Minding those microbial mineral-makers.

If a mineral shows up unexpectedly or disappears too readily from its environment, then it's a good bet that some bacteria are to blame. That's the message of two new reports examining chemical cycles in marine environments.

While studying the Black Sea in 1988, U.S. researchers were puzzled by the paucity of sulfur compounds known to be intermediate products in the cycling of this element from sulfate to hydrogen sulfide in salt water. They discovered that a versatile bacterium, Shewanella putrefaciens, that thrives in both oxygen-rich and oxygen-poor water actually uses these intermediates to help break down organic material. This species can take advantage of many oxidants, such as oxygen, manganese, or iron, in water. But unlike most bacteria, which also use sulfate, this one relied on the intermediates, Karen A. Perry of the University of Delaware's College of Marine Studies in Lewes and her colleagues report in the Feb. 5 SCIENCE.

"Apparently, there's significant microbiological control of sulfur speciation," says chemist George W.. Luther III, a Delaware collaborator. "It's not just chemical."

Other scientists investigating the anomalous buildup of an iron carbonate mineral called siderite also fingered versatile bacteria as the cause. Siderite nodules typically form where bacteria are producing methane, not where bacteria use sulfate to decompose organic material into carbon dioxide, says Derek R. Lovley, a microbiologist at the U.S. Geological Survey in Reston, Va. However, he and his colleagues discovered that, if given the opportunity. several of these so-called sulfate-reducing bacteria will instead use iron oxide to accept electrons generated in the decay process. Lovley and his colleagues describe their findings in the Feb. 4 NATURE.

"It shows a whole new class of microorganisms that might be involved in metal reduction," Lovley says. "There's a lot of unexpected things that microorganisms are doing."

Metal-reducing microbes are also proving capable of removing uranium from contaminated water, he adds.

Microbial mineral-makers can come from diverse backgrounds. For example, comparisons of RNA from ribosomes of various bacteria that orient via magnetic fields (SN: 5/16/92, p.330) reveal that this ability evolved more than once in these microbes. Most of these "magnetotactic" bacteria make iron oxide. But those that make iron sulfide particles come from a different twig on the evolutionary tree, according to Edward E DeLong of the University of California, Santa Barbara. In the Feb. 5 SCIENCE, he and his colleagues report that these microbes come from two subdivisions of Proteobacteria, even though they solved the need to orient in similar ways. The use of iron oxide or iron sulfide may have evolved because of the sediment types readily at hand, they suggest.