Making big mountains out of tiny bacteria.

The Dolomites, a mountain range spectacularly carved by erosion, symbolize an enduring riddle in geology. For more than two centuries, scientists have pondered how Earth constructs dolomite, a mineral that makes up more than 10 percent of all sedimentary rock, including much of the rock in these northern Italian peaks (SN: 11/30/85, p.343).

In the laboratory, using the high temperatures typical of the planet's interior, geologists can synthesize dolomite with ease. But they have never been able to manufacture the mineral at the much lower temperatures of Earth's surface, where geological evidence indicates that the vast majority of dolomite formed. Dolomite, or calcium magnesium carbonate, is a close cousin of limestone (calcium carbonate), and geologists suspect that most dolomite forms when limestone somehow incorporates magnesium ions derived from seawater.

Now, with the aid of unusually small bacteria found in sludge from a lagoon in Brazil, researchers from Switzerland have finally created dolomite in the laboratory at low temperatures.

"What they've found is tantalizing.... We've focused on trying to explain [dolomite] with classical inorganic chemistry. This upsets the applecart," says Lynton S. Land of the University of Texas at Austin.

Judith A. McKenzie of the Swiss Federal Institute of Technology in Zurich and her colleagues took sludge samples from a lagoon in Brazil where modern dolomite has formed. They placed samples of bacteria from the sludge in vials, along with a growth medium similar to the lagoon's water. After refrigerating the vials for a year, the researchers found dolomite crystals encrusting the bacteria. "Lo and behold, they produced dolomite. If we don't have bacteria [in the vial], nothing happens," says McKenzie. The team's results appear in the Sept. 21 Nature.

McKenzie notes that Robert L. Folk, a retired geologist, recently revived the century-old idea that bacteria help make the mineral after he had examined high-magnification images of dolomite that geologists think formed relatively recently. "I saw tiny bacteria in the dolomite crystals," says Folk.

McKenzie and her colleagues suggest that their bacteria might help explain another mystery surrounding dolomite: The mineral is much more abundant in ancient rocks than in modern ones. Conceivably, McKenzie says, dolomite-producing bacteria are less widespread today.

Some geologists, however, categorize the Brazilian bacteria as a curiosity. "I think [their discovery] really doesn't have a lot to do with the dolomite problem. Most dolomite doesn't form in lakes," says Bruce H. Wilkinson of the University of Michigan in Ann Arbor.

In addition to looking at how the bacteria make dolomite, McKenzie and her colleagues hope to develop probes that will enable them to determine whether similar bacteria exist at other modern dolomite formations. "I could spend the next 10 years on the dolomite problem," says McKenzie.