Biotic pathways

Microbes in wetlands degrade organic componnds for the following reasons (see Chapter 5)  

In general, microbial transformations of organic pollutants can be classified as (1) biodegradation, 

Acclimation. Time required for microorganisms to adapt to a new compound or lag period 

Cometabolism. It is transformed by metabolic reactions but does not serve as an energy source for the microorganism 

FIGURE 13.3 Microbial degradation of the toxic organics governed by the amount in the bioavailable pool. 

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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... 

Denitrification can be supported by electron donors other than organic carbon such as Fe(II), Mn(ll) and H2S, and they can proceed by both abiotic and biotic pathways. Abiotic reduction of no " to N2 coupled to Fe(II) oxidation may occur at low rates in the pH range 2-7 (Postma, 1990) ... 

This ionic mercury (Hgll) adheres to aerosols and thus has a short (days to weeks) residence time in the atmosphere rainfall delivers it to the local soils and rivers. Ionic mercury is readily methylated (eqn. 5.24) by both abiotic and biotic pathways. However, most scientists now agree that methylation by anaerobic sulphate reducing bacteria (SRB) is most important. 

Fairchild, W.L., D.C.G. Muir, R.S. Curri, and A.L. Yarechewski. 1992. Emerging insects as a biotic pathway for movement of 2,3,7,8-tetrachlorodibenzofuran from lake sediments. Environ.Toxicol. Chem. 11 867-872. 

TEDs include hydrogen, natural organic carbon, humic material, and nonhalo-genated fuel compounds. For reductive dehalogenation to occur, there must be both a TEA and a TED. For the biotic pathway, organisms must derive energy from the reaction. 

As shown in Figure 4, 1,1,1 TCA is transformed by parallel abiotic and biotic pathways. Dehydrohalogenation yields 1,1-DCE [45]. As shown in pathway 1, 1,1-DCE is susceptible to reductive dehalogenation to vinyl chloride. Abiotic or biotic hydrolysis of TCA is illustrated by pathway 3 of Figure 4 [11]. Reductive dechlorination of TCA to 1,1-DCA [27,31,34,69] and chloroethane (CA) is illustrated in pathway 2 [31,34, 69,70]. CA apparently undergoes hydrolysis to ethanol (a nonvolatile product), followed by oxidation of the ethanol to CO [31]. In many systems, the above pathways operate simultaneously. For example, Clostridium sp. strain TCAIIB, transformed TCA to 1,1-DCA ( 30-40%), acetic acid (7%), and unidentified products (45%) [71]. 

Figure 4. Abiotic and biotic pathways for transformation of 1,1,1-TCA (a=abiotic, b=biotic, p=pure culture, M=mixed culture, mg=methanogens).

In general, synthetic pyrethroids are degraded by both abiotic and biotic pathways. Even though residues of synthetic pyrethroids have frequently been detected in soils, sediments, natural waters and agricultural products (Wang et al. 2011). The pyrethroid insecticides have low mammalian toxicity and persistence in... 

On the other hand, Se speciation, mobihty and bioavailability are highly affected by the presence of miCToorganisms in the environment. Their main influence on the bioavailabihty is through the control of Se oxidation state, which directly relates to the solubility of different Se compounds. Different biotic pathways have been identified nowadays by which a Se oxyanion can be reduced to Se or Se(-II) (Femandez-Martmez and Charlet 2009). 

Interaction between abiotic pathways and reductive biotic pathways... 

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