New bug found on
bugMarine microbe sets
miniaturization records.
2
May 2002
JOHN
WHITFIELD
 |
| The new-found bacteria
(red) hitched to their hosts (green). |
| © H. Huber et al.
| | |
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.
|
Similarly unusual groups
could be under our noses
|
| Ford Doolittle, Dalhousie
University,
Canada | | |
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. |
