Microbially Mediated Processes

The interpretation of previous attempts at measuring the impact of metals on microbially mediated processes has been hindered by the use of a wide range of experimental conditions and measurements. Already, a shift from studies based on total metal concentration to those based on bioavailable metal concentrations has occurred. The next step will entail accurately predicting and measuring metal speciation patterns in order to identify microbial responses to metal speciation. Only then will it be possible to develop more effective methods to quantify and mitigate deleterious effects of metals on the myriad processes that microbes mediate in the environment. 

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. 

Other microbially mediated processes can control the release of P from sediments. For example, in fresh to brackish waters Fe-reducing aerobic bacteria convert amorphous Fe(III) into Fe(II)-releasing Fe-associated PO/j3-. 

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

BNF is a microbially mediated process that occurs in several types of bacteria and blue-green algae. This process uses the enzyme nitrogenase in an anaerobic environment to convert N2 to NH3. The microbes can be free-living or in a symbiotic association with the roots of higher plants. Legumes are the best-known example of this type of relationship (Schlesinger, 1997 Mackenzie, 1998) ... 

Given the low concentrations of oxygen in the subsurface environment, along with its poor solubility and poor mass-transfer efficiency, anaerobic treatment is an attractive alternative, provided that it is biologically possible. Studies by other researchers and in our own laboratory indicate that nitrate is an electron acceptor that can satisfactorily substitute for oxygen in microbially mediated processes and allow the degradation of a number of components of BTX (40-43). 

The nature of the soil environment can also affect metal speciation and microbial distribution and therefore both individual and community responses. Recent research has demonstrated that bacterial conmiunities are not distributed randomly in soil, and spatial patterns have been identified in the distribution of bacteria and bacterial function at scales from several millimeters to several meters (Foster, 1988 Robertson et al., 1997 Nunan et al., 2002, 2003). Evidence for bacterial patchiness at scales below 1 mm is now available (Grundmann et al., 2001 Nunan et al., 2002, 2003). It has been suggested that microscale spatial patterns may have a regulatory effect on bacterial activity as a result of the diffusional limitations imposed on nutrient uptake and on metabolite excretion and dispersion. These may have rate-limiting or stimulatory effects on several microbially mediated processes (Darrah et al., 1987 Grundmaiui et al., 2001). 

The Effect of Xenobiotics on Microbial-Mediated Processes References... 

Biological. Microbial mediated processes can transform organic contaminants into benign compounds or oxidize or reduce the toxic metals into a less toxic and mobile form. 

Microbially Mediated Processes in the Soil Phosphorus Cycle... 

The microbial-mediated processes of methylation and demethylation are influenced by factors such as redox potential, pH, sulfate concentration, and microbial activity. In a study conducted by DeLaune et al. (2004), methylation of added Hg in sediment was greater under reduced conditions... 

Figure 1 Schematic of the multi-step process of organic degradation in anaerobic systems, indicating the influence of H2. Boxes indieate major chemical pools, arrows with text indicate microbially-mediated process. Superscripts on arrow text indicate the mode of control by H2 (F)—facultative interspecies H2 transfer (O)—obligate interspecies H2 transfer (B)—H2 can be a by-product, shifting the product yield (U)— the reaction becomes thermodynamically unfavorable when H2 concentrations are too low (R)—the process is demonstrated or hypothesized to be reversible, based on H2 concentrations. Note ( ) important inorganic oxidants may include NO, Mn , Fe , and various oxidized sulfur speeies (e.g., S04 ) Xred corresponds to the redueed form (e.g., H2S). Adpated from Ref. [19].

Precipitation is an important factor in leachate attenuation processes within landfills and is especially important in attenuating heavy metals. Most heavy metals are characteristically more soluble at low pH values and therefore may be expected to be more mobile during the early stages of waste stabilisation. However, as the waste decomposition proceeds, the pH rises causing the solubility of most heavy metals to decrease. Also under conditions of lower redox potential, the precipitation of heavy metals as insoluble sulphides and carbonates may occur (Rees, 1982), thus reducing the concentration of heavy metals in the leachate. According to Pohland (1991) inorganic heavy metals codisposed with MSW are attenuated by the microbially-mediated processes of reduction, precipitation. 

Microbially mediated processes other than oxidation may also occur with water pollutants. In anoxic regions of water where O2 is absent, reduction may take place. One of the most common biochemical processes mediated by microorganisms and operating on pollutants in water is hydrolysis in which a molecule is split in two with the addition of a water molecule. The products of hydrolysis are often more amenable to further biodegradation, such as oxidation, than is the parent compound. 

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