Iron might hold ancient signatures of microbes
Date: 17-Sep-99
Country: USA
Author: Maggie Fox
If the traces turn out to be valid, they could also help settle
arguments about whether there was ever life on Mars, the team at the
University of Wisconsin said.
"This could be an ideal biosignature," Brian Beard, who led the study,
said in a statement.
Writing in the journal Science, Beard and colleagues said they found
evidence that iron in older sedimentary rocks had been digested by
microbes. Their method provides a way to look for the very oldest traces
of life.
"If you see something that looks like a seashell, it is not too far a
leap to say it is in this 4 million-year-old rock so it probably is 4
million years old," Beard, a geochemist, said in a telephone interview.
But when a rock has been through the pressure and heating and folding
that goes with being buried under the surface of the planet for hundreds
of millions of years, it is unlikely to hold any fossil that looks like
a fossil.
"This is a way we can see through that veil of expanse of time," Beard
said.
IRON IS KEY TO NEARLY ALL LIFE
Nearly all life, including plants, animals and the tiniest microbes,
uses iron.
"Iron has had a dramatic effect on how organisms have evolved," Beard
said. "Microorganisms fight for iron and some have developed a chemical
compound that allows them to grab iron and store it for future use."
This processing should leave traces.
Beard's team reports looked at sedimentary rock and igneous rock.
Sedimentary rock, as the name implies, is laid down over time from
various deposits. It hardens into rock and includes shale, sandstone and
other rock.
Igneous rock comes from volcanic activity. It is melted at such high
temperatures that no known life could survive.
Beard's team, who included scientists at NASA's Jet Propulsion
Laboratory, theorised that signs of life might be evident in the
sedimentary rock but not in igneous rock.
They used a new technique involving a mass spectrometer to sort the
charged particles - isotopes - of iron from both kinds of rock.
"Measurable isotopic variations can be seen," Beard said. "The mass
differences are small, but large enough that a microorganism could have
made the difference."
When they tested bacteria in the lab, they found the microbes created
isotopes of iron similar to those seen in the sedimentary rock but
different from igneous rock.
Beard said his group has started looking at a piece of meteorite from
Mars that some researchers say contains evidence of tiny microbes that
would have lived millions of years ago. Other experts contest this.
In a second study published in Science, Kevin Mandernack at the Colorado
School of Mines in Golden and colleagues examined marine bacteria that
produce long chains of the mineral magnetite.
Magnetite is made up of iron and oxygen. But magnetite made by the
bacteria had a different oxygen isotope than naturally occurring
magnetite, they reported.
Looking at magnetite in the Mars meteorite might provide a way of
telling if little worm-like structures in it were indeed made by
exceptionally tiny bacteria, as NASA scientists contend.
"If we find an isotope fractionation that matches an isotope
fractionation that was done in our laboratory experiment, and those two
agree, then I think the case for life on Mars would be strongly
strengthened," Beard said.
He also wants to look at other chemical evidence of the earliest life on
Earth.
In 1996 researchers found ancient rocks in Greenland that they dated to
3.85 billion years ago. To their amazement, the rocks contained carbon
isotopes that indicated the carbon had once been part of something alive
- probably a microbe.
"We want to look at some of those samples," Beard said.
