Pathways of Sulfide Oxidation

The examples given above illustrate that sulfide produced by dissimilatoiy sulfate reduction, in particular in organic-rich layers or within the zone of AOM, can produce a profound diagenetic alteration of the sediment up to thousands or hun- 

Vast amounts of sulfide, corresponding to 7 megaton of HjS daily, are generated in marine sediments as the product of bacterial sulfate reduction. A small fraction of this sulfide is trapped within the sediment, mainly by reaction and precipitation with iron to form pyrite, or by the sulfidization of organic matter, and it thereby becomes buried in 

The short-term (years to thousand years) burial of sulfide in the form of pyrite in ocean margin sediments is more effieient and generally accounts for 5-20% of the entire sulfide production (Jorgensen et al. 1990 Lin and Morse 1991 Canfield and Teske 1996). This burial provides a sink in the dynamic cycling of snlfur that is limited by the availability of reactive iron to bind the large exeess of sulfide. Raiswell and Canfield 

In oxie marine sediments, a brown layer rich in iron and manganese oxides generally separates and HjS and thereby prevents a direct sulfide oxidation by oxygen (e.g. Thamdrup et al. 1994a). In this snboxie zone, neither nor H S is present 

Most sulfide oxidation in marine sediments is anoxic (i.e. takes place in the absence of oxygen) 

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Oxidation of thiosulfate also produces small amounts of trithionate (SjOl ), tetrathionate (S4OI"), and pentathionate (SsOl ) (Goldhaber and Kaplan 1974). Summarized in Fig. 12.17 are possible oxidation and disproportionation pathways of reduced sulfur species leading toward sulfate that may be mediated by Thiobacilli. (Disproportionation pathways involve no electron transfer see also O Brien and Birkner 1977 Morse et al. 1987.) More recently, Jorgensen (1990) used radioactive to unravel the complex pathways of sulfide oxidation in sediments. He showed that thiosulfate disproportionation to sulfate and sulfide species... 

Furukawa Y, Inubushi K (2002) Feasible suppression technique of methane emission from paddy soil by iron amendment. Nutr Cycl Agroecosyst 64 193-201 Fuseler K, Krekeler D, Sydow U, Cypionka H (1996) A common pathway of sulfide oxidation by sulfate-reducing bacteria. FEMS Microbiol Lett 144 129-134 Galushko AS, Schink B (2000) Oxidation of acetate through reactions of the citric acid cycle by Geobacter sulfurreducens in pure culture and in syntrophic coculture. Arch Microbiol 174 314-321... 

The pathways of sulfide oxidation in nature are varied, and in fact poorly known, but include (1) the inorganic oxidation of sulfide to sulfate, elemental sulfur, and other intermediate sulfur compounds, (2) the nonphototrophic, biologically-mediated oxidation of sulfide (and elemental sulfur), (3) the phototrophic oxidation of reduced sulfur compounds by a variety of different anoxygenic phototrophic bacteria, and (4) the disproportionation of sulfur compounds with intermediate oxidation states. The first three of these are true sulfide-oxidation pathways requiring either the introduction of an electron acceptor (e g. O2 and NO3 ), or, in the case of phototrophic pathways, the fixation of organic carbon from CO2 to balance the sulfide oxidation. The disproportionation of sulfur intermediate compounds requires no external electron donor or electron acceptor and balances the production of sulfate by the production of sulfide. This process will be taken up in detail in a later section. A cartoon depicting some of the possible steps in the oxidative sulfur cycle is shown in Figure 6. 

In marine coastal sediments typically 90% of the sulfide produced during snlfate reduction is reoxidized (Canfield and Teske 1996). The pathways of snlfide oxidation are poorly known bnt inclnde oxidation to sulfate, elemental snlfm and other intermediate componnds. Systematic studies of sulfm isotope fractionations dnring sulfide oxidation are still needed, the few available data snggest that biologically mediated oxidation of snlfide to elemental snlfur and sulfate lead to only minimal isotope fractionation. 

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. 

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. 

B) This figure com s the fractionations imparted by sulfate reduction (csr) with with depletion of into pyrite (o " Ssuifate - 8 " Spyrite) as a function of sulfate reduction rate (SRR). As sulfate reduction rates increase, the difference between the isotopic composition of pyrite and the fractionation imposed by sulfate-reducing organisms becomes smaller. This relationship probably reflects a higher proportion of sulfide oxidation through disproportionation pathways at low sulfate reduction rates, and more direct sulfide oxidation to sulfate at high rates of sulfate reduction. Replotted after Habicht and Canfield (2001). 

The major metabolic pathway for hydrogen sulfide in the body is the oxidation of sulfide to sulfate, which is excreted in the urine (Beauchamp et al. 1984). The major oxidation product of sulfide is thiosulfate, which is then converted to sulfate the primary location for these reactions is in the liver (Bartholomew et al. 1980). 

The major metabolic pathway of hydrogen sulfide is the oxidation of the sulfide to sulfate in the liver (Beauchamp et al. 1984). Methylation also serves as a detoxification route. Hydrogen sulfide is excreted primarily as sulfate (either as free sulfate or as thiosulfate) in the urine. 

Scheme 13 Different pathways of oxidation of alkyl aryl sulfides.

The established mechanism of sulfide photo-oxidation in solution invokes the novel formation of two intermediates a persulfoxide. A, and a hydroperoxy sulfonium ylide, B, (Fig. 13A) [25], In this mechanism the sulfide substrate intercepts the second intermediate, k o, and does not competitively inhibit the predominant sulfone forming pathway, kso2. As a consequence, the sulfone/sulfoxide ratio is independent of sulfide concentration. In contrast, the results in the zeolite are inconsistent with this mechanism but are consistent with the trapping of a single intermediate with both sulfide and adventitious sulfoxide (Fig. 13B). 

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. 

In combination with H2O2 (salen)Mn(III) complexes 173a, b, i-n have also been employed by Jacobsen and coworkers as catalysts for the asymmetric oxidation of sulfides to sulfoxides, without a need for additives. From the structurally and electronically different Mn-salen catalysts screened, 173i turned out to be the most active and selective one (equation 58) . While dialkyl sulfides underwenf uncafalyzed oxidation with H2O2, aryl alkyl sulfides were oxidized only slowly compared wifh fhe cafalyzed pathway. Using... 

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... 

For a detailed discussion of the various pathways and stoichiometries, see reference 14. However, the question remains open as to which redox process provides the electrons for reaction 6. When buried in sediments, ferric iron may be used by microorganisms as an electron acceptor (15-17). On the other hand, it also comes into contact with reductants like H2S (18, 19). Although microbial reduction of ferric oxides using sulfide as the reductant has not yet been documented (17), various studies support a purely chemical interaction between these two compounds (20-22). 

As this short discussion shows, the kinetics of formation of the single parameters (Fe2+ and H2S) may control the extent and the pathway of pyrite formation. Oxidation of sulfide by elemental sulfur to form poly sulfides (pathway 1) should predominate at the oxygen-sulfide interface of very productive... 

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