Chemical 'crumbs' from microbes' snacks provide a trail to ancient, remote life

April 06, 2000

Researchers are turning their attention to the culinary habits of microbes in their search for a few chemical "crumbs" of evidence of ancient, remote, and even extra-terrestrial life. Scientists are analyzing rocks from the furthest reaches of Earth, and beyond, using new and sensitive instrumentation to check for tell-tale signs of ancient life.

In a paper in the April 8 issue of Science, a University of Rochester team has contributed to the effort. The scientists announced that they were able to re-create in the laboratory chemical signatures that were previously thought to be produced only by living organisms.

"The situation is a little more complicated than some people have thought," says geochemist Ariel Anbar, the lead investigator and an assistant professor of earth and environmental sciences and chemistry. "It's like someone saying that they have found a way to use fingerprints to identify people, and we're saying we can produce those same fingerprints in the lab without having that person around."

The work is part of an expanding worldwide effort to use increasingly sensitive chemical tools to look for signs of past life in remote places. Anbar's research focuses on the evidence that microbes leave behind after they munch on metals like molybdenum, copper, iron and zinc. The scientists say it's likely that as tiny organisms scour for these nutrients, they select certain isotopes of elements (isotopes are forms of the same element with a slightly different mass) over others, and the evidence is locked into rocks for millions of years. Researchers measure the amounts of different isotopes of certain metals as a way of knowing whether bacteria or other organisms once lived there.

It's a little bit like coming downstairs in the morning and finding the kitchen a mess, perhaps with the lid off a hidden cookie jar and crumbs on the counter. If the husband is the only person in the home who loves cookies and knows where the goodies are kept, then the missing cookies and the crumbs would implicate him as the midnight snacker.

Anbar's team found an "imposter," a chemical process that doesn't necessarily involve any living organisms but nevertheless leaves evidence that scientists had thought might only be due to microbes. The team turned its attention to iron, a particularly appealing indicator for scientists because it's present in a majority of rocks, creating many potential candidates for signs of ancient life. Researchers studied the properties of different chemical forms of iron and found that simple chemical processes in the laboratory can account for the proportions that scientists previously believed indicated past life. The scientists suspect the same chemical signatures could be created naturally in groundwater and sediments whether life is present or not.

Much of the laboratory work was done by chemistry graduate student Jo Roe. She fed different forms of iron into a small vertical column, then measured the rate at which different isotopes of iron traveled through this chemical environment. Formerly a nurse, Roe decided to switch careers and obtained bachelor's and master's degrees in chemistry before joining Anbar's laboratory and joining the hunt for exotic life. Also contributing to the project were research scientist Jane Barling of the Department of Earth and Environmental Sciences, and scientist Kenneth Nealson of the Jet Propulsion Laboratory in California.

The work comes as scientists are rapidly expanding their criteria for environments that can support life. Living organisms have been found at the bottom of the ocean, far from any light source, and in highly acidic environments. With each of these findings, the possibility that life exists or once existed in a harsh or remote outpost of our solar system, like Mars or one of Jupiter's moons, moves up a notch. Says Anbar: "Life can exist in a much wider variety of conditions than we thought before. People are optimistic that the conditions for life are out there."

Among the group's next research projects is the analysis of an iron meteorite that is part of the rubble from the formation of our solar system. The team will study the rock to develop techniques for tracking the chemical activity of the metals within, then hopes to use those methods to study other meteorites that may hold signs of life.

"Understanding biological fingerprinting is a little like deciphering an ancient treasure map," says Anbar, whose work is supported by the NASA Astrobiology Institute and the National Science Foundation. "We do our best to understand the map, but we don't really know where it will lead."

University of Rochester

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