Planet of the viruses

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:

Fig 2

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.

15 Comments

  1. Deb Tromb
    Posted October 11, 2012 at 7:03 am | Permalink

    Wasn’t a lowly microbe what brought down the invaders in “The War of the Worlds”?

  2. Posted October 11, 2012 at 7:06 am | Permalink

    We’ve a lander on Mars looking for signs of life. I suspect that had we spotted any human size animals we’d definitely be interested in them and think it a planet of animals and not of viruses.

  3. logicophilosophicus
    Posted October 11, 2012 at 7:21 am | Permalink

    “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.”

    Reminds me of the idea – Richard Dawkins? – that if everything but the nematodes became invisible there’d still be a fairly complete image of the Earth’s surface/biosphere. Anybody know the source?

  4. Posted October 11, 2012 at 8:24 am | Permalink

    I believe that the only truly accurate estimation possible would be that there is a metric fuckton of shit out there.

    Now, if you’ll excuse me, I need to hit the shower….

    Cheers,

    b&

  5. invivoMark
    Posted October 11, 2012 at 8:50 am | Permalink

    I did a low-end estimate, using several generous assumptions:

    1) Airborne viruses only cover one meter of air above ground, and they only cover land, not oceans.

    2) Average virus density is their lowest measured density of 1.7 million/m^3.

    This gives the number of airborne viruses at close to 2.5*10^20, which is very close to your number.

    Using an average molecular weight of 680g/mol per base pair, and an average length of 10kb per virus, I’m getting around 3kg of DNA from all those virions.

    Using an average virus diameter of 50nm, I’m getting .01 cubic meters of virus, or the size of a cube 20cm on a side.

    Keep in mind, that’s a low-end estimate. The real numbers are likely a lot bigger.

  6. Posted October 11, 2012 at 10:31 am | Permalink

    This is very timely, as I’m writing about man’s surprisingly lowly place in an evolved biosphere and how evolution is all about genetic replication, not us. It’s been my understanding from Eugene Koonin’s book The Logic of Chance (2011) that Pelagibacter ubique, a photosynthetic marine bacterium, is the most prolific single organism on the planet. I was also planning to mention the virus as the simplest of all replication vehicles used by the genes, but this article (“our planet is covered with microbial life and above all with viruses”) makes me wonder if the virus is actually the champion of the population contest.

    Any advice on this from all the experts here would be appreciated!

    • Matthew Cobb
      Posted October 11, 2012 at 1:07 pm | Permalink

      I think your best bet would be to have a look at Carl Zimmer’s “A Planet of Viruses” in which he gives the low-down on the stuff I’ve just alluded to. You can get it on Kindle, and in dead tree version, and a second-hand copy would be cheap – it may even be in your library. I note that Carl estimates the number of viruses in the ocean to be 1,000,000,000,000,000,000,000,000,000,000 (10^30)
      which is a lot more than I estimate for the air…

      • Posted October 11, 2012 at 1:11 pm | Permalink

        Thank you! I’ll have a look at it.

      • invivoMark
        Posted October 11, 2012 at 2:23 pm | Permalink

        And just for kicks… that number of viruses is the volume of a small house.

        That’s a lot of virus.

        • Gregory Kusnick
          Posted October 11, 2012 at 6:36 pm | Permalink

          Something’s not adding up. If there are ~10^10 more viruses in the ocean than in the air, that would correspond to a viral volume of ~10^8 cubic meters, or 0.1 cubic kilometer — the size of a small city.

          That’s a lot of virus.

          • invivoMark
            Posted October 11, 2012 at 10:33 pm | Permalink

            Whoops! I used 10^24 for number of viruses in the air. That’s a brain fart, considering I had just calculated the 10^20 number!

  7. Alex SL
    Posted October 11, 2012 at 2:28 pm | Permalink

    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.

    They’d probably be familiar with that situation from their own planet… even if their biology would be quite different from ours, which I don’t consider plausible, you will probably get more of the tiny things than of the big things in every biosphere.

    As a schoolboy, I bought a three volume German DTV Atlas zur Biologie as a reference for study. At the end of the third volume is the system of life. This book being from 1988, it is non-phylogenetic and severely outdated. But the whopper is that it speaks of an estimated 2700-3600 species of Eubacteria (the chapter on viruses does not give an estimate).

    Even before I started studying biology, I found that number just laughable. There are millions of plant, fungal and animal species, and extrapolating from the number of viruses and bacteria that attack humans surely the reasonable conclusion is that there must be millions of species of microbes, at a minimum? More likely, at least one order of magnitude more. (Caveat: might be more difficult to define species there.)

  8. Posted October 12, 2012 at 5:06 am | Permalink

    In case it is interesting to someone – I always find it amusing – the idea that there are life forms of all sizes, “down” to the infinitessimal, was one that Leibniz believed. He felt vindicated by what Leeuwenhoek had found.

  9. Posted October 12, 2012 at 12:22 pm | Permalink

    If I remember history right, a lot of what Leeuwenhoek was looking at may not have been microbes. The lens’ back then were pretty awful and there may have been a lot of visual artifacts in his field.

    I’m curious–I’m assuming they used something like ImageJ to help sort through images. How did they filter the sample, and how did they decide whether something was a virus or a particle of junk?

    It seems to me like you should be able to do some sort of biochemical analysis to look for a certain percentage of P as a proxy for how much DNA is in the sample. Maybe there’s nothing sensitive enough for that though?

  10. marksolock
    Posted October 12, 2012 at 10:10 pm | Permalink

    Reblogged this on Mark Solock Blog.


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