Sulfate reduction isotope effects

Habicht KS, L Sailing, B Thamdrup, DE Canfield (2005) Effect of low sulfate concentrations on lactate oxidation and isotope fractionation during sulfate reduction by Archaeoglobus fulgidus strain Z. Appl Environ Microbiol 71 3110-3111. 

Microbial reduction of nitrate to N2, known as denitrification, is similar. It is kinetically inhibited in the absence of bacteria and is known to induce a kinetic isotope effect (Blackmer and Bremner 1977 Kohl and Shearer 1978 Mariotti et al. 1981 Bryan et al. 1983 Htibner 1986 Mariotti et al. 1988). W N shifts ranging from 6.5%o to 20%o have been observed experimentally. As with sulfate, microbial fractionations appear to depend on the metabolic states of the microbes. 

Startup effects. Startup effects must also be considered in the interpretation of laboratory experiments. For example, during sulfate reduction, the first small amormt of sulfur to pass through the chain of reaction steps would be subject to the kinetic isotope effects of all of the reaction steps. This is because it takes some time for the isotopic compositions of the pools of intermediates to become enriched in heavier isotopes as described above for the steady-state case. Accordingly, the first HjS produced would be more strongly enriched in the lighter isotopes than that produced after a steady state has been approached. This principle was modeled by Rashid and Krouse (1985) to interpret kinetic isotope effects occurring during abiotic reduction of Se(IV) to Se(0) (see below). Startup effects may be particularly relevant in laboratory experiments where Se or Cr concentrations are very small, as is the case in some of the studies reviewed below. 

Bruchert V, Knoblauch C, Jorgensen BB (2001) Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments. Geochim Cosmochim Acta 65 763-776 Bryan BA, Shearer G, Skeeters JL, Kohl DH (1983) Variable expression of the nitrogen isotope effect associated with denitrification of nitrate. J Biol Chem 258 8613-8617 Canfield DE (2001) Biogeochemistry of sulfur isotopes. Rev Mineral Geochem 43 607-636 Chau YK, Riley JP (1965) The determination of selenium in sea water, silicates, and marine organisms. Anal Chim Acta 33 36-49... 

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... 

Kitchen NE, Valley JW (1995) Carbon isotope thermometry in marbles of the Adirondack Mountains, New York. J metamorphic Geol 13 577-594 Kiyosu Y, Krouse HR (1990) The role of organic acid in the abiogenic reduction of sulfate and the sulfur isotope effect. Geochem J 24 21-27... 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

The oxidation of methane was very slow under the experimental conditions employed The slowest rates are those with anhydrite as oxidant. Because the ratio of the rate constants, a, is dependent upon the oxidant, it is difficult to estimate the carbon isotope selectivity during sulfate reduction at temperatures relevant to TSR in sour gas occurrences. However, the effects are substantial with the cupric oxide-manganese dioxide and hematite-anhydrite trends in Figure 2 giving extrapolated a-values of about 1.02 and 1.04 respectively at 200°C. 

Sulfur isotopes can effectively be used to examine important geochemical processes associated with redox changes in sedimentary environments. The speciation of sulfur is strongly affected by redox potential, pH, productivity, microbial sulfate reduction, and iron availability (Berner, 1984). More details are provided on the sulfur cycle in chapter 12. In general, during microbial dissimilatory sulfate reduction there is fractionation of sulfur... 

The isotope ratios of the sulfur isotopes are also affected by kinetic and equilibrium isotope effects. Kinetic isotope effects are marked in the reduction of sulfates to hydrogen sulfide by bacteria (enrichment of the lighter isotopes in H2S). The equilibrium isotope effect in the reaction... 

Studies of are equally few. They have been used to show that sulfide is the main source of S04 in metamorphic and sedimentary catchments. There may be some kinetic effects on isotope fractionation if only the surfaces of the sulfides are being oxidized, and the S04 may become enriched in if sulfate reduction occurs (Bottrell et al., submitted). 

Details of sulfur isotope geochemistry are presented elsewhere in this volume (see Chapter 7.10) and are only highlighted here as related to paleo-environmental interpretations of finegrained siliciclastic sequences. Formation of sedimentary pyrite initiates with bacterial sulfate reduction (BSR) under conditions of anoxia within the water column or sediment pore fluids. The kinetic isotope effect associated with bacterial sulfate reduction results in hydrogen sulfide (and ultimately pyrite) that is depleted in relative to the ratios of residual sulfate (Goldhaber... 

Metastable Iron Sulfides Organic Sulfur Elemental Sulfur MECHANISM OF PYRITE FORMATION. 4.1 Evidence from Experimental Studies. 4.2 Isotope Effects during Experimental Pyrite Formation. 4.3 Origin of Morphological Variations in Pyrite SULFUR DIAGENETIC PROCESSES IN MARINE SEDIMENTS. 5.1 Depth Distribution of Diagenetic Sulfur Products. 5.2 Rates of Sulfate Reduction... 

Since first-order kinetics with respect to sulfate ion is indicated under some conditions for bacterial sulfate reduction, it is useful to consider the isotopic behaviour of a simple one-step first-order conversion (Fig. 6.2.4a). The term kinetic isotope effect describes the competing reactions (1) and (2),... 

There is considerable evidence that, in nature, bacterial sulfate reduction plays an important role in the formation of some deposits of elemental sulfur. Free sulfur is not, however, produced by sulfate-reducers per se and its formation depends, therefore, on chemical or biological oxidation of sulfide. Microorganisms capable of effecting the latter reaction are discussed in Chapters 6.1 and 6.3 while isotopic selectivities associated with this conversion are summarised in Table 6.4.2 (see p. 406). As discussed in Chapters 6.1 and 6.3 colourless sulfide-oxidising bacteria, e.g. Beggiatoa, and Thio-bacillus, inhabit aerobic zones of ponds, etc. while in the underlying anaerobic zones, where light can penetrate, photosynthetic oxidisers, such as Chro-matium and Chlorobium, are active. 

Since transport and a considerable time span were involved, the isotopic composition of the sulfide is consistent with a summation of the kinetic isotope effects in both reductions. The initial sulfates, intermediate sulfates and product-sulfide ions had 6 S values typically near +20, 0 to 13, and —25 to —40%o, respectively. itborite spring had an unusually low 5 S value of —62%o for dissolved sulfide. 

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