Sulfate reduction, pathways

Direct effects of HS may be reflected in changes in photosynthesis and ATP formation in treated plants. In the case of photosynthesis, our information is still fragmentary and not very recent. Ferretti et al. (1991) showed that HS, applied to the culture medium, increased the activities of the enzymes involved in the photosynthetic sulfate reduction pathway, whereas Merlo et al. (1991) observed in maize leaves a decrease in starch content accompanied by an increase in soluble sugars. These positive biological effects appeared to be mediated by changes in the activity... 

Howes, B.L., Dacey, J.W.H., and King, G.M. (1984) Carbon flow through oxygen and sulfate reduction pathways in salt marsh sediments. Limnol. Oceanogr. 29, 1037-1051. 

Finally, acetaldehyde can become bound to SO2, derived from the sulfate reduction pathway or added by winemakers as an antioxidant and antimicrobial compound prior to fermentation (refer to Sect. 8D.4.5). Prefermentation additions of SO2 increase the concentration of the acetaldehyde-hydroxysulfonate adduct and... 

Linderholm, A. L., Olineka, T. L., Hong, Y., Bisson, L. F. (2006) Allele diversity among genes of the sulfate reduction pathway in wne stains of Saccharomyces cerevisiae. American Journal of Etiology and Viticulture, 57, 431-440... 

The role of methionine and cysteine in juices has received some attention as possible controlling factors in the sulfate reduction pathway. There have been some... 

Isotope fractionation during sulfate reduction by the hyperthermophilic Archaeoglobus fulgidus varied with the concentration of sulfate, and it was suggested that different pathways were operative at concentrations >0.6 or <0.3 mM (Habicht et al. 2005). 

This lack of a concentration dependence contrasts with the sulfur isotope literature, which suggests that sulfur isotope fractionation induced by sulfate reduction decreases as the sulfate concentration decreases below 0.2 mmol/L (Canfield 2001 Habicht et al. 2002). This difference may reflect differences between S(VI) and Se(VI) reduction pathways or possible adaptations of bacteria to low Se concentrations, but at present no clear explanation has emerged. 

However, in contrast to microbiological experiments and near-surface studies, modelling of sulfate reduction in pore water profiles with in the ODP program has demonstrated that natural populations are able to fractionate S-isotopes by up to more than 70%c (Wortmann et al. 2001 Rudnicki et al. 2001). Brunner et al. (2005) suggested that S isotope fractionations of around -70%c might occur under hyper-sulfidic, substrate limited, but nonlimited supply of sulfate, conditions without the need of alternate pathways involving the oxidative sulfur cycle. 

Plants and bacteria produce the reduced sulfur required for the synthesis of cysteine (and methionine, described later) from environmental sulfates the pathway is shown on the right side of Figure 22-13. Sulfate is activated in two steps to produce 3-phosphoadeno-sine 5 -phosphosulfate (PAPS), which undergoes an eight-electron reduction to sulfide. The sulfide is then used in formation of cysteine from serine in a two-step pathway. Mammals synthesize cysteine from two amino acids methionine furnishes the sulfur atom and serine furnishes the carbon skeleton. Methionine is first converted to 5-adenosylmethionine (see Fig. 18-18), which can lose its methyl group to any of a number of acceptors to form A-adenosylhomocysteine (adoHcy). This demethylated product is hydrolyzed to free homocys-... 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Oxidation of sulfide will affect rates of sulfate reduction only if sulfate is the end product of such oxidation. Many compounds with oxidation states intermediate between sulfide and sulfate may be formed instead. Although many details of the oxidation pathways remain to be clarified, evidence suggests that sulfate is formed. Oxidation of sulfide by phototrophic microorganisms results in production of elemental sulfur, sulfate, or polythionates (e.g., 171). Members of each of the three families of phototrophic sulfur-oxidizing bacteria are capable of carrying the oxidation all the way to sulfate elemental sulfur and polythionates are intermediates that are stored until lower concentrations of sulfide are encountered (131, 171). Colorless sulfur... 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

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. 

The second pathway by which sulfate is reduced is the dissimilatory pathway in which sulfate is the terminal electron acceptor and leads to the formation of large quantities of H2S. During the dissimilatory reduction of sulfate, APS is formed as in Eq. (6). Then APS is reduced directly to sulfite and AMP by the enzyme APS-reductase. Table XVIII shows the data of Peck 373) on the pathway of sulfate reduction in various microorganisms. 

Pathway of Sulfate Reduction in Various Types of Microorganisms ... 

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... 

Jprgensen B. B. and BakE. (1991) Pathways and microbiology of thiosulfate transformations and sulfate reduction in a marine sediment (Kattegat, Denmark). Appl. Environ. Microbiol. 57, 847—856. 

The most important metabolic reaction is the assimilation of sulfur into organic forms which ultimately require the reduction of oxidized sulfur to the oxidation level of sulfide. This reduction is effected by the majority of microorganisms (bacteria, algae, fungi) and plants and, because of its abundance, sulfate is the dominant precursor of reduced sulfur. Pathways of assimilatory sulfate reduction are discussed briefly in Chapter 6.2 and depicted in Fig. 6.2.1 (p. 317). 

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