Microbe-mediated reactions

Figure 20.8 Examples of common classes of reactions in enzyme- and microbe-mediated stereochemical synthesis...

Heterotrophic and autotrophic bacteria are important participants in the restoration industry. Both types are indigenous to almost every site. The subsurface environment includes many thousands of species of microbes, which act in harmony to support each other. Waste products from one group become nutrients for another. When free oxygen is depleted, anaerobic activity increases. Thus, it is often convenient to consider microbiological activity as a series of processes resulting from bacterially mediated oxidation-reduction reactions. 

Table 2 illustrates the effect of the Gibbs free energy on the spontaneity of a chemical/biochemical reaction and the resulting release of energy. Thus, it is useful to use AG values for any biochemical reaction mediated by microbes to determine whether energy is liberated for work, and how much energy is liberated. 

A simplified depiction of the marine nitrogen cycie iiiustrating redox and phase transitions mediated by microbes. The boxes contain the nitrogen species and its oxidation number. The arrows represent transformation reactions as foiiows (1) nitrogen fixation, (2) soiubiiization, (3) ammonification,... 

Figure 4.3 Free energy changes in redox reactions mediated by microbes, (a) Oxidation of reduced inorganic compounds linked to reduction of O2. (b) Oxidation of organic matter CH2O linked to reduction of various organic and inorganic oxidants. pH = 7 and unit oxidant and reductant activities except (Mn +) = 0.2mM and (Fe +) = ImM...

Singlet molecular oxygen, Oj, is also thought to arise, perhaps in reaction 4 or 5 (in place of dioxygen). The addition of 02 completes the cast of characters comprising molecules which may mediate the effects of O , since whenever O is formed, H2O2, OH and O2 all may exist in aqueous solutions in the presence of micromolar concentrations of ionic iron which contaminate many buffers. Armed with the capacity to form species with such a variety of affinities for electrons, the PMN is endowed with the ability to initiate a formidable array of chemical reactions, not merely between the various species themselves but also with its own constitutents and those of ingested microbes. 

Formation of the hydroxyl radical. Because the killing of certain microbes was dependent on the ability of PMNs to undergo the respiratory burst, the actual microbicidal species was sought. It had been shown that a potent oxidant was formed during the catalytic action of xanthine oxidase on xanthine an enzymic reaction which, like the PMN, produces both 07 and H2O2. This potent oxidant was proposed to be the hydroxyl radical formed by the reaction between O7 and H2O2 (reaction 4). Because such an oxidant seemed a likely candidate to mediate the microbicidal activity of PMNs, the formation of OH was assessed in PMNs. 

The PMN cells include neutrophils, eosinophils, and basophils. Neutrophils make up 60-75% of circulating WBCs and provide the first line of defense against microbes that penetrate the normal barriers of skin. They are extremely efficient phagocytes and are a source of inflammatory cytokines and lipid mediators such as PGs, LTs, and platelet-activating factor. Eosinophils are involved in allergy and provide protection against parasites. Basophils contain vasoactive amines such as histamine, serotonin, and heparin, as well as precursors for PGs and LTs. Release of these pharmacological materials by the basophils is responsible for the anaphylactic reaction. These factors also serve as chemoattractants for neutrophils and eosinophils to sites of inflammation. 

The environmentally significant and most extensively studied electron acceptors used, along with the reactions that can be mediated by microbes, are illustrated in equations 1-8. 

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