WHAT is the commonest living thing on Earth? Until now, those in the know would probably have answered Pelagibacter ubique, the most successful member of a group of bacteria, called SAR11, that jointly constitute about a third of the single-celled organisms in the ocean. But this is not P. ubique’s only claim to fame, for unlike almost every other known cellular creature, it and its relatives have seemed to be untroubled by viruses.

As Jonathan Swift put it in a much-misquoted poem, “So, naturalists observe, a flea/Hath smaller fleas that on him prey”. Parasites, in other words, are everywhere. They are also, usually, more abundant than their hosts. An astute observer might therefore have suspected that the actual most-common species on Earth would be a “flea” that parasitised P. ubique, rather than the bacterium itself. The absence of such fleas (in the form of viruses called bacteriophages, that attack bacteria) has puzzled virologists since 1990, when the SAR11 group was identified. Some thought the advantage this absence conferred explained the group’s abundance. But no. As they report in this week’s Nature, Stephen Giovannoni of Oregon State University and his colleagues have discovered the elusive phages. Swift’s wisdom, it seems, still holds good.

Tracking down a particular virus in the ocean makes finding a needle in a haystack look a trivial task. A litre of seawater has billions of viruses in it. Modern genetic techniques can obtain DNA sequences from these viruses, but that cannot tie a particular virus to a particular host.

To do so, Dr Giovannoni (pictured) borrowed a technique from homeopathy: he diluted some seawater to such an extent that, statistically speaking, he expected a 100-microlitre-sized aliquot to contain only one or two viruses. The difference between his approach and a homeopath’s was that what homeopathy dilutes almost to nothing are chemicals, and thus cannot breed. A virus can, given a suitable host. So he mixed each of several hundred aliquots into tubes of water containing P. ubique. Then he waited.

The race is to the Swift

After 60 hours, he looked to see what had happened. In most cases the bacteria had thrived. In a few, though, they had been killed by what looked like viral infection. It was these samples that he ran through the DNA-sequencing machine, in the knowledge that the only viral DNA present would be from whatever it was had killed the bacteria.

His reward was to find not one, but four viruses that parasitise P. ubique. He then compared their DNA with databases of DNA found in seawater from around the world, to find out how abundant each is. The upshot was that a virus dubbed HTVC010P was the commonest. It thus displaces its host as the likely winner of the most-common-living-thing prize.

That does depend, of course, on your definition of “living thing”. Some biologists count viruses as organisms. Some do not. The reason is that a virus relies for its growth and reproduction on the metabolic processes of the cell it infects. This means viruses themselves are hard to parasitise, since they do no work on which another organism can free-ride. Which is why the next two lines of Swift’s poem, “And these have smaller fleas to bite ’em/And so proceed ad infinitum”, are wrong—and why, because HTVC010P itself can have no parasites, it probably really is the commonest organism on the planet.