New bug found on bug

Marine microbe sets miniaturization records.
2 May 2002

Researchers have found a strange and tiny new group of microbes living on another microbe at the bottom of the sea.

The organisms are about 400 billionths of a metre across - more than six million would fit on the head of a pin.

The new bugs seem unable to survive alone. It's not clear what these infinitesimal hangers-on get from their hosts, or if they do any damage. Their hosts do fine without them.

The microbe has one of the smallest genomes known, at half a million DNA letters. Its sequence, currently underway, may point to the minimum number of genes needed for an organism to sustain itself.

The new microbes are called Nanoarchaeota. They belong to a group called the Archaea, one of the three giant branches of life along with bacteria and eukaryotes, which contains us and other animals, plants and fungi.

The Nanoarchaeota are smaller, and have a shorter genome, then any other archaean. "It's a new continent of microbes," says one of their discoverers, Karl Stetter of the University of Regensburg, Germany.

"This is as novel as you can get," agrees microbiologist Carl Woese of the University of Illinois at Urbana-Champaign. "In my book it'd be a new kingdom of life." The Nanoarchaeota might even lie between "cells as we know them and earlier intermediate forms", he speculates.

Nanoarchaeota could be a primitive and ancient group. Or they might have lost much of their genome and shrunk on route to their dependent lifestyle. Many other microbes with small genomes are parasites, having discarded DNA they don't need during evolution.

But the groups' history and position in the tree of life is still a mystery. Any ideas advanced so far "are out there for people to shoot at", says Woese.

Digging deep

Stetter and his colleagues found the Nanoarchaeota 120 metres down in the sea off the north coast of Iceland. Here, volcanic activity heats the water close to boiling point.

They noticed the tiny blobs on the surface of an Archaean called Ignicoccus, whose cells are about 2 millionths of a metre across. Each cell sported 30 to 50 blobs. Staining showed that the blobs contained DNA.

Early attempts to identify this DNA failed. Researchers tailor sequences of the related molecule, RNA, to stick to microbial RNA and so tell them what they've got. But all conventional probes drew a blank.

"We couldn't believe it," says Stetter. "We weren't sure if we had a virus or a living organism."

Using less choosy methods the team eventually caught and sequenced the blobs' genetic material. This revealed them to be Archaea - albeit unusual ones. Since then, the researchers have grown Nanoarchaeota with Ignicoccus in the lab.

The finding suggests that other similarly unusual groups could be under our noses, says evolutionary biologist Ford Doolittle of Dalhousie University in Halifax, Canada. "There's probably a lot of stuff out there that we haven't discovered yet," he says.