Biologically mediated

TABLE 25-40 Minor and Micro Nutrients Required for Biologically Mediated Reactors... 

Nitrification. Nitrification refers to biologically mediated oxidation of reduced nitrogen to higher oxidation states ... 

Acid-base, hydrolysis, hydration, neutralization, oxidation-reduction, polymerization, thermal degradation Adsorption-desorption, precipitation-dissolution, immiscible-phase separation, biodegradation, complexation Acid-base, neutralization, oxidation-reduction (most inorganic and some biologically mediated), adsorption-desorption, precipitation-dissolution, complexation Hydrolysis, oxidation-reduction (biodegradation of anthropogenic inorganics), immiscible-phase separation... 

Table 20.5 also indicates whether a process is biotic (mediated or initiated by organisms in the environment), abiotic (not involving biological mediation), or both. Biotic processes are limited to environmental conditions that favor growth of mediating organisms. Abiotic processes occur under a wide range of conditions. Adsorption, precipitation, complexation, and neutralization are abiotic all other processes in Table 20.5 may be either. 

Some claims have, however, been made that the methylation of lead was not biologically mediated. In 1980, Craig126 studied the methylation of trimethyllead acetate in a lake sediment (Lake Minetonka, Minn., U.S.A.), and concluded that it is not necessary to invoke a biological route to the methylation and that the results in this case can be explained entirely by a disproportionation process126. Craig and Wood also proposed an abiotic methylation route of lead127. 

Johnson, D.L. and R.M. Burke. 1978. Biological mediation of chemical speciation. II. Arsenate reduction during marine phytoplankton blooms. Chemosphere 8 645-648. 

Psychological stress may influence the immune system by activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic-adrenal-medullary axis (SAM). The well-described innervation of primary and secondary lymphoid tissues by the autonomic nervous system also has been implicated in stress-related modulation of the immune response. These pathways operate by producing biological mediators that interact with and affect cellular components of the immune system.13... 

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

In the case of iron and manganese, most of these metals are removed from the hydrothermal fluids and converted to particulate form close to their point of entry. Some of these removals are in the form of sulfides, which fc>rm as the fluids emerge into the deep sea. The rest occurs as the fluids mix with cold, oxic, alkaline seawater, which promotes the oxidation of reduced metals. Thus, Fe (aq) and Mn (aq) are transformed into insoluble iron and manganese oxides, forming colloids and particles, the latter of which eventually settle onto the sediments. As described in the next chapter, at least some of these oxidation reactions are biologically mediated. Some of... 

This has been whimsically described as the Great Particle Conspiracy. An important set of these processes involves the incorporation of trace metals into biogenic hard and soft parts. Another involves the adsorption of metals onto the surfaces of particles. Although some metals can theoretically be removed by abiotic precipitation into mineral phases, continuing research confirms that most of the trace metal removal is biologically mediated. 

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. 

Soil In soil and water, chemical and biological mediated reactions can transform ethylene dibromide in the presence of hydrogen sulfides to ethyl mercaptan and other sulfur-containing compounds (Alexander, 1981). 

Soil In both soils and water, chemical and biological mediated reactions can transform thiram to compounds containing the mercaptan group (Alexander, 1981). Decomposes in soils to carbon disulfide and dimethylamine (Sisler and Cox, 1954 Kaars Sijpesteijn et al., 1977). When a spodosol (pH 3.8) pretreated with thiram was incubated for 24 d at 30 °C and relative humidity of 60 to 90%, dimethylamine formed as the major product. Minor degradative products included nitrite ions (nitration reduction) and dimethylnitrosamine (Ayanaba et al., 1973). 

The calculated hydrolysis half-life at 25 °C and pH 7 is 262 d (Ellington et al., 1988). The hydrolysis half-lives of methomyl in a sterile 1% ethanoEwater solution at 25 °C and pH values of 4.5, 6.0, 7.0, and 8.0 were 56, 54, 38, and 20 wk, respectively (Chapman and Cole, 1982). In both soils and water, chemical- and biological-mediated reactions transformed methomyl into two compounds — a nitrile and a mercaptan (Alexander, 1981). 

In many cases, the subsurface is contaminated by a mixture of toxic chemicals with components having different physical and chemical properties. Therefore, a contaminant undergoes transformations controlled by the properties of each substance, the characteristics of the subsurface, and the ambient conditions. Physicochemically mediated and biologically mediated degradation are the main processes involved in such transformations. 

---