Anaerobic sulfide oxidation

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. 

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. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

In general, sulfide is oxidized via sulfite to sulfate, while thiosulfate can either be oxidized only to tetrathionate or may be split into sulfide and sulfite. Both compounds are then further oxidized to sulfate. During anaerobic sulfide or thiosulfate oxidation, elemental sulfur appears as sulfur globules inside or outside the cells. Sulfur metabolism in Anoxyphotobacteria has two main functions ... 

Anaerobic Sulfide Oxidation. An alternative explanation is that sulfide is oxidized anaerobically in association with phototrophic reduction of C02 to organic carbon (46, 49). This hypothesis is supported by the discovery of considerable quantities of bacteriochlorophyll pigments within and below the suboxic zone (50). The integrated quantities of these pigments appear to exceed that of the chlorophyll a in the overlying oxygenated portion of the eu-photic zone. The light levels at the depth of the bacteriochlorophyll maximum, however, are very low (<<0.1% where Ia is the incident radiation) and the carbon assimilation rates necessary to verify the hypothesis are difficult to calculate or measure. 

The use of Thiobacillus species has been studied quite extensively. Sublette and Sylvester especially focused on the use of Thiobacillus denitrificans [50-52] for aerobic or anaerobic oxidation of sulfide to sulfate. In the anaerobic oxidation NOs was used as an oxidant instead of oxygen (confirm Table 2). Buisman used a mixed culture of Thiobacilli for the aerobic oxidation of sulfide to elemental sulfur and studied technological applications [53-55]. Visser showed the dominant organism in this mixed culture to be a new organism named Thiobacillus sp. W5 [6]. 

Fry B, Gest H, Hayes JM (1984) Isotope effects associated with the anaerobic oxidation of sulfide by the purple photosynthetic bacterium, Chromatium vinosum. FEMS Microbiol Lett 22 283-287 Fry B, Gest H, Hayes JM (1985) Isotope effects associated with the anaerobic oxidation of sulfite and thiosulfate by the photosynthetic bacterium, Chromatium vinosum. FEMS Microbiol Lett 11 111-Til Fry B, Gest H, Hayes JM (1988a) " S/ S fractionation in sulfur cycles catalyzed by anaerobic bacteria. Appl Environ Microbiol 54 250-256... 

Fig. 14.22 Schematic illustration of gas hydrate deposits and biogeochemical reactions in near-surface sediments on southern Hydrate Ridge. High gradients in pore water sulfate and methane are typical of methane hydrate-rich environment close to sulfate-rich seawater. At the sulfate-methane interface (also named sulphate-methane transition in earlier chapters of the book) a microbial consortium of methanothrophic archaea and sulfate-reducing bacteria (Boetius et al. 2000) perform anaerobic oxidation of methane (AOM) leading to carbonate precipitation. AOM rates influence hydrogen sulfide fluxes and gradients, which are reflected on the seafloor by the distribution of vent communities around active gas seeps and gas hydrate exposures (Sahling et al. 2002).

Sulfide is oxidized in the rhizosphere and the resulting sulfate is taken up by the plants. Alternatively, sulfide can be taken by the plant and oxidized in the intercellular airspaces of roots (Carlson and Forrest, 1982). Sulfide precipitation with metals as metal sulfides (insoluble precipitates under anaerobic conditions) decreases pore water concentrations. However, metal sulfides formed in... 

Other potential forms of copper include sulfide, hydroxide, oxide, and carbonate forms. Under reducing conditions, copper sulfide is formed. Sulfides are stable under anaerobic conditions. Sulfides can be oxidized with electron acceptors of higher reduction potentials by sulfide-oxidizing bacteria to form sulfate, releasing the metal ion back into the solution. Under alkaline conditions, the hydroxide, oxide, or carbonate can be formed so that dissolved copper ions can be removed from the solution and precipitated in this form. 

Fry B., Qest H. and Hayes J. M. (1984) Isotope effeet associated with the anaerobic oxidation of sulfide by the purple photosynthetic bacterium Chromatium vinosum. Microbiol. Lett. 22, 283-287. 

The transformation from anaerobic sites to aerobic sites is a drastic one, with high CP current demand and extremely high corrosion rates. Iron (II) sulfides are oxidized to iron (III) oxides and sulfur species. In turn, sulfur is ultimately oxidized to sulfate. 

Nelson, M. B., Davis, J. A., Benjamin, M. M. and Leckie, J. O. (1977). The Role of Iron Sulfides in Controlling Trace Heavy Metals in Anaerobic Sediments Oxidative Dissolution of Ferrous Monosulfides and the Behavior of Associated Trace Metals." Air Force Weapons Laboratory, Technical Report 425. 

It is also often taken for granted that many of the Earth s subsystems are exposed to free oxygen (O2), leading to a range of one-way reactions of reduced materials (such as organic carbon or metal sulfides) to an oxidized form. As pointed out many times in earlier chapters, the oxidation-reduction status of the planet is the consequence of the dynamic interactions of biogeochemical cycles. As is the case with the acid-base balances, there is considerable sensitivity to perturbations of "redox" conditions, sometimes dramatically as in the case of bodies of water that suddenly become anaerobic because of eutrophication. Another extreme... 

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