A Bacterium as Large as the Head of a Fly
This microscope photograph shows the end two bacteria in a string of
Thiomargarita namibiensis. The little balls are globules of sulfide, the
bacteria's food. (Science)
By Kenneth Chang
ABCNEWS.com
April 15 - You know how some people consider "jumbo shrimp" an oxymoron? Here's
another: giant bacteria.
Writing in this week's issue of the journal Science, an international team
of researchers report on a single-cell organism that is a leviathan among
microbes.
How Big Is It?
It's so big you can see it. It's as big as the head of a fruit fly or the period
that ends this sentence.
"It's the largest bacteria that is known," says Heide Schulz, a Ph.D.
student at the Max Planck Institute for Marine Microbiology in Germany and lead
author of the Science article. Schulz, drawing a comparison with the largest
animal ever, says, "It is interesting for microbiologists like the blue whale
would be interesting to zoologists."
The red dots indicate where Thiomargarita namibiensis was found in the
continental shelf sediments. (ABCNEWS.com / Magellan Geographix)
Schulz spotted the bacteria behemoths, up to three-quarters of a millimeter
in diameter, in sediments scooped up off the coast of Namibia during a 1997
expedition. When she first examined them, "I thought it might be a kind of ill
bacteria," she says. "But then I realized they were all that big."
An analogy: If an ordinary bacterium were scaled up to the size of newborn
mouse, the new bacterium, dubbed Thiomargarita namibiensis would as big as a
blue whale. The previous record holder - Epulopiscum fishelsoni, which lives in
the guts of surgeonfish - would be about as big as a lion.
"It's certainly in The Guinness Book of World Records by a factor of 100,"
comments Doug Nelson, a microbiologist at University of California, Davis, who
studies similar bacteria. "I think all of these giant bacteria help us to think
about what the upper limit for bacterialike organisms is."
Eating Sulfur, Breathing Nitrogen
The bacterium's vastness comes from how it makes a living. Off the coast of
Namibia is an upwelling of deep ocean water, which brings up decaying organic
material that has drifted to the ocean bottom. The nutrient-rich waters nurture
the growth of phytoplankton - so much so that the almost all the oxygen in the
water gets used up.
A trio of Thiomargarita photographed next to a fruit fly. The head of the fly
and the largest white cell have about the same diameter, about half a
millimeter. (Science)
The bacteria living in the olive-green mud therefore need to "breathe"
something else. For Thiomargarita, chemicals known as nitrates in the water
serve as their oxygen, while hydrogen sulfide, produced by other bacteria in the
sediment, is their food.
At the center of each Thiomargarita is a sac of nitrate-rich water, which
takes up 97 percent of its volume. "What this bacterium does is it carries
around its own oxygen," Nelson says. "If this bacteria were a scuba diver, it
would only have to come up to the surface only six to 10 times a year to fill up
its tank. Most of it is a scuba tank of nitrate."
String of Pearls
The immobile organism also stores globules of sulfide in its body, giving it a
glistening blue-green whiteness, and individual bacteria often line up in chains
of a dozen or longer. "They look like a necklace," Schulz says.
Thiomargarita namibiensis means "Sulfur Pearl of Namibia."
A typical chain of Thiomargarita under a microscope. In the middle, a
Thiomargarita is dividing. (Science)
The research team, which included scientists from Max Planck, University of
Oldenburg in Germany, University of Barcelona in Spain and Woods Hole
Oceanographic Institution in Massachusetts, had originally gone to the Namibian
waters to search for other sulfur-eating bacteria they had studied off the west
coast of South America.
They found few of the South American-type bacteria. Mostly the sediment
teemed with Thiomargarita.
In the more fluid sediments and turbulent waters off Namibia, Thiomargarita
's sit-wait-store strategy may be more effective than that of the smaller South
America bacteria, which move up and down in the sediments in search of nitrates
and sulfides.
"This bacterium literally just sits there and takes what nature gives it,"
Nelson says. "It's the logical extension of adapting to that sort of regime."
Scientists also know that there are many more bacteria to study. According
to Nelson, "99.9 or 99 percent of bacteria have never been studied in pure
culture."
Maybe even larger microbes lurk out there.