Oxidation microbially mediated

Chapnick, S.D., Moore, W.S., Nealson, K.H. 1982. Microbially Mediated Manganese Oxidation in a Freshwater Lake. Limnology and Oceanography, 27, 1004-1014. 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

The refractory compounds in the HMW DOM pool seems to be generated through abiotic reactions that act to link degradation products into macromolecules. These new chemical bonds create molecular structures that enhance the overall refractory nature of the DOM. The chemical changes lead to increased crosslinking, aromaticity, cyclization, esterification, and nitrogen depletion. The general types of chemical reactions responsible are oxidations, polymerizations, and condensations. Considerable debate exists as to whether these reactions are wholly abiotic or whether they are, at least in part, microbially mediated. 

Polymerization, or conjugation, is the process in which toxic organic molecules undergo microbially mediated transformation by oxidative coupling reactions. In this case, a contaminant or its intermediate product(s) combines with itself or other organic molecules (e.g., xenobiotic residues, naturally occurring compounds) to form larger molecular polymers that can be incorporated in subsurface humic substances. 

Nitrification-denitrification involves the conversion of NH " to NO , the oxidation of NOj" to N03 , and the reduction of NOj" to NO ". The gases N O and are used in the microbially mediated processes involved in the nitrification-denitrification phenomenon. 

Anaerobic conditions often develop in hydrocarbon-contaminated subsurface sites due to rapid aerobic biodegradation rates and limited supply of oxygen. In the absence of O, oxidized forms or natural organic materials, such as humic substances, are used by microorganisms as electron acceptors. Because many sites polluted by petroleum hydrocarbons are depleted of oxygen, alternative degradation pathways under anaerobic conditions tend to develop. Cervantes et al. (2001) tested the possibility of microbially mediated mineralization of toluene by quinones and humus as terminal electron acceptors. Anaerobic microbial oxidation of toluene to CO, coupled to humus respiration, was demonstrated by use of enriched anaerobic sediments (e.g., from the Amsterdam petroleum harbor). Natural humic acids and... 

From the study of a microbially mediated oxidation of arteether 28b, sufficient quantities of 7a-hydroxy 180 and 15-hydroxy derivatives 182 were obtained to employ them as intermediates for the preparation of fluorinated compounds. The hydroxyl groups were oxidized to the corresponding aldehyde 187, or ketone 188, with catalytic quantities of tetra- -propylammonium perruthenate (TPAP) in the presence of excess iV-methylmorpholine A -oxide. On reaction with DAST, 187 and 188 were converted into the corresponding geminal difluoro derivatives, 189 (63%) and 190 (42%). In addition to 190, a monofluoro olefin 191 was obtained in 25% yield from 188 on reaction with DAST <1995JME4120>. 

Francis (1990) has summarized the numerous possible microbially mediated reactions resulting in the mobilization or immobilization of metals and found that major interactions include oxidation-reduction processes, biosorption and immobilization by cell biomass and exudates, and mobilization by microbial metabolites. A profound issue in metal remediation is that through microbial action, metals can readily be re-mobilized, creating toxicity issues in sites where metals are not completely removed. 

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. 

The oxidation of Mn(II) by oxygen in homogeneous solution is extremely slow (on a time scale of years) (46), but it is catalyzed by the adsorption of Mn(II) on oxide surfaces (47, 48). The main oxidation pathway under natural conditions, however, is assumed to be the microbially mediated oxidation that occurs on the time scale of hours to days. The importance of microbial oxidation of Mn(II) in natural environments has been demonstrated in a number of studies (49-54). 

In the dissolved phase, few alternative abiotic oxidants are available in the natural environment. Nitrate, sulfate, and other terminal electron acceptors used by anaerobic microorganisms are thermodynamically capable of oxidizing some organic contaminants, but it appears that these reactions almost always require microbial mediation. 

Many of these reactions involving Mn and Fe have been shown to be linked with microbially mediated (e.g., chemolithotrophic) processes, in particular, bacterial reduction of Fe and Mn oxides are also capable of oxidizing sulfides to SO42-, and NH3 to NO3- under anaerobic conditions. Other important reactions involving metal oxides include the oxidation of NH3 to N2 via Mn oxides in the presence of oxygen. 

Moffett, J.W. and Ho, J., Microbially mediated incorporation of trace-elements into manganese oxides in seawater, Abstr. Pap. Am. Chem. Soc., 209, 103-Geoc, 1995. 

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

Microbial-mediated production of sulfides and further oxidation to sulfuric acid ... 

However, the time scales for adsorption and desorption reactions are much shorter than those for microbially mediated arsenic species transformations. Adsorption and desorption reactions reach equilibrium over a period of 24 hours or less (32, 33). On the other hand. Wool son ( ) estimated conversion rates of 0.067 to 0.404 % day"l for oxidative metabolism of cacodylic acid to arsenate in model aquatic systems. 

Reeburgh et al. (1993) estimated the role of microbially mediated CH4 oxidation using limited data on oxidation rates in environments representing the main CH4 budget source terms. 

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. 

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). 

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