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Study Of Sulfides In Bacteria Casts Doubt On Evidence Of Life In

Arizona State University

Contact: James Hathaway, 602-965-6375

Source: Dr. Peter Buseck, 602-965-3945

May 8, 1998

Study of sulfides in bacteria casts doubt on evidence of life in Martian
meteorite ALH84001

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.