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Sulfate-Reducing Bacteria

Sulfur occurs in living matter chiefly as a component of certain amino acids. Sulfur is present in the soil primarily as a part of proteins and is taken up by plants in its oxidized form, sulfate (SO₄^2-). 

The sulfur- containing proteins are first degraded into their constituent amino acids by enzymes and then excreted by numerous soil organisms as hydrogen sulfide (H2S). Hydrogen sulfide (H2S), immediately identified by its pungent odor, is highly toxic to most biological systems, if it was ever to accumulate. 

Under aerobic conditions, H2S oxidizes spontaneously to its most readily utilized form sulfate, by sulfate bacteria. However, under anaerobic conditions, sulfate-reducing bacteria can reduce sulfate to H2S.

In cooling systems, corrosion and fouling are frequently attributed to sulfate-reducing bacteria. The organism is often found in sludge in the basins of cooling towers. In these systems the bacterium, using sulfate as a hydrogen acceptor, produces hydrogen sulfide and ferrous sulfide, which in combination form a malodorous black slime. In severe cases of biological fouling, bubbles of hydrogen sulfide rise through the water in the basin causing objectionable odor in the area.

Corrosion occurs where biological matter settles or attaches to surfaces. Biologically produced chemicals such as ammonia, hydrogen sulfide and acids can increase general corrosion rates in a system, near and away from the generating organisms. The corrosion of iron by sulfate-reducing bacteria is rapid, and unlike ordinary rust, is not self-limiting. 

Tubercles formed by sulfate-reducing bacteria consist of an outer shell of red ferric oxide mixed with black magnetite iron oxide. Beneath the shell is a soft black center of ferrous sulfide, under which there is usually a deep pit. When cleaned, the interior surface of the pit usually has a characteristic bright silvery appearance.

Even though the sulfate-reducing bacteria are obligate anaerobes, they may survive but not actively grow if exposed to oxygen. The bacterium is present in most natural waters including fresh, brackish and seawater, and most soils and sediments contain sulfate reducers. Sulfate or sulfite must be present for active growth. The bacteria may tolerate temperatures as high as 176°F and a pH from about 5 to 9.

Control of sulfate-reducing bacteria requires a comprehensive biocide program and regular bacteria levels monitoring.