Iron and sulfur bacteria are a group of morphologically and physiologically heterogeneous organisms that can transform large amounts of iron, manganese, or sulfur, usually depositing it in the form of objectionable slimes. They may cause, or be associated with, fouling and plugging wells. Their growth also may result in consumer complaints of red, black, or brown water in potable-water distribution systems. Sulfate-reducing bacteria may cause rusty water; pipe tuberculation; and odor, taste, frothing, color, and turbidity problems in waters.
The designation “iron and sulfur bacteria” is historical and imprecise. It includes microflora that may be filamentous or single-celled, autotrophic or heterotrophic, aerobic or anaerobic. Some iron bacteria deposit manganese oxides. (Chemically, manganese behaves similarly to iron in the environment, although it oxidizes under more highly oxidizing conditions than iron.) The taxonomic position of these bacteria is diverse; in some cases, it is uncertain or in dispute.
Because many of these bacteria can use reduced iron or sulfur as their primary energy source, they can grow under highly oligotrophic conditions when attached to a substrate in flowing water that contains iron or sulfur. Temperature, light, pH, and oxygen supply are critical to the growth of iron and sulfur bacteria. Under different environmental conditions, some bacteria may appear either as iron, manganese, or sulfur bacteria. For example, Acidithiobacillus (formerly Thiobacillus) ferrooxidans, which contributes to the problem of acid mine drainage, can be identified by the test for transforming ferrous to ferric iron, but it can also grow via oxidation of reduced sulfur compounds at low pH.
Thus, for both practical and historical reasons, these bacteria are treated collectively here as iron and sulfur bacteria. These elements often co-occur, and their transformations may be important in the operation and maintenance of water treatment and distribution systems. They may be especially bothersome in industrial waters (e.g., cooling and boiler waters).