Sulfate dissimilatory

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... 

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

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. 

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. 

Newman DK, EK Kennedy, JD Coates, D Ahmann, DJ Ellis, DR Lovley, EMM Morel (1997) Dissimilatory arsenate and sulfate reduction in Desulfotomaculum auripigmentum sp. nov. Arch Microbiol 168 ... 

Widdel F, G-W Kohring, F Mayer (1983) Studies on dissimilatory sulfate-reducing bacteria that decompose fatty Acids. Arch Microbiol 134 286-294. 

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

Rabus R., T. A. Hansen and F. Widdel, 2006, Dissimilatory sulfate- and sulfur-reducing prokaryotes. In The Prokaryotes An Evolving Electronic Resource for the Microbiological Community, http //www.springerlink.-com/content/nl084686101028pj/fulltext.pdf, Springer, New York. 

Several proteins were reported to function as enzymes for the dissimilatory reduction of metals and nonessential elements. As Usted in Table 16.4, the most frequently reported proteins involved in metal reduction are the cytochromes from sulfate-reducing bacteria. The focus on these cytochromes supports the initial papers by Lovley and colleagues in which they reported that reduced cytochrome Cs from Desulfovibrio vulgaris Hildenborough reduces uranyl salts (Lovley et al. 1993a) and chromate (Lovley and PhUhps 1994). 

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... 

Dissimilatory sulfate- or sulfur-reducing bacteria. Desulfovibrio... 

Removal of sulfate from the water column can occur by either assimi-latory or dissimilatory reduction. Assimilatory reduction occurs in the water column, whereas uptake by plankton results in the formation of organic S. 

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... 

Oremland, R.S., Dowdle, P.R., Hoeft, S. et al. (2000) Bacterial dissimilatory reduction of arsenate and sulfate in meromictic Mono Lake, California. Geochimica et Cosmochimica Acta, 64(18), 3073-84. 

The existence of sulfate-reducing bacteria as old as 2.8 Ga and possibly 3.2 Ga has been established by means of 34S/32S ratios from sedimentary pyrites found in Precambrian rocks. The values show dissimilatory fractionation similar to those produced by extant sulfate-reducing bacteria. Thus it appears that this type of mineralization process, may have evolved in the early Precambrian (Schidlowski et al., 1983) 87). 

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. 

Dissimilatory Reduction in Surficial Sediments. Porewater profiles from a number of sites throughout Little Rock Lake show that sulfate is always depleted below the sediment-water interface (Figure 3). Sulfate depletion in porewaters occurs not onlyin the soft gyttja but also in sandy, littoral sites with organic contents < 10%. The observed depletion of sulfate and the occurrence of H2S indicate that the sediments are anoxic immediately below the sediment-water interface and that sulfate reduction occurs in surficial sediments. 

Seston Sulfur. Much of the sulfur that is immobilized by assimilatoiy uptake or dissimilatory reduction is oxidized and reenters the water column as sulfate. We have approached the question of seston-S recycling by comparing carbon/sulfur (C/S) ratios in seston and sediment and by following the fate of 35S in labeled algae added to laboratory sediment-water microcosms. 

Relative Importance of Seston Deposition and Dissimilatory Reduction In-I-alce Sulfate Sinks... 

In little Rock Lake, seston deposition appears to be a more important sulfate sink than does dissimilatory reduction. Several previous studies (2.41 have concluded that dissimilatoiy reduction is the major mechanism for sulfate retention, and Cook et al. (2) concluded that seston deposition was a minor sulfate sink in experimentally acidified Lake 223. The C/S ratio calculations discussed above snow that approximately 29% of the total S in recent sediments at SB-5 is excess-S derived from dissimilatory reduction and the remaining 71% originated from seston deposition. 

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. 

In-lake processes remove approximately half of the sulfate inputs from the water column of Little Rock Lake. Two processes, seston deposition and dissimilatory reduction, are responsible for sulfate retention. For the preacidified lake, seston deposition probably is the dominant sink, accounting for 70% of net retention. Preliminary data and theoretical considerations suggest that the diffusive flux of sulfate to sediments will increase during experimental acidification, and we believe that dissimilatoty reduction is the dominant sulfate sink in lakes with elevated sulfate concentrations. 

After the formation of APS, the enzymes ADP-sulfurylase or a dissimilatory ATP-sulfurylase now catalyze the exchange of sulfate with inorganic phosphate and inorganic pyrophosphate, respectively, on the APS molecule. 

If phototrophic bacteria possess a dissimilatory ATP-sulfurylase, they convert APS with pyrophosphate directly to ATP and sulfate, without the help of an additional enzyme. Such an enzyme is necessary, if the organisms like Chlorobium vibrioforme f. thiosulfatophilum (Table IV) contain only the ADP-sulfurylase, because this enzyme liberates only ADP and sulfate from APS in the presence of inorganic phosphate. In this case, the organisms gain one ATP molecule from 2 molecules of ADP. This reaction is catalyzed by adenylate kinase which converts 2 ADP into 1 ATP and 1 AMP (38). 

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). 

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. 

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