses. For most biologists, viruses aren’t actually alive – they are bits of DNA (or sometimes RNA) and protein that pirate cell machinery to reproduce. However, whatever their formal status, their consequences on health and well-being can be devastating.
In a recent study published in the Journal of Virology, Tae Woong Whon and his co-workers from Kyung Hee University in South Korea decided to try and estimate how many viruses there are in a cubic metre of air, and to see how that density varies in space and time.
They collected air samples from just above the ground in three different environments – a residential area, a forest and a factory, and also collected rainwater from the residential area. They then filtered their collections and then – like Leeuwenhoek – looked at what they had found, and then used those sampling counts to estimate how many viruses and bacteria there would have been in each of the land sites. Here are some examples of the viruses they saw, imaged through transmission electron microscopy:
Intriguingly, they found no differences between the three kinds of sites, and overall the number of particles was lowest in the winter (temperature and vapour pressure seem to be the key factors, but not humidity).
The overall numbers are mind-boggling:
The number of viruses in a m3 of air ranged from 1,700,000 to 40,000,000, while the number of bacteria ranged from 860,000 to 11,000,000. Even though these things are very small, that’s still an awful lot!
In a healthy young adult, the “tidal volume” – the amount of air you breathe in on an average breath – is about 500 ml, or 1/2000th of a cubic metre. So if you were to lie close to the ground and just breathe, you could be inhaling up to 20,000 viruses and 5,500 bacteria with each breath.
Things might not be quite so alarming, however, as most of the viruses the team identified were not human viruses. They sequenced the bits of DNA they were able to capture, and many of them were known to infect bacteria, plants, fungi and various birds. However, over 50% of the sequences could not be identified.
So how many viruses and bacteria might there be around us? The volume of the troposphere (the lower part of the atmosphere) is around 8 x 1018 m3. If we assume that the average density of viruses in a m3 of troposphere is, say, 1 millionth of that found by Whon and his colleagues, then that would mean that, swirling about above our heads, there would be about 320,000,000,000,000,000,000 viral particles.
The amazing density of bacterial and viral DNA covering the planet – including in the sea, which isn’t included in these guestimates – suggests that if aliens were to look down at our world, they wouldn’t identify it as the planet of the humans, or of the cats, or even of the insects. It would be the planet of the viruses.
Reference: Tae Woong Whon, Min-Soo Kim, Seong Woon Roh, Na-Ri Shin, Hae-Won Lee and Jin-Woo Bae (2012) Metagenomic Characterization of Airborne Viral DNA Diversity in the Near-Surface Atmosphere. J. Virol. 86:8221-8231
h/t Pierre Barthélémy (@PasseurSciences)
PS I may well have tripped up with the guestimate calculations. Feel free to chip in either to correct (nicely!) or to add – measures of the number of viruses in the sea would be good, as would some kind of guess of the weight of all that DNA.
EDIT (13 October): Carl Zimmer, author of the excellent A Planet of Viruses (see comment thread) has just had this comment tweeted: “If you stacked all viruses on Earth end-to-end the stack would be 200 million lightyears tall.” Wow.