Chemistry of uranium-eating microbes.

Scientists who demonstrated the potential of a particular bacterium to remove heavy metals from polluted streams and to decontaminate nuclear waste have figured out how these microbes do their dirty work.

In weeks, this Citrobacter species can accumulate 9 grams of uranium for each gram of its own dry weight without suffering ill effects, says Lynne E. Macaskie of the University of Birmingham, England. The microbe makes a metal phosphate, she and her colleagues report in the Aug. 7 SCIENCE.

Macaskie's group observed the bacteria through an electron microscope. In 12 hours, the cells became opaque to electrons, especially along the edges. Dried cells turned yellow and emitted fluorescence characteristic of uranium compounds. X-ray data indicated that the cells had simultaneously used equal amounts of phosphate and uranium, the researchers report. Further analysis confirmed that the bacteria made crystals of a metal phosphate that chemists often synthesize.

"The more you know about the mechanism, the better your chances of improving on the process," comments Derek Lovley, a microbiologist with the U.S. Geological Survey in Reston, Va.

Macaskie suggests that enzymes in the bacterium make a negatively charged phosphate that then automatically joins with a positively charged uranium-oxygen complex to form a crystal. Water surrounding these chemicals helps stabilize their fibrous crystal structures, she says. Macaskie has watched these fibers grow out of the cell surface once the bacterium starts to accumulate material. Those whiskers may appear where the enzyme is making phosphate available to combine with uranium, the researchers note.

In 1991, Lovley discovered a different bacterium useful for removing uranium. Instead of storing uranium in its cells, this microbe uses enzymes to convert uranium ions to an insoluble form. Thus, dissolved uranium settles out as uraninite (uranium ore) and can be filtered from water. "It's a very stable process; we don't even need a living cell," he notes.

In a report coming out this fall in ENVIRONMENTAL SCIENCE AND TECHNOLOGY, Lovley and his colleagues describe their success in using this microbe to decontaminate nuclear waste sites.

Also, in the May ENVIRONMENTAL SCIENCE AND TECHNOLOGY, Lovley's team reported the discovery of bacteria that get rid of chlorofluorocarbons, common chemicals blamed for destroying ozone in the atmosphere. These bacteria do not accumulate chlorofluorocarbons but probably break them down into carbon dioxide and chloride or fluoride salts, Lovley says.