Analyses of 3.5-billion-year-old rocks from Australia indicate that some of the microorganisms living when those rocks formed were able to derive energy from sulfur, the first time such a metabolic feat has been chronicled in rocks of that age.
Because bacteria have no hard parts, they don’t fossilize well. Nevertheless, signs of ancient life are often recorded in a rock’s chemistry. For example, bacteria that extract energy by metabolizing sulfate minerals leave behind sulfides partially depleted of the heavier isotopes of sulfur, says Pascal Philippot, a geochemist at the Paris Geophysical Institute.
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To discern the influence of such bacteria on ancient rocks, scientists look for a lower-than-normal concentration of the isotope sulfur-34 relative to that of sulfur-32.
In rocks more than 2.7 billion years old, sulfides typically show no sign of sulfur-34 depletion, says Philippot. The absence of that signal in such rocks could mean that sulfate-consuming microbes hadn’t yet evolved, he notes.
However, in 2001, a team of scientists reported evidence of sulfate-consumers in 3.5-billion-year-old rocks unearthed at a site in western Australia. Now, additional analyses of rocks from that site by Philippot and his colleagues suggest that the sulfides were indeed created by microbes—but microbes with a highly unusual metabolism.
In the new tests, the researchers also measured the rocks’ concentration of sulfur-33, a rare isotope that accounts for less than 1 percent of the element’s atoms. If sulfate-consuming bacteria had created the Australian sulfides, the ratio of sulfur-33 to sulfur-32 in the rocks should have been lower than normal. In some samples, however, the researchers found sulfur-33 concentrations that were as much as 6 parts per thousand above normal. That’s a clear sign that the sulfides weren’t produced by sulfate consumers, says Philippot.
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Instead, the sulfides came from microorganisms that derived their energy by processing pure sulfur, the researchers propose in the Sept. 14 Science. The sulfur would have formed in the atmosphere as ultraviolet light broke down sulfur-bearing volcanic gases, a process that would have produced abundant sulfur-33, says Philippot. If bacteria had then consumed such material, the resulting sulfides would have been enriched in sulfur-33 but depleted in sulfur-34.
Sulfur-consuming bacteria “are the only available explanation for the isotopic trends” seen in the Australian sulfide samples, says Bo Thamdrup, a geochemist at the University of Southern Denmark in Odense. Scientists have isolated only three species of sulfur-consuming bacteria, Thamdrup notes, and their requirements are spartan indeed: The microbes need only sulfur, water, carbon dioxide, and inorganic nutrients to survive.
Despite the simplicity of such microbes, geochemical evidence for their existence has extended back only 1.3 billion years, says Timothy W. Lyons, a geochemist at the University of California, Riverside. “This [finding] is a big step.”