MADISON - 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 this week in the journal Science, a team of scientists led by University of Wisconsin-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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