Microorganisms, atrazine-degrading

Taken together, these lines of evidence support the idea that evolution of metabolic pathways occurred after the introduction of atrazine (Shapir et al., 2007). Moreover, they may also indicate that certain use patterns of atrazine could potentially increase the ability of soil microorganisms to degrade the herbicide rapidly (Entry et al., 1995a Vanderheyden et al., 1997 Shaner and Henry, 2007). 

Increase in production of atrazine degradation [191 metabolites by rhizosphere microorganisms in the presence of decomposing roots. 

The widely-used herbicide atrazine poses a threat to environmental health if point sources due to spills are not efficiently dealt with. Numerous atrazine-degrading microorganisms have been been isolated, and there is much known about the genetics and biochemistry of atrazine biodegradation. A variety of studies have e qilored the utility of atrazine-degrading bacteria for soil bioremediation. The effects of various rate controlling factors, pH, availability of nutrients, terminal electron acceptor and efficacy of different bacteria have been explored. Finally, the advent of molecular methods used to monitor the persistence and assess die activity of bioremediating bacteria is likely to inqirove the success of bioremediation. 

Triazines pose rather more of a problem, probably because the carbons are in an effectively oxidized state so that no metaboHc energy is obtained by their metaboHsm. Very few pure cultures of microorganisms are able to degrade triazines such as Atrazine, although some Pseudomonads are able to use the compound as sole source of nitrogen in the presence of citrate or other simple carbon substrates. The initial reactions seem to be the removal of the ethyl or isopropyl substituents on the ring (41), followed by complete mineralization of the triazine ring. 

Stratton, G.W. 1984. Effects of the herbicide atrazine and its degradation products, alone and in combination, on phototrophic microorganisms. Arch. Environ. Contam. Toxicol. 13 35-42. 

Oscarson DW, Huang PM, Liaw WK, Hammer UT (1983) Kinetics of oxidation of arsenite by various manganese dioxides. Soil Sci Soc Am J 47 644-648 Pang LP, Close M, Flintoft M (2005) Degradation and sorption of atrazine, hexazinone and pro-cymidone in coastal sand aquifer media. Pest Man Sci 61 133-143 Paris DF, Lewis DL (1976) Accumulation of metoxychlor by microorganisms isolated from aqueous systems. BuU Environ Contam Toxicol 13 443-450 Parr JF, Smith S (1976) Degradation of toxaphene in selected anaerobic soil environments. Soil Science 121 52-57... 

Transformation of triazines is primarily the result of degradation caused by microorganisms. However, triazines are also subject to a slow chemical degradation process known as hydrolysis. Chemical hydrolysis of atrazine, for example, is a process where the chlorine atom is removed from the atrazine molecule and replaced with a hydroxyl (OH) group. Chemical hydrolysis is relatively fast in acidic and alkaline soils, but it is relatively slow in neutral soils. In neutral soils, the rate of chemical hydrolysis of triazines increases when the triazine is adsorbed on the surfaces of soil particles. Hydroxytriazines, the products of chemical hydrolysis, are very strongly held by soil surfaces and hence move very slowly in soils. The hydroxytriazines have no biological activity. 

The interactions between y-triazines and microorganisms have been studied over nearly 50 years and new research has led to important discoveries. The isolation of pure cultures that are able to modify or completely mineralize y-triazines has led to the discovery of new genes and enzymes that are involved in the degradation and mineralization of y-triazines by soil bacteria. Studies carried out in soils with a history of repeated y-triazine applications indicate that rapid degradation and mineralization of atrazine developed in various soils (Barriuso and Houot, 1996 Bradley et al, 1997 Pussemier et al, 1997). 

Other representative pesticides that have also been shown to be mineralized include glyphosate, parathion, carbaryl, EPTC, isofenphos, and propachlor. Pesticides that are susceptible to mineralization are not typically found in, or considered to be a threat to, groundwater suppHes because of their rapid degradation, ie, nonpersistence. Microorganisms can evolve, that is, develop metaboHc pathways for the mineralization of previously persistent compounds. For example, there have been several reports documenting the existence of atrazine-mineralizing microoiganisms (21). 

Stratton GW (1984) Effects of the Herbicide Atrazine and its Degradation Products, Alone and in Combination, on Phototrophic Microorganisms. Arch Environ Contam Toxicol 13 35-42... 

The metabolism of chloro-S-triazines in soil involves reactions of dealkylation, deamination, hydroxylation, and ring cleavage [171]. Dealkylation of chloro -triazines does not remove their toxidly, which has been, instead, attributed to the release by chemical hydrolysis of active chknine [172]. In a study on the effects of atrazine and its degradation products on phototrophic microorganisms, the most toxic degradation product was deethylated atrazine, vdiidi was 2 to 7 times more effective towards cyanobacteria than deisopropylated atrazine. On the contrary, diamino and hydro atrazine were non-toxic [173]. 

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