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Tracks In Iron Provide An Insightful Map Of Microbial World

Office of News and Public Affairs
University of Wisconsin-Madison

September 16, 1999

Tracks in iron provide an insightful map of microbial world

Reading the narrow bands of iron found in some sedimentary rocks, scientists
may have found a way to assess microbial populations across time and space,
opening a window to the early history of life on Earth and possibly other
planets. 

Writing in the Friday, Sept. 17 issue of the journal Science, a team of
scientists led by UW-Madison geochemist Brian L. Beard describes a
geochemical signature in iron indicative of life. If the technique is
confirmed and refined, it could be used to trace the distribution of Earth's
microorganisms in the distant past, and could help resolve disputes about
the existence of past life on other planets such as Mars. 

"This could be an ideal biosignature," Beard says in describing a set of iron
isotope-sorting experiments designed to determine if iron found in different
kinds of rocks has been metabolized by microorganisms. 

Iron is vital to plant, animal and microbial life. Nearly all organisms ingest
it in the course of daily life. If scientists can devise a method to distinguish
between iron that has been processed by a living organism and iron that has
not been metabolized, they will have a way to measure the distribution of
microbes on Earth billions of years ago. 

Because iron is common on the moon, planets and other objects in space, the
technique could be used to detect signs of past life beyond our own planet. 

Beard's group measured the isotopic composition of iron from two distinct
sources: sedimentary rock and igneous rock. Sedimentary rock reflects the
accumulation of sediments, including organic material and trace elements
such as iron. Igneous rock is forged deep in the Earth at very high
temperatures where life is absent. It also can contain iron. 

Working in collaboration with scientists from NASA's Jet Propulsion
Laboratory and the Institute for Great Lakes Study at UW-Milwaukee, the
Wisconsin team sampled the isotopic composition of iron from the two
sources by incinerating samples of iron and measuring charged particles
from the reaction is a mass spectrometer, a device that sorts and counts
ionized particles. 

"Measurable isotopic variations can be seen," says Beard. "The mass
differences are small, but large enough that a microorganism could have
made the difference." 

Isotopes from sedimentary rock, says Beard, match the isotopic signature
of iron ingested and metabolized by bacteria in the lab: "What we found in
the biological experiments was that microbes produce a measurable iron
isotope fractionation. We wondered if inorganic processes might have the
same effects, but we found that the isotopic composition of iron in igneous
rocks is constant." 

Knowing this, it may now be possible for scientists to look at sedimentary
rock and gain a sense of the worldwide ebb and flow of microbial populations
in the distant past, perhaps as far back as 2 billion years ago, when the
Earth's oceans were full of soluble iron. Such insight may help show how
life evolved on Earth. 

"Iron has had a dramatic effect on how organisms have evolved," Beard says.
"Microorganisms fight for iron and some have developed a chemical
compound that allows them to grab iron and store it for future use." 

Beard says his group next plans to apply the technique to a piece of the Mars
Rock, a controversial meteorite that some scientists believe harbors evidence
of past microbial life on the Red Planet. It could also be used to screen
samples brought back to Earth from planned NASA missions to Mars. 

Co-authors of the paper published in Science include Clark Johnson, a
professor of geology and geophysics at UW-Madison; Lea Cox, Henry Sun and
Kenneth Nealson of NASA's Jet Propulsion Laboratory in Pasadena, Calif.; and
Carmen Aguilar of the Institute for Great Lakes Study at UW-Milwaukee.

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