Ocean bug has 'smallest genome'

By Roland Pease
BBC science correspondent

Scientists study genomes of the sea (Image: Science)

Small but perfectly formed, Pelagibacter ubique is a lean machine stripped down to the bare essentials for life.

Humans have around 30,000 genes that determine everything from our eye colour to our sex but Pelagibacter has just 1,354, US biologists report in the journal Science.

What is more, Pelagibacter has none of the genetic clutter that most genomes have accumulated over time.

There are no duplicate gene copies, no viral genes, and no junk DNA.

'Chicken soup'

The spareness of its genome is related to its frugal lifestyle. The shorter the length of DNA that needs to be copied each generation, the less work there is to do.

Pelagibacter has even gone one step further. It has chosen where possible to use genetic letters - or base pairs - which use less nitrogen in their construction: nitrogen is a difficult nutrient for living things to obtain.

The result is one of the most successful organisms on the planet. Pelagibacter feeds off dead organic matter that is dissolved in ocean water - lead researcher Stephen Giovannoni of Oregon State University likens it to a very thin chicken soup.

The dissolved carbon is always there, so there is no need to build in special metabolic circuits to adjust between periods of feast and famine. Indeed, in laboratory studies, the Oregon biologists have found that adding nutrients to the broth has no effect on the microbe's vigour.

Self-sufficient

The sheer abundance of Pelagibacter - there are an estimated 20 billion billion billion Pelagibacter microbes scattered throughout the world's oceans - is probably what has allowed the organism to streamline its genes.

With so many copies in the ocean, there are plenty of opportunities for random mutations to try out more thrifty combinations.

There are organisms with smaller genomes - Mycoplasma genitalium has about 400 genes. But these are all obligate parasites or symbionts, relying on other organisms to do the jobs they have abandoned. Pelagibacter is entirely self-sufficient.

There is a great deal of interest in finding out how few genes a living organism can get away with. Bio-entrepreneur Craig Venter is trying to create an artificial version of a bacterium, aiming for as few as 300 genes.

Stephen Giovannoni says the synthetic one will barely function. But Pelagibacter on the other hand, accounting for a quarter of all organisms in the ocean, is a shining example of Darwin's principle, the survival of the fittest.