Biotic Reduction

Interaction between abiotic pathways and reductive biotic pathways... 

Abiotic catalysis is generally less important than biotic but may be important. Examples are Mn(lll,IV) and Fe(III) reduction by microbial metabolites, and Fe(ll) oxidation which is catalysed by sorption onto soil particles. 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

The rate of the biotic reduction of Fe oxides by a strain of Corynehacterium under 02-free conditions followed the order natural ferrihydrite > synthetic goethite > hematite (Fischer (1988) (Fig. 12.29) in accordance with the sequence in reducibility by Fe-reducing bacteria isolated from a eutrophic lake sediment (Jones et al., 1983). Iron from ferrihydrite reduced by Shewandla alga was found to be isotopically lighter than that of the ferrihydrite Fe by a 5 ( Fe/ " Fe) of 1.3 %o This difference may be used to trace the distribution of microorganisms in modern and ancient earth (Beard etal. 1999). 

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. 

The need for biological mediation of most redox processes encountered in natural waters means that approaches to equilibrium depend strongly on the activities of the biota. Moreover, quite different oxidation-reduction levels may be established within biotic microenvironments than those prevalent in the over-all environment diffusion or dispersion of products from the microenvironment into the macroenvironment may give an erroneous view of redox conditions in the latter. Also, because many redox processes do not couple with one another readily, it is possible to have several different apparent oxidation-reduction levels in the same locale, depending upon the system that is being used as reference. 

Peijnenburg, W.J., Hart, M.J., den Hollander, H., de Meent, D., Verboom, H., and Wolfe, N., QSARs for predicting biotic and abiotic reductive transformation rate constants of halogenated hydrocarbons in anoxic sediment systems, Sci. Total Environ., 109/110, 283-300, 1991. 

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

In long-term evolutionary scales, humans now have the abilities to intervene rapidly in this interdependent relationship and alter the stability of the rates of metabolism of organic matter. For example, reduction of ozone in the stratosphere and associated increased UV irradiance could lead to accelerated photolytic degradation of macromolecules of DOM by both abiotic and biotic pathways to C02. In addition, the photolytic enhancement of substrates for bacterial metabolism by UV photolysis can result in accelerated rates of biogeochemical cycling of nutrients and stimulated... 

DOM can also act as an electron acceptor for biotically mediated oxidation reactions. Many active microorganisms, particularly phototrophs, produce reductants in excess of metabolic needs that must be regenerated by transfering electrons to acceptors in the environment via membrane-spanning reductases (Price and Morel, 1990). It has been discovered that some iron-reducing bacteria use humic and fulvic acids as terminal electron acceptors for their respiratory transport systems (Coates et al., 1998). 

The C02 flux at the atmosphere vegetation cover boundary is determined in many respects by the soil processes involved in organic matter transformation. To better understand the biotic and abiotic mechanisms that control C02 emission from the soil, Jassal et al. (2005) compared measured C02 fluxes in a forest with their distribution profile in the soil of a 54-year-old coniferous forest on the eastern coast of Vancouver. It was established that C02 concentration grows at all depths of the soil layer with rising temperature and humidity. This is explained by soil diffusion reduction and changes in soil ecosystem functioning. It was noted that more than 75% of C02 emitted from the soil was generated at a depth of 20 cm, and almost total C02 flux forms from the 0 cm-50 cm layer. 

Matheson LJ. Abiotic and Biotic Reductive Dehalogenation of Halogenated Methanes. Ph.D thesis, Oregon Graduate Institute, 1994. 

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