5.4.1: Sulfate reduction

Microorganisms can reduce sulfate to sulfide \((\text{S(-II)})\) for assimilatory or dissimilatory purposes. During assimilation, the sulfide is incorporated into sulfur-containing amino acids: methionine, cysteine, homocysteine, and taurine (Brosnan and Brosnan, 2006). During dissimilatory sulfate reduction, the sulfide is released into the environment, where it can speciates among \(\text{H}_{2}\text{S}\), \(\text{HS}^{-}\), and \(\text{S}^{2-}\) and other aqueous species as determined by solution pH and composition. Assimilatory sulfate reduction occurs in anoxic and oxic environments. However, dissimilatory sulfate reduction primarily occurs in anoxic environments that lack nitrate, much like ferric iron reduction.

Dissimilatory sulfate reduction is a major driver of the global carbon cycle. In marine sediments, for example, it is estimated to account for more than 50% of the organic matter oxidation (Jorgensen, 1982). Electron donors used by sulfate-reducing microorganisms include dihydrogen and simple organics produced by fermenting and syntrophic microbes, such as ethanol, formate, lactate, pyruvate, malate, succinate, acetate, benzoate, and phenol (Muyzer and Stams, 2008). Some example reactions follow:

\[\begin{align} & 4 \ \text{H}_{2} + \text{SO}_{4}^{2-} + \text{H}^{+} \longleftrightarrow 4 \ \text{H}_{2} \text{O} + \text{HS}^{-} \\ & 2 \ \text{HCOO}^{-} + \text{SO}_{4}^{2-} + \text{H}^{+} \longleftrightarrow 2 \ \text{HCO}_{3}^{-} + \text{HS}^{-} \\ & \text{CH}_{3} \text{COO}^{-} + \text{SO}_{4}^{2-} \longleftrightarrow 2 \ \text{HCO}_{3}^{-} + \text{HS}^{-} \\ & 2 \ \text{CH}_{3} \text{CHOHCOO}^{-} + \text{SO}_{4}^{2-} \longleftrightarrow 2 \ \text{CH}_{3} \text{COO}^{-} + 2 \ \text{HCO}_{3}^{-} + \text{HS}^{-} + \text{H}^{+} \\ & 4 \ \text{CH}_{3} \text{CH}_{2} \text{COO}^{-} + 3 \ \text{SO}_{4}^{2-} \longleftrightarrow 4 \text{CH}_{3} \text{COO}^{-} + 4 \ \text{HCO}_{3}^{-} + 3 \ \text{HS}^{-} + \text{H}^{+} \\ & \text{CH}_{4} + \text{SO}_{4}^{2-} \longleftrightarrow \text{HCO}_{3}^{-} + \text{HS}^{-} + \text{H}_{2} \text{O} \end{align}\]

Some sulfate reducers degrade organics completely to carbon dioxide/bicarbonate whereas others degrade organic compounds incompletely to acetate (reactions \((\PageIndex{4})\) and \((\PageIndex{5})\)).

The sulfide produced by dissimilatory sulfate reduction can accumulate in solutions and gas phases. It may also form sulfide minerals. Sulfide can precipitate with numerous transition metals and metalloids. Precipitation with iron is common in the terrestrial subsurface, where iron is commonly present and iron reduction may also be occurring (Berner, 1970; Kirk et al., 2016; Postma and Jakobsen, 1996). Typically, the first iron-sulfide to form in low-temperature systems is mackinawite \((\text{FeS})\), which can transform into greigite \(\left(\text{Fe}_{3} \text{S}_{4}\right)\) and then pyrite \(\left(\text{FeS}_{2}\right)\) over time (Benning et al., 2000). More information about sulfide biomineralization is available in Section 12.1.4.

Numerous organisms can use sulfate reduction as a source of energy. Many of the groups that are capable of sulfate reduction include ‘desulf’ in their name. For example, Desulfovibrio is a well-known genus of gram-negative bacteria that can use sulfate reduction in dissimilatory metabolism (Devereux et al., 1990). Most microorganisms capable of sulfate reduction belong to five lineages of Bacteria and two lineages of Archaea (Muyzer and Stams, 2008). Bacterial groups include genera within Deltaproteobacteria, Clostridia, Nitrospirae, Thermodesulfobacteria, and Thermodesulfobiaceae (Muyzer and Stams, 2008). Archaeal groups include the genus Archaeoglobus in Euryarchaeota and the genera Thermocladium and Caldirvirga in Crenarchaeota (Muyzer and Stams, 2008). However, more recent research by Anantharaman et al (2018) indicates that, in addition to these well-recognized groups, many other lineages contain species capable of dissimilatory sulfate reduction.

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