Dissimilatory sulfate, sulfide

Dissimilatory sulfate reducers such as Desul-fovibrio derive their energy from the anaerobic oxidation of organic compounds such as lactic acid and acetic acid. Sulfate is reduced and large amounts of hydrogen sulfide are generated in this process. The black sediments of aquatic habitats that smell of sulfide are due to the activities of these bacteria. The black coloration is caused by the formation of metal sulfides, primarily iron sulfide. These bacteria are especially important in marine habitats because of the high concentrations of sulfate that exists there. 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

More direct biological channels also seem promising as sources. Land plants release H2S, but the process has not been considered for marine algae ( ). Intermittent deep sulfide maxima could be connected with anoxic microenvironments recently located in marine snow. These organic particulates accumulate in the pycnocline and offer potential sites for contrary redox reactions such as dissimilatory sulfate reduction (34). 

Thus, the appearance of free sulfate does not require the advent of free oxygen in the Archean environment. Certainly sufficient free sulfate had appeared in the hydrosphere prior to development of the pathway of dissimilatory sulfate reduction. Schidlowski (1979) argues that the small fractionations observed between sulfide and sulfate 834s values of pre-2.7 billion year rocks (Figure 10.11) are consistent with the hypothesis that the oxidation of sulfide to sulfate by photosynthetic bacteria preceded the bacterial pathway of dissimilatory sulfate reduction and may have been responsible for early free dissolved sulfate concentrations in the hydrosphere. 

When obligate anaerobic bacteria carry out dissimilatory sulfate reduction, they are referred to as sulfate reducers or sulfidogens. The traditional sulfate-reducing genera are Desulfovibrio and Desulfotomaculum. Sulfate reduction results in the production of hydrogen sulfide ... 

In contrast to the specialized dissimilatory sulfate reducers, many organisms (humans as well) are capable of assimilatory sulfate reduction. This process, which requires chemical energy in the form of ATP and a series of transfer reactions, can occur anaerobically and aerobically. It produces low concentrations of hydrogen sulfide that are immediately incorporated into organic compounds. Many microbes, plants, and animals have such a metabolic ability. 

The biochemical pathway of both assimilatory and dissimilatory sulfate reduction is illustrated in Figure 1. The details of the dissimilatory reduction pathway are useful for understanding the origin of bacterial stable isotopic fractionations. The overall pathways require the transfer of eight electrons, and proceed through a number of intermediate steps. The reduction of sulfate requires activation by ATP (adenosine triphosphate) to form adenosine phosphosulfate (APS). The enzyme ATP sulfurylase catalyzes this reaction. In dissimilatory reduction, the sulfate moiety of APS is reduced to sulfite (SO3 ) by the enzyme APS reductase, whereas in assimilatory reduction APS is further phosphorylated to phospho-adenosine phosphosulfate (PAPS) before reduction to the oxidation state of sulfite and sulfide. Although the reduction reactions occur in the cell s cytoplasm (i.e., the sulfate enters the cell), the electron transport chain for dissimilatory sulfate reduction occurs in proteins that are peiiplasmic (within the bacterial cell wall). The enzyme hydrogenase... 

Sulfide methylation reactions couple dissimilatory sulfate reduction to DMS production and determine the rates of DMS emission in freshwater wetlands. This process involves acetogenic bacteria, some of which degrade aromatic acids to acetone. In soils, freshwater, and marine ecosystems a wide diversity of other anaerobic and aerobic bacteria can contribute to sulfur gas production. In addition, diverse aerobes (e.g. methylotrophs and sulfate oxidizers) and anaerobes (e.g. methanogenes) consume S gas, thereby regulating fluxes in the atmosphere-biosphere system. 

Dissimilatory sulfate (S04 H2S) Dimethyl sulfide production (CHaljS Metal cycles Iron and manganese oxidation and reduction... 

Similar to assimilatory sulfate reduction, dissimilatory sulfate reduction to sulfide involves eight-electron transfer from reduced compounds (organic carbon sources) to sulfate. Dissimilatory sulfate reduction plays a major role in the organic matter oxidation and nutrient mineralization in wetland environment. The key requirements for inorganic sulfur reduction in a wetland ecosystem are... 

Gaseous sulfur compounds produced in wetlands are either intermediate metabolites or end products of biological processes. Hydrogen sulfide (H2S) produced by dissimilatory sulfate reduction in anaerobic environment was originally thought to be the primary gaseous sulfur source emitted to the atmosphere (Rodhe and Isaken, 1980). 

Clostridia cannot carry out dissimilatory sulfate reduction. This will not only separate them from SRB (and especially Desulfotomaculum sp., which are also spore formers) but also explain why metal sulfide corrosion products are not found where these bacteria exist. 

Sulfate respiration, dissimilatory sulfate reduction a form of respiration in which the sulfate ion replaces oxygen as the terminal electron acceptor (see Sulfate reliction). The sulfate ion must first be activated (see Sulfate activation). S,r. is an anaerolric process in which sulfate is reduced to hydrogen sulfide, which is excreted. Ecologically, S.r. contributes to desulfurication, and is important for the sulfur cycle of the biosphere. 

In agreement with the statements of Trueper (1) one can say that principally different dissimilatory sulfur metabolic pathways exist in Anoxyphotobacteria for the oxidation of sulfite to sulfate (via APS or directly), the utilization of thiosulfate (splitting or formation of tetrathionate), and the oxidation of sulfide or elemental sulfur (by a "reverse" siroheme sulfite reductase or other mechanisms). 

Sulfur isotopic measurements can shed light on the origin of sulfur in coal. The 34S/32S ratio depends on the source of sulfur and the geologic processes involved during coal formation. For example, isotopic compositions are different for the two principal sources of sulfur in coal 1) the sulfur preserved from the precursor plant material, and 2) the sulfur derived from the bacterial reduction of dissolved sulfate in ambient waters. Plant assimilation of sulfur results in a slight depletion of 34S (4-4.5%c) relative to the 834S in the dissolved sulfate source (102.103). In contrast, the dissimilatory bacterial reduction of sulfate results in a large isotopic fractionation sulfide sulfur can be depleted as much as 60%o in the heavy isotope (89.104-106). 

Dissimilatory BSR under pure-culture laboratory conditions can produce sulfide depleted in by 2-46%o relative to the parent sulfate (Chambers et al, 1975 Canfield, 2001 Detmers et al, 2001). Although this range is generally accepted, controls on the magnitude of this fractionation are subjects of recent debate. For example, contrary to a long-held assumption, the isotopic offset between parent sulfate and HS-produced during BSR (A " S) may not vary with a simple inverse relationship to the rate of sulfate... 

Of more immediate geochemical relevance is the dissimilatory reduction of sulfate to hydrogen sulfide which is a major topic for discussion in Chapter 6.2. Sometimes called sulfate respiration the process involves oxidation of organic matter (or hydrogen) with the transfer of electrons to sulfate instead of oxygen as in the majority of respiratory systems. The process is accompanied by a net release of free enei which is utilized by the org m-ism for growth (e.g. eqn (1), Wake et al., 1977). 

SRB are essentially ubiquitous in aqueous environments that contain organic carbon and sulfate (e.g., subsurface aquifers and lake sediments). Moreover, analysis of a key gene associated with sulfate reduction (dissimilatory sulfite reductase) indicates that microbial sulfate reduction is an ancient trait, suggesting that organisms may have contributed to sulfide mineral formation throughout much of Earth history (Wagner et al. 1998). SRB are tolerant to environmental extremes of heat (some are hyperthermophiles) and salinity (some are halophiles). 

Since sulfate instead of oxygen serves as an oxidant for organic compounds like CH2O in the above back reaction, the dissimilatory process may be regarded as a form of anaerobic respiration or sulfate respiration. In the early Earth s history this reaction would have played a cmcial role as it would have been responsible for large-scale transformations of sulfide to sulfate in biological mediation of the sulfur cycle. 

The iron is especially important. In freshwater ecosystems, fluxes of hydrogen sulfide are also relatively small owing to the lack of sufficient sulfate as a substrate for dissimilatory reduction and to the relatively greater incorporation of the available sulfur into biomass. However, the release of hydrogen sulfide is significant from wetlands. In addition, H2S emission from plant canopy occurs when S plant uptake is in excess of biosynthetic demands. The latter process may account for as much as 40% of total natural S emission. 

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