Atrazine

Atrazine, 2-chloro-4-(ethylamine)-6-(isopropylamine)-s-triazine, is an s-triazine-ring herbicide that is used globally to stop pre- and post-emergence broadleaf and grassy weeds in major crops. Atrazine binds to the plastoquinone-binding protein in photosystem II, inhibiting electron transport. Atrazine is one of the most widely used herbicides and according to the Environmental Protection Agency (EPA) the US used 77 million lb of Atrazine in 2003.

The half-life of atrazine in soil is 15 to 100 days. Atrazine and its derivatives are used in many industrial processes as well, including use in dyes and explosives. Hydroxyatrazine is unregulated and its possible health effects are not known. However, atrazine is the most widely used herbicide in conservation tillage systems, which is a system designed to help prevent soil erosion and runoff by as much as 90 percent.

The oral LD50 for atrazine is 3090 mg/kg in rats, 1750 mg/kg in mice, 750 mg/kg in rabbits, and 1000 mg/kg in hamsters. The dermal LD50 in rabbits is 7500 mg/kg and greater than 3000 mg/kg in rats. The 1-hour inhalation LC50 is greater than 0.7 mg/L in rats. The 4-hour inhalation LC50 is 5.2 mg/L in rats.

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Biodegradation

The start of atrazine biodegradation can occur by three known ways. Atrazine can be dechlorinated and then the other ring substituents are removed by amidohydrolases. These steps are performed by AtzA-C respectively, which are commonly produced by a single organism. The end product, cyanuric acid, is then used as a carbon and nitrogen source. The most characterized organism that performs this pathway is Pseudomonas sp. strain ADP. The other mechanism involves dealkylation of the amino groups. In this mechanism dechlorination can be performed in the second step to eventually yield cyanuric acid, or the end result is 2-chloro-4-hydroxy-6-amino-1,3,5-triazine, which currently has no known path to further degradation. This path can occur by a single Pseudomonas species or by a number of bacteria.^[1][2]

Sorption of atrazine in soil determines the bioavailability to degradation, which is performed mostly by microbes. Low atrazine biodegradation rates are a product of low solubility and sorption to areas inaccessible by bacteria. The addition of surfactants increases the solubility, increasing catalysis. Before use the surfactant must be evaluated for its effect on the environment as well as its use as a preferential carbon and energy source must be evaluated. Atrazine itself is a poor energy source due to the highly oxidized carbons in the ring. It is catabolized as a carbon and nitrogen source in limiting environments although the optimum carbon and nitrogen availability is not known. It has been shown that inorganic nitrogen increases atrazine catabolism while organic nitrogen decreases it. Low concentrations of glucose can have the effect of decreasing bioavailability though formation of bound atrazine, while higher concentrations promote the catabolism of atrazine.^[3]

The genes AtzA-C have been found to be highly conserved in atrazine degrading organisms worldwide. This could be due to the mass transfer of AtzA-C on a global scale. In Pseudomonas sp. ADP, the atz genes are located non-contiguously on a plasmid with mercury catabolism genes as well. This plasmid is conjugatable to Gram negative bacteria in the lab and could easily lead to the worldwide distribution with the amount of atrazine and mercury being produced. AtzA-C have also been found in a Gram positive bacterium, but chromosomally located.^[4] This is not surprising due to the presence of insertion elements flanking each gene and the detection of these genes on different plasmids. Their configurations on these different plasmids suggest the insertion elements are involved in the assembly of this specialized catabolic pathway.^[2] Two options exist for degradation of atrazine using microbes: bioaugmentation or biostimulation.^[2]

Controversy

  Atrazine has been banned in the European Union.^[6][7] However, the European Union scientific review stated, “It is expected that the use of atrazine, consistent with good plant protection practice, will not have any harmful effects on human or animal health or any unacceptable effects on the environment.” A very similar product to atrazine, called terbuthylazine, is used in the EU today. It is used in about 80 countries worldwide.^[8]

Atrazine is one of the most widely used herbicides in the United States, with 76 million pounds of it applied each year.^[9] It is probably the most commonly used herbicide in the world.^[8]

Atrazine has been shown in some experiments by UC Berkeley biologist Tyrone Hayes to be a teratogen, and, even at concentrations as low as 0.1 part per billion, to emasculate male frogs by causing their gonads to produce eggs – effectively turning males into hermaphrodites.^[10] Hayes also found that atrazine causes male frogs' testosterone levels to dip below those of females.^[8] The Environmental Protection Agency (EPA) and its independent Scientific Advisory Panel (SAP) examined all available studies on this topic - including Hayes' work - and concluded there is "currently insufficient data" to determine if atrazine affects amphibian development. Hayes, formerly part of the SAP panel, resigned in 2000 to continue studies independently.^[11]

In 2003, the EPA classified the herbicide as "not likely" to cause cancer in humans, stating it did "not find any results among the available studies that would lead us to conclude that a potential cancer risk is likely from exposure to atrazine."^[12] After a 10-year science review, the EPA recommended atrazine's re-registration in October 2003.^[13]

The EPA considered the re-registration final when it issued the cumulative risk assessments in 2006 on the triazine herbicides, of which atrazine is one, and concluded that the herbicides pose "no harm that would result to the general U.S. population, infants, children or other...consumers."^[14]

Synonyms

2-chloro-4-(2-propylamino)-6-ethylamino-s-triazine, 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine,2-chloro-4-ethylamino-6-isopropylamino-s-triazine, 2-Chloro-4-(isopropylamino)-6-ethylamino-s-triazine, Ortho St. Augustine Weed and Feed, 6-chloro-N-ethyl-N'-isopropyl-1,3,5-triazine-2,4-diamine, A 361, aatrex, AAtrex 4L, AAtrex 80W, aatrex nine-o, actinite pk, akticon, aktikon, aktikon pk, aktinit a, aktinit pk, argezin, atazinax, Atranex, atrasine, atrataf, Atratol, atratol a, Atrazine, Atrazine 4L, Atrazine 80W, Atrazine, Atrazines, ATRAZINE (PRIMATOL), Atred, Atrex, Attrex, ATZ, Azinotox 500, Candex, cekuzina-t, chromozin, Crisamina, crisatrina, crisazine, Crisazina, Cyazine, Extrazine II, farmco atrazine, fenamine, Fenatrol, Fogard, g 30027, geigy 30,027, gesaprim, gesaprim 50, gesaprim 500, gesoprim, Griffex, Griffex 4L, hungazin, hungazin pk, inakor, Laddock, maizina, Mebazine, oleogesaprim, oleogesaprim 200, pitezin, primatol, Primatol A, primaze, radizine, Radazine, Scotts Bonus Type S, strazine, triazine a 1294, Vectal, Vectal SC, Vectral SC, Weedex, weedex a, Wonuk, zeaphos, zeapos, zeazin.

References

1. ^ Zeng Y, Sweeney CL, Stephens S, Kotharu P. (2004). Atrazine Pathway Map. Wackett LP. Biodegredation Database.
2. ^ ^{a b c} Wackett LP, Sadowsky MJ, Martinez B. (2002). Biodegradation of atrazine and related s-triazine compounds: from enzymes to field studies. Applied Microbiology and Biotechnology. 58:39-45. Abstract
3. ^ * Ralebitso TK, Senior E, van Verseveld HW. (2002). Microbial aspects of atrazine degradation in natural environments. Biodegradation. 13:11-19. Abstract
4. ^ Cai B, Han Y, Liu B, Ren Y, Jiang S. (2003). Isolation and characterization of an atrazine-degrading bacterium from industrial wastewater in China. Letters in Applied Microbiology. 36:272-276. Abstract
5. ^ USGS Pesticide Use Maps
6. ^ Lee, Jennifer. "Popular pesticide faulted for frogs' sexual abnormalities", New York Times, 2003-06-19, pp. 20. Retrieved on 2007-04-27. 
7. ^ "EU ban on spray 'wrecks' crop trial", Financial Times, 2003-10-13, pp. 25. Retrieved on 2007-04-27. 
8. ^ ^{a b c} Briggs H. (April 15, 2002), Pesticide 'causes frogs to change sex'. BBC News. Retrieved on 2007-10-16.
9. ^ Walsh, Edward. "EPA Stops Short of Banning Herbicide", Washington Post, 2003-02-01, pp. A14. Retrieved on 2007-04-27. 
10. ^ Hayes T, Haston K, Tsui M, Hoang A, Haeffele C, and Vonk A (2003). "Atrazine-Induced Hermaphroditism at 0.1 ppb in American Leopard Frogs". Environmental Health Perspectives 111.
11. ^ Weedkiller 'threatens frogs', BBC News
12. ^ EPA Interim Reregistration Eligibility Decision (IRED), January 2003.
13. ^ Revised EPA Interim Reregistration Eligibility Decsion (IRED), October 2003.
14. ^ Triazine Cumulative Risk Assessment and Atrazine, Simazine, and Propazine Decisions, June 22, 2006, EPA.

Further reading

• Environ Health Perspect. 2007 May;115(5):720-7. Epub 2007 Feb 5. PMID 17520059