Dissimilatory metabolism

Trudinger, P.A., 1969. Assimilatory and dissimilatory metabolism of inorganic sulfur compounds by microorganisms. Adv. Microb. Physiol., 3 111—158. 

Predicting which links of biogeochemical processes driven by microbes, predominate under given natural conditions requires understanding of the energetics of dissimilatory metabolism. There are two aspects of the problem to discuss ... 

Besides dioxygen, nitrogen oxides can serve as electron acceptors in reactions catalyzed by copper enzymes. The copper-containing nitrite reductase (NIR) from denitrifying bacteria such as Achromobaaer, Pseudomonas, or Rhodobacter is part of the dissimilatory metabolic pathway of these bacteria. The enzyme catalyzes the one-electron reduction of NO2 to NO and water according to Equations (14) and (15). 

Nitrous oxide reductase (N2OR) catalyzes the last step of the dissimilatory metabolic pathway of denitrifying bacteria (e.g.. Pseudomonas stutzeri), for a recent review see Eady et al This is a two-electron reduction from N2O to N2 and water according to Equation (16)... 

Depending on the bacteria and soil conditions which can be described appropriately as an extremely heterogeneous system, these transformations may be assimilatory or dissimilatory metabolic functions. Based on the recent field surveys and laboratory studies, the bacterial-environmental interactions, with reference to the cycles of sulfur and other elements, in corrosion on buried pipes are shown as Figure 1. This Figure demonstrates that microbiologically influenced corrosion (MIC) results from the activities of a microbial community. 

Lovley DR, EJP Phillips (1988) Novel mode of microbial energy metabolism organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl Environ Microbiol 54 1472-1480. 

Lovley, D. R and E. J. P. Phillips (1988), "Novel Mode of Microbial Energy Metabolism Organic Carbon Oxidation Coupled to Dissimilatory Reduction of Iron or Manganese", Applied and Environ. Microbiology 54/6, 1472-1480. 

Lovley DR, Phillips EJP (1988) Novel mode of microbial energy metabolism organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. App Environ Microbio 54 1472-1480 Lovley DR, Stolz JF, Nord Jr GL, Phillips EJP (1987) Anaerobic production of magnetite by a dissimilatoiy iron-reducing microorganism. Nature 330 252-254... 

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. 

Kinetic isotope effects during microbial processes. Micro-organisms have long been known to fractionate isotopes during their sulfur metabolism, particularly during dissimilatory sulfate reduction, which produces the largest fractionations in the sulfur cycle... 

This requires a biomass which can be metabolized. The process usually involves enzymatic transfer of electrons by micro-organisms from the decomposing biomass (represented in the above equation as CH2O) to the Fe " in Fe " oxides. As seen from eq.16.3, reduction consumes protons and is, therefore, favoured, the lower the pH (see also Chap. 12). It usually takes place when all pores are filled with water (see reviews by Fischer, 1988 and Van Breemen, 1988). Biotic reduction of Fe oxides is now recognized as an important process in the oxidation (metabolism) of organic pollutants in soils by dissimilatory, iron-reducing bacteria. 

Enzymatic Steps and Dissimilatory Sulfur Metabolism by Whole Cells of Anoxyphotobacteria... 

The following presentation will give a general overview of our current knowledge of the in vivo and in vitro dissimilatory sulfur metabolism in Anoxyphotobacteria. 

Table I. Enzymes of Dissimilatory Sulfur Metabolism of Rhodospirillaceae (data taken from 9.14.23.341...

In agreement with the statements of Trueper (1) one can say that principally different dissimilatory sulfur metabolic pathways exist in Anoxyphotobacteria for the oxidation of sulfite to sulfate (via APS or directly), the utilization of thiosulfate (splitting or formation of tetrathionate), and the oxidation of sulfide or elemental sulfur (by a "reverse" siroheme sulfite reductase or other mechanisms). 

In contrast to the specialized dissimilatory sulfate reducers, many organisms (humans as well) are capable of assimilatory sulfate reduction. This process, which requires chemical energy in the form of ATP and a series of transfer reactions, can occur anaerobically and aerobically. It produces low concentrations of hydrogen sulfide that are immediately incorporated into organic compounds. Many microbes, plants, and animals have such a metabolic ability. 

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