Sulfide oxidation pathways

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. 

Aerobic prokaryotes utilising (later) copper released from its sulfide by oxygen. Increasing use of oxygen and its environmental products was seen in oxidative pathways and synthesis (around two or so billion years ago). 

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. 

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. 

Figure 2. Aerobic catabolism of methylated sulfides (adapted from Kelly, 1988). 1) DMSO reductase (Hyphomicrobium sp.) 2) DMDS reductase (Thiobacillus sp. 3) trimethylsulfonium-tetrahydrofolate methyltransferase (Pseudomonas sp.) 4) DMS monooxygenase 5) methanethiol oxidase 6) sulfide oxidizing enzymes 7) catalase 8) formaldehyde dehydrogenase 9) formate dehydrogenase 10) Calvin cycle for CO2 assimilation (Thiobacillus sp.) 11) serine pathway for carbon assimilation (Hyphomicrobium sp.).

The initial six-electron oxidation of sulfide to sulfite is catalysed by a soluble, dis-similatory sulfite reductase that contains siroheme and at least one iron-sulfur center as prosthetic groups [83-85]. While similar enzymes in plants and in non-pho-totrophic bacteria usually function to reduce sulfite to sulfide in assimilatory pathways, the enzyme in photolithoautotrophically grown C. vinosum appears to function in the reverse direction, with the electrons from sulfide oxidation being delivered to an as yet unidentified acceptor. Evidence is also available for the... 

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... 

Polysulfides can be generated via two major pathways. First, polysulfides can be formed by the oxidation of dissolved sulfide and sulfide minerals(l, 2). Second, they can be formed by the reaction of elemental sulfur with bisulfide ion(35). Polysulfide levels can be predicted for the second process as described in previous studies(, 36-38). Equilibrium calculations as described in a previous study(22) were performed for the polysulfide levels in these samples. The ratio of S(0) experimental to S(0) calculated for all samples from Great Sippewissett were 0.145 (4-8 cm), 0.137 (8-13 cm) and 0.128 (23-28 cm). Because these ratios are less than 1.0, these results indicate that polysulfides should form primarily from the reaction of bisulfide ion with elemental sulfur(5) rather than sulfide oxidation. This data set is... 

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... 

One of the most important anaerobic pathways of decomposition in marine sediments is sulfate reduction (Berner, 1964 Goldhaber and Kaplan, 1974 J0rgenson, 1977). Proof that sulfate reduction is taking place in surface sediments at each station of this study comes from the abundance of fixed sulfur in the solid phase and the presence in the pore waters of dissolved sulfide (Figs. 31-34 Appendix B Goldhaber et ai, 1977). Because sulfate reduction presumably dominates the anaerobic decomposition reactions over most of the sampled sediment regions, reaction 5 of Table IV will be assumed as the major model reaction to aid in the interpretation of pore-water and solid-phase property distributions. Additional decomposition reactions involving sulfide oxidation, specific interaction with Fe and Mn oxides, and fermentation (Presley and Kaplan, 1968) occur, but will not be emphasized here. 

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). 

Organic matter is oxidized in the suboxic zone through iron or manganese reduction. This may be a direct oxidation by metal reducing bacteria or an indirect oxidation via sulfate reduction and sulfide oxidation. Does it matter for the end products which pathway dominates ... 

The (peroxo)di-iron(III) species (38b) and (38b ) " effect enantioselective sulfoxidation of aryl sulfides. In the catalytic reaction with FI2O2 as a terminal oxidant, saturation kinetics were observed with respect to both [FI2O2] and [sulfide]. The peroxo species (38b) + (38b ) generated in situ at 0°C decays in the presence of sulfide, following a saturation kinetics with respect to [sulfide]. For p-bromophenyl methyl sulfide, the ee approaches ca. 40%, providing compelling evidence for a metal-based oxidation pathway involving a (peroxo)di-iron(III)-sulfide binary complex embedded within chiral environment. 

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