Masses of red slime, oily-looking films, and black-coated rocks in natural waters often cause worry because people think that the water is polluted. When they learn these coatings and films occur in unpolluted water also, they get curious. Throughout the Anacostia watershed, along the Atlantic Coastal Plain, in the Piedmont and Shenandoah regions, throughout the fens of Iowa, around the Great Lakes, up in the Colorado Rockies, and in Hawaii and Alaska, red slime, oily films, and black-coated rocks can be found anywhere that ground water, which lacks oxygen and carries iron and manganese, discharges into a stream.

Slimes, oil-like films, and rock coatings are often made by bacteria that are reacting to the presence of iron and manganese in the water. Bacteria live on the water surface, in the water column, in the sediment, and at the sediment-water interface. Certain bacteria, the oxidizers, fix oxygen onto iron and manganese. Other bacteria, the reducers, remove the oxygen. In fixing or removing oxygen, some are getting energy and others are performing other life functions. Bacteria have been involved in the iron and manganese cycles for billions of years.

Huntley Meadows, Va.
Oil-like film on surface of quiet water at Huntley Meadows Park, Va. The film is often made by Leptothrix discophora. In the weeks between rainfalls, you can watch the film get redder and redder as more and more iron oxide forms.

Oily-looking films are usually the most worrisome substances. The way to tell the difference between a bacterial film and oil floating on the water is to break the film. If the oily film stays broken, it is a natural bacterial film. If it flows back into place, it is petroleum, which indicates pollution.

Collecting the bacteria and viewing them through a microscope is one way to tell important differences in the water chemistry from one site to the next. Some bacteria thrive in near-neutral waters, and others thrive in acid waters. Acid waters can be a result of mining activities.

Looking at these microorganisms in the water is both a growing environmental field and an outdoors activity that can give hours of enjoyment. Techniques for collecting and studying the iron bacteria are simple and are presented here. With only minor modification (such as using plastic equipment instead of glass), these techniques can be incorporated easily into projects for children.

These collecting techniques also work where the precipitate colors are different. Turquoise blue films may be made by bacteria that precipitate copper minerals. Other bacteria participate in the making of white slimes of aluminum, sulfur, or calcium minerals. Some green and purple slimes are made by bacteria that thrive where sulfur is present. Other green and blue-green slimes are made by algae and cyanobacteria (also known as. "blue-green algae") that use chlorophyll to participate in photosynthesis. These are not mineral slimes.

Some people think that all bacteria are bad, but the bacteria that fix metals that are carried by water are very useful because they remove harmful materials. Who knows? You could be making valuable observations and collecting new bacteria that could turn water-borne metals into useful products.

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Contact: Norrie Robbins
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