The electron donors and acceptors for oxidation-reduction reactions can be either organic or inorganic compounds. When oxygen is present, aerobic respiration is the dominant respiratory process. Oxygen serves as the electron acceptor and provides the greatest energy yield (Figure 2). Oxygen's high affinity for electrons (tendency to be reduced from O to O is so strong that it can oxidize reduced forms of all the other elements (e.g., Fe2+, S2~, and CH4) shown in Figure 2. Only when oxygen concentrations drop below ^30 p.M are other electron acceptors (e.g., nitrate) potentially capable of competing with oxygen for available electrons. When no nitrogen oxides are present, the threshold for alternative electron acceptors may even lower to about 10 p,M of oxygen. Thus, the distributions of the terminal electron-accepting reactions are dictated by geochemical gradients, by the relative abundance of the various electron acceptors, and by the amount and availability of the electron-donor supply.
Information in Table 1 (see below) extends and reinforces several key rules of microbial physiology that allow an ecologist to predict where and why a given biogeo-chemical process will occur in a given ecological setting. Reduced forms of C, N, and S are good electron donors that are thermodynamically unstable in the presence of electron acceptors farther up the redox scale shown in Figure 2. For instance, if molecular oxygen is comingled with reduced C, N, or S in an aquatic habitat, oxygen will serve as the dominant electron acceptor for chemolitho-trophic microorganisms (ones that derive ATP from oxidation of inorganic compounds) in their respiratory chains. ATP will be generated, and the oxidized waste products will be CO2, NO 3, and SO42 , respectively.
The fully oxidized forms of C, N, and S (metabolic waste products in aerobic habitats produced by hetero-trophic and chemolithotropic microorganisms) can be transported in adjacent oxygen-free environments that are inhabited by microbial populations whose activities are limited by the availability of final electron acceptors. In this new context, CO2, NO3~, and SO42~ will be viewed by resident anaerobic microorganisms as physiologically valuable final electron acceptors. The order in which these electron acceptors will be used is predicted by the thermodynamic hierarchy in Figure 2.
It is the constantly shifting dynamics between oxidized and reduced forms of biosphere compounds as they change positions between aerobic and anaerobic habitats that drive microbially mediated biogeochemical reactions. Thus, a combination of the information in Table 1 and Figure 2 assists in predicting the occurrence of ecologically significant microbial processes.
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