Sulfate dissimilatory 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 

as sulfate reduction occurs under highly reducing conditions with redox potentials lower than 

FIGURE 11.8 Effect of soil redox potential on sulfate reduction or sulfide production. (From Connell and Patrick, 1968.) 

Group I—Dissimilatory sulfate reducers nonacetate oxidizers Desulfovibrio Desutfomonas Desulfotomaculum Desulfobulbus 

Group II—Dissimilatory sulfate reducers acetate oxidizers Desulfotomaculum Desulfobacterium Desulfococcus Desulfonema Desulfosarcina 

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Dissimilatory sulfate reduction (SO - - H2S) Sulfate-reducing bacteria... 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

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. 

Steuber, J. and Kroneck, P.M.H. (1998). Anaerobic dissimilatory sulfate reduction. Inorg. Chim. 

Kinetic isotope effects during microbial processes. Micro-organisms have long been known to fractionate isotopes during their sulfur metabolism, particularly during dissimilatory sulfate reduction, which produces the largest fractionations in the sulfur cycle... 

Sulfate Reduction. Dissimilatory sulfate reduction, anaerobic respiration with sulfate as the terminal electron acceptor, is performed by relatively few genera of bacteria (84). Many bacteria and algae are able to... 

Mass balance calculations clearly show that sulfate is removed from the water column by in-lake processes. Three processes are potentially important 1) diffusion of sulfate into sediments and subsequent reduction, 2) sedimentation of seston, and 3) dissimilatory sulfate reduction in the hypolimnion. 

Seston-S deposition probably is a more important process than dissimilatory reduction in lakes with low [SO42 ]. As lakewater sulfate concentrations increase, seston deposition reaches a plateau limited by the overall primary production rate and the maximum algal S content, but diffusive fluxes continue to increase in direct proportion to [SO42 ]. Thus, in highly acidic lakes (pH 3 5 [SOjt2 J > 100 peq/L), such as McCloud Lake, Florida and Lake 223, Ontario, dissimilatory sulfate reduction probably is the major sulfate sink. Nriagu and Soon (131 concluded that endproducts of dissimilatory reduction and elevated sediment S content would not be observed below S mg/L (240 / eq/L), but we see clear evidence of dissimilatory reduction in Little Rock Lake at concentrations of approximately SO /teq/L. 

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). 

Little or no fractionation accompanies the uptake of sulfate in soils by plants during ASR (60.611. Chukhrov et al. ( Q) showed that in cases where atmospheric sulfate is not subject to bacterial reduction in the soil, the value of the plant sulfur was identical to rainfall sulfur. In soils subject to dissimilatory sulfate reduction, the 6 S value of plant sulfur differed from that of local rainfall. Additionally, Chukhrov et al. (60) found that plants from oceanic islands had a sulfur content with higher values than those from continental areas, which they attribute to the relative influence of marine sulfate to these areas. 

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. 

Sulfur isotopes can effectively be used to examine important geochemical processes associated with redox changes in sedimentary environments. The speciation of sulfur is strongly affected by redox potential, pH, productivity, microbial sulfate reduction, and iron availability (Berner, 1984). More details are provided on the sulfur cycle in chapter 12. In general, during microbial dissimilatory sulfate reduction there is fractionation of sulfur... 

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 ... 

The primary process by which sulfur is fractionated in hydrologic environments is via biologically mediated sulfur transformations. The most important of these is the dissimilatory sulfate reduction (DSR) reaction... 

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... 

Dissimilatory sulfate reduction is a rare metabolic process which is carried out by a few bacterial species belonging to the genera Desulfovibrio, Desul-fotomaculum and the newly-described Desulfomonas. The bacteria, however, are widely distributed in the natural environment and they are probably responsible for most of the H2S formation on earth at temperatures below about 100°C. Their activities have been detected under environmental conditions of Eh, +350 to —500 mV pH, 4.2 to 10.4 pressure, 0.1 to 100... 

Chambers, L.A. and Trudinger, P.A., 1975. Are thiosulfate and trithionate intermediates in dissimilatory sulfate-reduction J. Bacteriol., 123 36—40. 

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 reduction coupled to oxidation of organic acid, and particularly acetate and lactate in sulfate-reducing bacteria (SRBs) this utilized in further reactions but not fixed into biomass. 

DISSIMILATORY SULFATE REDUCTION Ecological and phylogenetic diversity of sulfate reducers... 

Dissimilatory sulfate reduction is conducted by a specialized group of prokaryotes, who gain energy for their growth by catalyzing exergonic chemical reactions in which organic carbon or H2 (gas) is oxidized, while sulfate is reduced (Eqns. 9 and 10). 

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