More complicated than you think

BIODIVERSITY is not just a matter of tigers and whales, or butterflies and trees, or even coral reefs and tuna. It is also about myriad creatures too small to see that live in numbers too large to count in ways too numerous to imagine. It is easy to forget, especially at meetings like the one to discuss the Convention on Biological Diversity that has been taking place in Nagoya under the auspices of the United Nations, that most of biology is in fact microscopic. Indeed, the more microscopic biology gets, the more diverse it becomes.

In that context, the discovery by Curtis Suttle of the University of British Columbia and his colleagues of a critter they propose to call Cafeteria roenbergensis virus, or CroV, should not be surprising. But for those brought up on a textbook definition of what a virus is, it is still a bit of a shock. For CroV is not a very viruslike virus. It has 544 genes, compared with the dozen or so that most viruses sport. And it may be able to make its own proteins—a task that viruses usually delegate to the molecular machinery of the cells they infect.

CroV, as its full name suggests, is a parasite of Cafeteria roenbergensis, a single-celled planktonic organism that was itself discovered only in 1988. Despite the recentness of its discovery, C. roenbergensis is one of the commonest creatures on the planet. It is also reckoned by some, given that it hunts down and eats bacteria, to be the most abundant predator on Earth. It is found in every ocean. The samples of C. roenbergensis from which Dr Suttle and his team extracted their quarry were collected off the coast of Texas.

To see a world in a grain of sand

That quarry's nature, reported this week in the Proceedings of the National Academy of Sciences, is not a complete surprise. A larger virus, called Mimivirus, which lives in freshwater amoebas, turned up in 2003 and a few other, similar, viruses have been found since then. But CroV is by far the biggest to come out of the sea.

Those who like their categories cut and dried may wonder whether viruses are alive or not. Wise biologists do not struggle too much with such questions. Viruses have genes, can reproduce and are subject to the evolutionary pressures imposed by natural selection. That is enough for biology to claim them. As for CroV, those 544 genes (composed of 730,000 base pairs, the DNA letters in which the language of the genes is written) mean its genome is bigger than those of several bacteria—creatures which everyone agrees are alive.

The problem with categorical thinking in biology is that evolution does not work like that. It actually works by whatever works working. If an organism can successfully subcontract part of the business of metabolism to another while retaining the rest itself, rather than offloading the whole lot as most viruses do, then there are no rules to stop it happening.

CroV seems to do just that. Besides the genes that relate to protein synthesis it has others which encode DNA repair mechanisms and still others which are involved with protein recycling and signalling within cells. This is not mere hijacking. It is tantamount to a complete personality transplant for the infected cell.

About a third of CroV's genes are similar to Mimivirus genes, suggesting they share a distant ancestor. On the other hand, two-thirds are not. A significant chunk of them seem to have been copied from bacteria. But the majority are unique, and previously unknown to science. A whole new chapter of life, in other words, has been opened.

This discovery, then—and the earlier one of C. roenbergensis itself—speak volumes, albeit in a microscopic language, about biodiversity. Two centuries after Carl Linnaeus invented the system now used to describe it, and a century after Charles Darwin worked out what causes it, the ability of that diversity to surprise is still staggering.