Sulfide oxidation, microbial

Fig. 22.7. Thermodynamic driving forces for various anaerobic (top) and aerobic (bottom) microbial metabolisms during mixing of a subsea hydrothermal fluid with seawater, as a function of temperature. Since the driving force is the negative free energy change of reaction, metabolisms with positive drives are favored thermodynamically those with negative drives cannot proceed. The drive for sulfide oxidation is the mirror image of that for hydrogentrophic sulfate reduction, since in the calculation 02(aq) and H2(aq) are in equilibrium.

A few examples of chemoautolithotrophic processes have been mentioned in this chapter, namely anaerobic methane oxidation coupled to sulfate reduction and the ones listed in Table 12.2 involving manganese, iron, and nitrogen. Another example are the microbial metabolisms that rely on sulfide oxidation. Since sulfide oxidation is a source of electrons, it is a likely source of energy that could be driving denitrification, and manganese and iron reduction where organic matter is scarce. 

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

Measured rates of microbial oxidation of sulfide in lakes range from 0 to over 100,000 mmol/m2 per year (Table IV). These rates, which are comparable to measured rates of sulfate reduction (Table I), suggest that microbial oxidation of sulfide is capable of supplying sulfate at rates needed to sustain sulfate reduction. The majority of measurements are for photosynthetic bacteria in the water column. Symbiotic sulfate reduction and sulfide oxidation are known to occur and lead to dynamic cycling of S within anaerobic water... 

All measured profiles of sulfate reduction in sediments indicate that much sulfide production and, by inference, oxidation occurs in permanently anaerobic sediments (78, 73, 90,101). The two most likely electron acceptors for anaerobic sulfide oxidation are manganese and iron oxides. Burdige and Nealson (151) demonstrated rapid chemical as well as microbially catalyzed oxidation of sulfide by crystalline manganese oxide (8-Mn02), although elemental S was the inferred end product. Aller and Rude (146) documented microbial oxidation of sulfide to sulfate accompanied by reductive dissolution... 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

In reality, the oxidation of pyrite and other Fe(II) sulfides typically involves several intermediate reactions, which may be enhanced by microbial activity or various chemical species, such as bicarbonate (HCO3-) (Welch et al., 2000 Evangelou, Seta and Holt, 1998). The exact mechanisms of each intermediate reaction are often very complex and poorly understood (Rimstidt and Vaughan, 2003). Mostly likely, sulfide oxidizes in pyrite before iron. Fe(II) is then released into solution as shown by the following reaction involving oxygen and water (Gleisner and Herbert, 2002, 139-140) ... 

Sulfide oxidation, another microbially mediated process, also results in the production of acidity ... 

Sulfide oxidation and sulfate reduction are reactions that are usually microbially mediated in Earth-surface environments. It is likely that this is also true of subglacial environments. If this is the case, there is a requirement for nutrients, such as nitrogen and phosphorus. Snow- and ice melt provide limited quantities of nitrogen, mainly as NOJ and NH4, and it is likely that phosphorus is derived from comminuted rock debris. However, there may well be a rock source of NH4 from mica and feldspar dissolution (Holloway et al., 1998), and some may also be obtained from the oxidation of organic matter. The concentration of NOJ in glacial runoff is usually <30 p,eq L, and often between 0 p,eq and 2 peq L. On occasion, NO concentrations are below the detection limit, which may be evidence for microbial uptake in subglacial environments. 

Van den Ende F. P. and Van Gemerden H. (1993) Sulfide oxidation under oxygen limitation by a Thiobacillus thioparus isolated from a marine microbial mat. FEMS Microbiol. Ecol. 13, 69-78. 

Sulfide oxidation occurs both microbially and abiotically. In coastal sediments that are not subject to water column anoxia, burrowing animals mix the upper few centimeters of sediment (i.e., bioturbation), homogenizing the sedimentary solid-phase and pore water constituents (e.g., Gerino et al., 1998). In doing so, underlying anoxic (i.e., sulfidic) sediments are mixed with overlying oxic sediments thereby minimizing accumulation of sulfide via dilution and abiotic and microbially... 

The pre-1991 research involving microbial oxidation of 29 sulfide minerals of iron, copper, arsenic, antimony, gallium, zinc, lead, nickel, and mercury was compiled by Nordstrom and Southam (1997). The importance of microbially mediated sulfide oxidation has been recognized for several decades (Nordstrom and Southam, 1997). Bacteria catalyze the oxidative dissolution of sulfide minerals, increasing the production of acidity in mine wastes. In the absence of bacteria, the rate of sulfide oxidation stabilizes as the pH decreases below 3.5 (Singer and Stumm, 1970). 

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

The concept of microbially-mediated mineral sulfide oxidation as a particular case of electrochemical corrosion phenomena warrants considerable extension, and the techniques developed by Corrans, Vanselow and their collaborators could be usefully applied to a variety of other sulfide mineral systems. 

Allman, 1968 Davy, 1975). Transport by water movement of end-products of sulfide oxidation at one site to other locations can influence the development of microbial populations and may facilitate chemical interactions leading to the modification of as yet unoxidized components. For example, a metal interchange reaction of the kind indicated below can lead to a stabilization of the concentration of solubilized copper and a conversion of chalcopyrite to the more readily oxidizable covellite (E. Peters, 1976, personal communication) (eqn (17)) ... 

Bach W., and Edwards, K.J., 2003. Iron and sulfide oxidation within the basaltic ocean crust Implications for chemolithoautotrophic microbial biomass production. Geochimica et Cosmochimica Acta, 67 3871-3887. 

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