Sulfate Reducing Bacteria (SRB) are ubiquitous microbes foundational to the global sulfur cycle. As strict anaerobes, SRB thrive in anoxic conditions, such as deep sediments, waterlogged soils, and inside human and animal digestive tracts. Their primary metabolic output is hydrogen sulfide (\(\text{H}_2\text{S}\)), a corrosive gas that gives many anaerobic environments their characteristic rotten-egg smell.
Defining Sulfate Reducing Bacteria
Sulfate Reducing Bacteria are a diverse collection of microorganisms that derive energy through a process called dissimilatory sulfate reduction. This metabolic pathway allows them to use sulfate (\(\text{SO}_4^{2-}\)) as their terminal electron acceptor instead of oxygen. SRB consume organic compounds or hydrogen gas as electron donors, linking the degradation of organic matter to the sulfur cycle. Sulfate is chemically reduced, resulting in sulfide (\(\text{S}^{2-}\)), which forms hydrogen sulfide (\(\text{H}_2\text{S}\)) that is expelled as a waste product. This energy-generating process is distinct from assimilatory sulfate reduction, which only produces sulfur compounds for building cell components.
The Destructive Power of SRB
SRB activity represents a major threat to industrial infrastructure through Microbiologically Induced Corrosion (MIC). SRB are the most common biological agent responsible for MIC, particularly affecting iron and steel structures. These bacteria adhere to metal surfaces, forming a dense, protective biofilm that creates an anaerobic microenvironment. Within this biofilm, \(\text{H}_2\text{S}\) reacts with the iron, leading to the formation of iron sulfide (FeS) corrosion products. This often promotes severe localized damage, resulting in deep pitting corrosion beneath the biofilm.
Mechanisms of corrosion are complex, involving both the corrosive chemical action of sulfide and the bacteria’s ability to directly harvest electrons from the metal surface. SRB activity is especially costly in the oil and gas industry, where it causes reservoir “souring.” Souring occurs when the bacteria reduce sulfate present in injection water, contaminating crude oil or natural gas reserves with \(\text{H}_2\text{S}\). The presence of this gas necessitates expensive processing for its removal, and the resulting corrosion can lead to catastrophic pipeline failures. SRB also cause deterioration in concrete structures, such as sewer systems, as the \(\text{H}_2\text{S}\) is oxidized by other bacteria into highly corrosive sulfuric acid.
Harnessing SRB for Environmental Cleanup
The unique metabolic output of SRB is intentionally harnessed for beneficial environmental applications, particularly in bioremediation. The hydrogen sulfide produced by these bacteria is a powerful chemical agent used to treat water contaminated with toxic heavy metals. SRB are used extensively in the treatment of acid mine drainage and industrial wastewater. When SRB are introduced into contaminated water, the sulfide they generate reacts rapidly with soluble heavy metal ions, such as cadmium, copper, lead, and zinc. This reaction forms highly insoluble metal sulfide precipitates, effectively removing the metals from the water and soil and significantly reducing their bioavailability and toxicity.
Sulfate Reduction and Human Health
Sulfate Reducing Bacteria, including the genus Desulfovibrio, are common inhabitants of the human gastrointestinal tract, especially the large intestine, where they contribute to the complex gut microbiome. While their presence is considered normal in many healthy individuals, an imbalance or overgrowth of these bacteria is linked to inflammatory conditions.
Research indicates that an elevated presence of SRB is often observed in patients with Inflammatory Bowel Disease (IBD), including ulcerative colitis. In these individuals, the excessive production of \(\text{H}_2\text{S}\) may disrupt the intestinal lining. High concentrations of \(\text{H}_2\text{S}\) can inhibit the colon cells’ ability to oxidize butyrate, their preferred energy source. This energy deprivation can lead to cell death and chronic inflammation, though the exact relationship to IBD is still under investigation.