Geobiologists are announcing today their first major success in using a novel method of "growing" bacteria-infested rocks in order to study early life forms. The research could be a significant tool for use in better understanding the history of life on Earth, and perhaps could also be useful in astrobiology.
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.