# Monday Micro – 200 million light years of viruses?!

By Siouxsie Wiles 05/08/2014

Polio EM PHIL 1875 lores” by CDC/ Dr. Fred Murphy, Sylvia Whitfield – This media comes from the Centers for Disease Control and Prevention‘s Public Health Image Library (PHIL), with identification number #1875..

Over the weekend I got an email from broadcaster Graeme Hill telling me about an amazing statement he had heard about the number of viruses on the planet and if it could be true. The statement came from a 2010 BBC Horizon documentary about viruses which you can hear here: Viruses

“Viruses are the most abundant life form on Earth. If you laid all the viruses on the planet end to end, they’d form a line 200 million light years long.”

“Can this possibly be true?” Graeme asked.

Let’s take a look and see how the BBC came up with that astonishing factoid.

The figure seems to be based on the following equation:

10^31 viruses on Earth x 200 nm = 2 x 10^24 metres = 200 million light years

The estimate of 10^31 viruses on Earth appears quite a bit in the literature and seems to trace back to this paper (1) which bases it on the estimate for the number of bacteria on Earth from this paper (2). The logic behind this is that the vast majority of viruses that exist are likely to be preying on bacteria (so-called bacteriophages). So if that number is true, it doesn’t account for any of the other viruses on the planet.

The estimate of 200 nm for the average size of a virus is also a ‘guestimate’. Most viruses that have been discovered have a diameter that ranges from 20 and 300 nm, although the filamentous viruses that make up the Filoviridae family (of which Ebola is a member) can be up to 1400 nm in length. I would use 20 nm for the size estimate to be on the conservative size, but that would still put it at 20 million light year’s worth of viruses!

So instead of using estimates, is there any actual data out there quantifying viruses?

Wommack and colleagues looked at the abundance of viruses in Chesapeake Bay, an estuary of the coast of the USA (3). They collected water samples and visualised the viruses present by transmission electron microscopy after ultracentrifugation. Virus counts ranged between 2.6 x 10^6 and 1.4 x 10^8 viruses per ml of water, with a mean of 2.5 x 10^7 viruses per ml. Estimates for the amount of water in Chesapeake Bay put it at 18 trillion gallons which is 68 trillion litres. This means that, if we use the mean value from Wommack’s study and an average size of 20 nm, in Chesapeake Bay alone there are one twenty-fifth (0.004) of a light year’s length of viruses! Another study, this time along a transect in the western Gulf of Mexico found a similar value for the number of viruses present – from 10^7 to 10^8 per ml (4).

So if we take a value of 10^7 viruses per ml of seawater and multiply this by the estimate for the amount of water in the Earth’s oceans (about 10^21 litres) we get the equivalent of almost 3 million light year’s worth of 20 nm sized viruses. Just in the oceans. Looks like Horizon’s claim may actually be pretty close to the mark!

References: