Dissimilatory processes

A significant fraction of H2S produced by dissimilatory processes will also be oxidized by other reactions (Jprgensen, 1982). These intermediate sulfur species (e.g., elemental sulfur) from H2S oxidation may be enriched in 34S, also contributing to the overall 34S signal of H2S (Fry et al., 1988 Canfield and Thamdrup, 1994). Stable sulfur isotopes in... 

Because of the steep redox gradients in many sediments and the often rapid removal of the products of NTR by DNF and other dissimilatory processes, measurements of NTR remain problematic. NTR has been estimated in the presence and absence of specific inhibitors by the accumulation of NH4 (Henriksen et al., 1980 Caffrey et al., 2003) or by dark lT COj uptake into SOM (Dore and Karl, 1996). The inhibitors used include nitrapyrin (N-serve), methyl fluoride, dimethyl ether, acetylene, and aUylthiourea. Problems involved with these methods include nonspecificity of inhibitors and difficulty in determining the correct conversion factor for uptake to N1T4 oxidized, which has been observed to range from 8.3 to 42 mol N1T4 oxidized per mole fixed. Dore and Karl (1996) used an independent chemical assay of nitrite oxidation to verify the conversion factor used. NTR has also been estimated by the accumulation of nitrite in the presence versus absence of chlorate, an inhibitor of nitrite oxidation (Belser and Mays, 1980). NOa isotope dilution techniques, similar to those used to measure NMIN (see Section 5.2) have been used to measure NTR (Anderson et al., 1997 Risgaard-Petersen et al., 1994) however, when DNRA rates are high, the added NOa may disappear faster than it is sufficiently diluted to accurately estimate NTR. 

Fazzolari E., Mariotti A., and Germon J. C. (1990) Nitrate reduction to ammonia a dissimilatory process in Enter-obacter amnigenus. Can. J. Microbiol. 36, 779-785. 

The synthesis and breakdown of organosilicon compounds is fundamental to assimilatory uptake and utilization of siliceous materials and is involved also in certain dissimilatory processes. Studies of the kind described above provide insights into the mechanisms by which such processes operate, and they offer a basis for inferring the evolutionary history of certain modes of biosphere-silicate interaction. 

Since sulfate instead of oxygen serves as an oxidant for organic compounds like CH2O in the above back reaction, the dissimilatory process may be regarded as a form of anaerobic respiration or sulfate respiration. In the early Earth s history this reaction would have played a cmcial role as it would have been responsible for large-scale transformations of sulfide to sulfate in biological mediation of the sulfur cycle. 

Fig. 3.16 Generalized scheme of the role of bacteria in the carbon cycle and its coupling to the nitrogen and sulphur cycles (after Fenchel Jorgensen 1977 Jorgensen 1983a, b Parkes 1987 Fenchel Finlay 1995 Werne et al. 2002). For clarity, the forms of N, S and P liberated at each stage of mineralization are summarized on the left side of the diagram, where they contribute to the general mineral pools from which assimilation occurs. The nitrate reduction zone refers to the dissimilatory processes involved in denitrification.

Comparison of assimilatory with dissimilatory processes as source of sulfur... 

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. 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Figure 1. Schematic diagram of Fe redox cycling through biological processes. A large number of pathways are involved in dissimilatory Fe(III) reduction, as listed in Table 2. Processes that occur under oxic conditions are placed near the upper part of the diagram, and those that occur under anoxic conditions are placed in the lower part of the diagram. Major lithologic sources of Fe are noted for high and low oxygen environments.

Dissolution of minerals, such as may occur during dissimilatory Fe(lll) reduction, or precipitation of new biominerals during reductive or oxidative processing of Fe, represent important steps in which Fe isotope fractionation may occur. We briefly review several experiments that have investigated the isotopic effects during mineral dissolution, as well as calculated and measured isotopic fractionations among aqueous Fe species and in fluid-mineral systems. In some studies, the speciation of aqueous Fe is unknown, and we will simply denote such cases as Fe(lll)jq or Fe(ll)aq. 

Dissimilatory nitrogen reduction tends to be a sequential process in which the end products are the gases N2 and N2O. Conversion of DIN to N2 and N2O removes fixed nitrogen from the ocean. This is the major route by which nitrogen is removed from the sea as burial of fixed nitrogen in the sediments is minor (see Figure 24.2). 

Figure 4.1 shows that NOs" is the stable form of nitrogen over the usual range of pe + pH in aerobic environments. The fact that most of the N2 in the atmosphere has not been converted to NO3 therefore indicates that the biological mediation of this conversion in both directions is inefficient. Hence NO3 reduction to N2 occurs by indirect mechanisms involving intermediaries. Dissimilatory reduction of N03 (i.e. where the nitrogen oxide serves as an electron acceptor for the cell s metabolism but the N reduced is not used by the microbes involved) potentially occurs by two processes denitrification. 

Chromate is reduced to Cr(lll) in dissimilatory microbial reactions, but this process is inhibited at moderate concentrations of C(V1) and so is probably of limited value in detoxifying soils contaminated with Cr(Vl) (Lovley, 1993). However, Cr(VI) can also be reduced to Cr(lll) abiotically by oxidation of Fe(ll) Fe(III) in ferric oxide is reduced to Fe(ll) biotically ... 

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... 

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