Soils microbial degradation

Table 8.11 Reduction in compound availability for soil microbial degradation as a result of ageing. Reprinted with permission from Alexander M (2000) Aging, bioavaUability and overestimation of risk from environmental pollutants. Environ Sci Technol 34 4259 265. Copyright 2005 American Chemical Society...

Biodegradation in soil, microbial degradation occurs rapidly, t,/2 = 2.1 months for aerobic and t,/2 = 10 d for anaerobic metabolism (Tomlin 1994). 

Soil. Microbial degradation occurs, the principal metabolite being 3,6-dichlorosalicylic acid. DT50, <14 days... 

Animals. In rats, there is rapid and almost quantitative unchanged elimination in urine Plants. Not metabolized in plants Soil. Microbial degradation occurs. Major product is C02... 

Soil. Microbial degradation involves hydroxylation, decarboxylation, cleavage of the acid side-chain, and ring opening. Half-life in soil <7 days. Rapid degradation in the soil prevents significant downward movement... 

Plants. Three major metabolites have been identified, isopropyl water-soluble conjugates of glucose or other plant components Soil. Microbial degradation leads to the production of 3-chloroaniline by an enzymic hydrolysis reaction, with liberation of C02. DTJ0 in soil c. 65 days (15°C), 30 days (29°C)... 

The microbial metabolism of pesticides has often been subdivided into 2 distinct classes. The first of these is termed simply "catabolism". This process often results in the mineralization of some portion of an organic compound via enzymatic pathways to simple products of universal currency (C02, NH3). In some cases, one portion of the molecule may be mineralized and another portion may accumulate in soil. This is true for the soil microbial degradation of carbofuran (42,43). Therefore, catabolism should not be equated with mineralization or complete destruction of a pesticide. It should be pointed out, however, that mineralization of a pesticide in soil Is nearly always a consequence of microbial activity (44). The key to understanding catabolism is that it is primarily a process driven by the microbial quest for energy. Therefore, catabolism has come to be equated with utilization of a pesticide as an energy source and thus a growth substrate (40,41). Catabolism is most commonly linked to the conversion of pesticides into carbon skeleton... 

Hydrolyzed by strong acids and alkalis, and in aqueous solutions in sunlight. Thermally stable up to 200°C. In soil, microbial degradation involves hydrolysis to ethylmercaptan, carbon dioxide and di-isobutylamine. ti/2 (soil) 1.5 to 10 weeks Unstable in highly alkaline media... 

Unstable in alkaline media. Stable in acid and neutral media. Decomposes above 150°C. Most important metabolite is CO2, formed by microbiological degradation of the phenol compounds. ty2 (river water environmental conditions) 13.5 days (pH 7.5), and (pond water 26 to 30°C) 2.3 days (pH 7.8 to 8.5), and (deionized water 27 2°C) 36 days (pH 7), and (deionized water 27 2°C) 1.2 h (pH 10). ty2 (soil) 30 to 60 days Hydrolyzed slowly in acid and alkaline media. Stable to UV light. Decomposes above 150°C. In soil, microbial degradation yields 3-chloroaniline via an enzymatic hydrolysis reaction with release of CO2. ti/2 (distilled water)... 

Relatively stable to hydrolysis by acids and alkalis (pH 5 to 9) at 40°C. Stable for at least 2 years at room temperature and at least 2 months at 120°C. Unstable to light. In soil, microbial degradation involves hydrolysis to ethyl mercaptan, CO2 and dialkylamine. ty2 (aerobic soil pH 5 to 6) 8 to 25 days, (flooded soil)... 

Soil Microbial degradation of endrin in soil formed several ketones and aldehydes of which A to-endrin was the only metabolite identified (Kearney and Kaufinan, 1976). In eight Indian rice soils, endrin degraded rapidly to low concentrations after 55 days. Degradation was highest in a pokkali soil and lowest in a sandy soil (Gowda and Sethu-nathan, 1976). 

A recent suggestion has been to use plants to stimulate the microbial degradation of the hydrocarbon (hydrocarbon phytoremediation). This has yet to receive clear experimental verification, but the plants are proposed to help deUver air to the soil microbes, and to stimulate microbial growth in the rhizosphere by the release of nutrients from the roots. The esthetic appeal of an active phytoremediation project can be very great. 

In soils and sediments, microbial degradation and hydrolysis are important degradation processes. 

Sanchez MA, M Vasquez, B Gonzalez (2004) A previously unexposed forest soil microbial community degrades high levels of the pollutant 2,4,6-trichlorophenol. Appl Environ Microbiol 70 7567-7570. 

Mihelcic JR, RG Luthy (1988) Microbial degradation of acenaphthene and naphthalene under denitrification conditions in soil-water systems. Appl Environ Microbiol 54 1188-1198. 

Oremland RS, DJ Lonrergan, CW Culbertson, DR Lovley (1996) Microbial degradation of hydrochloroflnoro-carbons (CHCljF and CHCljCFj) in soils and sediments. Appl Environ Microbiol 62 1818-1821. 

DeRito CM, GM Pnmphrey, EL Madsen (2005) Use of field-based stable isotope probing to identify adapted popnlations and track carbon flow throngh a phenol-degrading soil microbial conunnnity. Appl Environ Microbiol 71 7858-7865. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

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