Study Of Sulfides In Bacteria Casts Doubts On Evidence Of Life In Martian Meteorite ALH84001

May 08, 1998

The Martian meteorite ALH84001 gave people hope that it was evidence for extraterrestrial life because minerals found in it resembled minerals created by some unusual earthly bacteria. Now it appears that the bacteria themselves contradict that claim.

In an article appearing in the May 8, 1998 issue of Science, a team led by two scientists from Arizona State University reports finding evidence for as many as three different iron sulfide minerals in two different bacteria known for generating magnetic compounds but not other iron sulfides normally found with them.

Using transmission electron microscope observations, the team, which includes ASU Geology and Chemistry/Biochemistry faculty members Mihaly Posfai and Peter R. Buseck, Iowa State University microbiologist Dennis A. Bazylinski, and California Polytechnic physicist Richard B. Frankel, found clear evidence of mackinawite and greigite and, possibly, cubic iron sulfide.

One of the iron sulfides they did not detect is pyrrhotite , a mineral that has been found in the now-famous Martian meteorite ALH84001 and that frequently occurs as a breakdown product of the other sulfides. Though pyrrhotite's presence in the meteorite has been cited as possible evidence of past Martian bacterial life, the study's evidence suggests that the bacteria may actually prevent its formation.

The study found evidence that the bacteria first produce mackinawite, a nonmagnetic iron sulfide, which then naturally converts to the magnetic greigite. It also suggests that this process may actually begin with cubic iron sulfide, which is unstable and rapidly becomes mackinawite. In the geological environment the bacteria are found in, the reaction sequence would also eventually lead to greigite breaking down into pyrite and pyrrhotite, but that reaction does not occur when the bacteria are present.

The team's research finding contradicts an earlier study that found pyrite and pyrrhotite present in the bacteria. As no subsequent study has been able to duplicate this result, the current team posits that earlier researchers may have confused cubic iron sulfide with these minerals, which give similar selected area electron diffraction patterns.
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Arizona State University

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