Reporting in the August 23 edition of the journal Geology, California Institute of Technology geobiology graduate student Tanja Bosak and her coauthors describe their success in growing calcite crusts in the presence and absence of a certain bacterium in order to show that tiny pores found in such rocks can be definitively attributed to microbial presence.

Micropores have long been known to exist in certain types of carbonate rocks that built up in the oceans millions of years ago, but researchers have never been able to say much more than that the pores were likely caused by microbes.

The new results show that there is a definite link between microbes and micropores.

In the experiment, Bosak and her colleagues grew a bacterium known as Desulfovibrio desulfuricans in a supply of nutrients, calcium, and bicarbonate that built up just like a carbonate deposit in the ancient oceans.

The mix that contained the bacteria tended to form rock with micropores in recognizable patterns, while the "sterile" mix did not.

"Ours is a very reductionist approach," says Dianne Newman, the Clare Boothe Luce Assistant Professor of Geobiology and Environmental Science and Engineering at Caltech and a coauthor of the paper.

"This work shows that you can study a single species to see how it behaves in a controlled environment, and from that draw conclusions that apply to the rock record. The counterpart is to go to nature and infer what's going on in a system you can't control."

"We were primarily interested in directly observing how the microbes disrupt the crystal growth of the carbonate rocks," adds Bosak.

In essence, the microbes are large enough to displace a bit of "real estate" with their bodies, resulting in a tiny cavity that is left behind in the permanent record.

The micropores in the study tend to be present throughout the crystals, and they not only mirror the shape and size of the bacteria, but also tend to form characteristic swirling patterns. If the micropores had been formed by some kind of nonliving particles, the patterns would likely not be present.

The next step in the research is to run the growth experiments with photosynthetic microbes. The information could help scientists determine which shapes found in certain types of rocks can be used as evidence of early life on Earth.

In the future, the information could also be used to study samples from other rocky planets and moons for evidence of primitive life.

Primarily, however, Newman says the technique will be of immediate benefit in studying Earth. "If you really want to look at life billions of years ago, in the Precambrian, you need to study microbial life.

"Even today the diversity of life is predominantly microbial," Newman adds, "so if we expand our perspective of what life is beyond macroscopic organisms, it's clear that microbes have been the dominant life form throughout Earth history."

In addition to Bosak and Newman, the other authors of the paper are Frank Corsetti of USC's department of earth sciences, and Virginia Souza-Egipsy of USC and the Center of Astrobiology in Madrid, Spain.

The paper is titled "Micron-scale porosity as a biosignature in carbonate crusts," and is available online at http://www.gsajournals.org/ .

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