If hell exists, then we know an awful lot about the microorganisms that might live there. Of the two dozen or so bug genomes decoded so far, eight belong to the temperature-loving 'thermophiles'. The genetic secrets of the latest -- Thermoplasma acidophilum -- are published this week in Nature1.

This hardy bug grows best at a blistering pH 2 and 59°C, and without the structural protection of a conventional cell wall. This has long intrigued physiologists, but other researchers have also been anxious to find out what makes this particular microorganism tick.

Evolutionary biologists, for example, have speculated that T. acidophilum is an ancient ancestor of nucleus-containing 'eukaryotes': cells that make up higher organisms such as people. And biotechnologists are eager to learn the secrets of the thermophile's enzymes, which thrive in conditions that usually turn proteins into mangled messes.

So, Andreas Ruepp of the Max-Planck-Institute, Martinsried, Germany, and colleagues found some surprises when they sequenced the 1.5 million base pairs that make up one of the smallest genomes of its type unravelled to date.

"The most startling observation is the high proportion of genes that seem to have been acquired from other species," comments Don Cowan, a molecular biologist at University College London, UK. About seventeen per cent of 'open reading frames' (sections of the genome that actually encode proteins), for example, have relatives in the bacterium Sulfolobus solfataricus.

And the evolutionary biologists may have to rethink their family trees. After comparing T. acidophilum genes with gene databases, the researchers say that the thermophile is more closely related to bacteria that eukaryotes. (Strictly, T. acidophilum is not a bacterium, but a member of the 'third domain of life' Archaea.) Key marker genes are missing from its genome, which just about defeats arguments for eukaryotic ancestory, says Cowan.

Ruepp's team believe that the genome can be broadly split down the middle. "Two classes of genes can be distinguished," they say. One mainly contains 'housekeeping' genes needed for fundamental, routine tasks. The second, 'life-style' class contains genes tailored to a specific environment which seem to revolve around metabolic functions. These life-style genes could be shared around to help organisms survive harsh environments, the researchers suggest.

On a different note, Cowan stresses that the newly announced sequence shows how little researchers understand gene function as a whole -- the purposes of almost half of the predicted 1,509 protein-coding regions in T. acidophilum are still a mystery, he says.