by Jim Caryl
IN today’s issue of Nature is an account of a virus that infects a virus, a virophage. We are all familiar with the ubiquitous plant and animal viruses. Many of us are familiar with the bacteriophage, viruses that specifically attack bacterial cells and are the most abundant organism on the planet; somewhere around 10 million virus particles in a drop of sea water.
Now we hear of a small virus, called Sputnik, which infects an enormous virus, called Mamavirus. This is a larger member of a class of giant viruses, originally discovered in 2003 in a cooling tower in Bradford. The original Bradford virus of that 2003 discovery, Mimivirus, was initially mistaken for a small bacterial cell such is its size. Viruses are parasites, incapable of replicating themselves without a host cell; a virus infects and subsequently usurps the cellular machinery of the host cell to make a virus factory, spewing out replica virus particles. The small Sputnik “virophage” is able to parasitise the large virus’s factory for its own ends.
Having realised that such parasitism exists, and adjusted their views to the sizes of particles involved, researchers believe that this phenomenon may be common in nature, and particularly important in oceanic plankton blooms; the knock on effects of which have implications in ocean nutrient cycles and climate, plankton being one of the major carbon sinks on the planet.
It is a fantastically interesting discovery by Didier Raoult and colleagues, from the University of the Mediterranean, and certainly raises some questions as to the nature of whether viruses are alive or not. If the large Mimivirus is capable of being mistaken for a bacterial cell, and being parasitised by a smaller virus, at what point to we conclude that viruses are a distinct living entities, all be it obligatorily parasitic ones?