Oxide coating may aid bacteria at mealtime.

Why do many different kinds of bacteria living in sediments oil, and water drape themselves in manganese oxides?

The question has bewildered researchers. After all, "It is a pretty drastic thing to coat yourself with a bunch of rocks," says environmental scientist William G. Sunda of the National Marine Fisheries Service in Beaufort, N.C.

Researchers have proposed many possible explanations. Some researchers say the manganese coat may protect the bacteria against toxic metals or ultraviolet light, while others suggest it may generally strengthen the capsule around the organism, note Sunda and David J. Kieber of the State University of New York in Syracuse in the Jan. 6 NATURE.

But Sunda and Kieber say they have a better hypothesis.

Humic substances, such as the humus gardeners use, abound in the environments where bacteria live. Microorganisms can't directly use these biologically inert, complex materials, considered the dregs of organic matter, Sunda and Kieber write. It would be useful for the bacteria if they could because humics are valuable sources of carbon.

However, the insoluble manganese oxides surrounding some bacteria may oxidize the humics, they argue. This would create low-molecular-weight organic compounds on which the bacteria can thrive. In fact, manganese oxides may be able to break down many types of complex organic compounds into simpler ones. This would make large stores of carbon accessible to the manganese-cloaked organisms, the two scientists contend.

The outer shells of manganese oxide are not impenetrable and thus may allow bacteria to feed off of the carbon, which it needs to grow, Sunda suggests.

Bacteria that don't have manganese coats can't tap into humic sources of carbon. They must use a lot of energy to create enzymes to break down the specific nonhumic organic compounds they feed on, the researchers suggest.

Sunda and Kieber have yet to prove their theory, but they have made an encouraging start, they and others say. To test their hypothesis, they added manganese oxides to filtered estuarine water that had a high concentration of humics. They also add manganese oxides and fulvic acid, a common humic compound, to water from the Gulf of Mexico, which contained a low concentration of humics.

In both cases, within four hours, they detected increased levels of low-molecular-weight carbonyls -- organic compounds that the bacteria can tap as food. Only insignificant concentrations of these carbonyls had existed in the water samples prior to the addition of manganese oxides, indicating that the manganese oxides helped produce them, they report. The carbonyls included pyruvate, acetone, formaldehyde, and acetaldehyde.

The idea advanced by Sunda and Kieber "seems so sensible a possibility," and they have "interesting data" to back it up, says James Cowen of the University of Hawaii in Honolulu.

A follow-up experiment planned by Sunda and Kieber will entail more work than the one described in NATURE, which took a mere week and no special funding, they say.

The new study will involve growing bacteria that can use manganese oxides in the presence of manganese and humic compounds containing radioactively labeled carbon. The team hopes to trace the movement of carbon from the humics into the bacteria. This would indicate that the carbon the bacteria grow on comes from the humics, Sunda says.

The oxidation of humics by the manganese may benefit more than just bacteria, Sunda and Kieber say. It may someday prove useful in pollution control efforts. Manganese oxides are not very picky about what they will oxidize, so they may help degrade different types of pollutants, Kieber speculates.