Atrazine

Atrazine
Atrazine
Atrazine
IUPAC name 1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine
Other names Atrazine See also synonyms
style="background: #F8EABA; text-align: center;" colspan="2" | Identifiers
CAS number	1912-24-9 Y
SMILES
style="background: #F8EABA; text-align: center;" colspan="2" | Properties
Molecular formula	C8H14ClN5
Molar mass	215.68 g mol−1
Appearance	colorless solid
Density	1.187 g/cm3
Melting point	175 °C (448 K)
Boiling point	200 °C (473 K)
Solubility in water	0.007 g/100 mL (?°C)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Atrazine, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, an organic compound consisting of an s-triazine-ring is a widely used herbicide. Its use is controversial due to its effects on nontarget species, such as on amphibians, and because of widespread contamination of waterways and drinking water supplies. Although it has been excluded from a re-registration process in the European Union, it is still one of the most widely used herbicides in the world. Like many herbicides, it is sold under numerous trade names.

Uses

Atrazine is used to stop pre- and post-emergence broadleaf and grassy weeds in major crops. The compound is both effective and inexpensive, and thus is well-suited to production systems with very narrow profit margins, as is often the case with maize. Atrazine is the most widely used herbicide in conservation tillage systems, which are designed to prevent soil erosion. About 76 million pounds of atrazine were applied in the United States in 2003.

Its effect on yields has been estimated from 6% to 1%, with 3-4% being the conclusion of one review. In another study looking at combined data from 236 university corn field trials from 1986–2005, atrazine treatments showed an average of 5.7 bushels more per acre than alternative herbicide treatments.

In its 2003 Interim Reregistration Eligibility Decision, U.S. EPA said "The total or national economic impact resulting from the loss of atrazine to control grass and broadleaf weeds in corn, sorghum and sugar cane would be in excess of $2 billion per year if atrazine were unavailable to growers." In the same report, it added the "yield loss plus increased herbicide cost may result in an average estimated loss of $28 per acre" if atrazine were unavailable to corn farmers.

Chemistry and biochemistry

Atrazine is prepared from cyanuric chloride, which is treated sequentially with ethylamine and isopropyl amine. Like other triazine herbicides, atrazine functions by binding to the plastoquinone-binding protein in photosystem II, which animals lack. Plant death results from starvation and oxidative damage caused by breakdown in the electron transport process. Oxidative damage is accelerated at high light intensity.

Biodegradation

Atrazine degrades in soil primarily by the action of microbes. The half-life of atrazine in soil ranges from 13 to 261 days. Atrazine biodegradation can occur by two known pathways: 1) Hydrolysis of the C-Cl bond, followed by the ethyl and isopropyl groups, catalyzed by the hydrolase enzymes called AtzA, AtzB, and AtzC. The end product of this process is cyanuric acid, itself unstable with respect to ammonia and carbon dioxide. The best characterized organisms that use this pathway are of Pseudomonas sp. strain ADP.

2) Dealkylation of the amino groups to give 2-chloro-4-hydroxy-6-amino-1,3,5-triazine, the degradation of which is unknown. This path also occurs in Pseudomonas species as well as a number of bacteria.

Rates of biodegradation are affected by atrazine's low solubility, thus surfactants may increase the degradation rate. Though the two alkyl moieties readily support growth of certain microorganisms, the atrazine ring is a poor energy source due to the oxidized state of ring carbon. In fact, the most common pathway for atrazine degradation involves the intermediate, cyanuric acid, in which carbon is fully oxidized, thus the ring is primarily a nitrogen source for aerobic microorganisms. Atrazine may be catabolized as a carbon and nitrogen source in reducing environments, and some aerobic atrazine degraders have been shown to use the compound for growth under anoxia in the presence of nitrate as an electron acceptor, a process referred to as a denitrification. When atrazine is used as a nitrogen source for bacterial growth, degradation may be regulated by the presence of alternative sources of nitrogen. In pure cultures of atrazine-degrading bacteria, as well as active soil communitites, atrazine ring nitrogen, but not carbon are assimilated into microbial biomass. Low concentrations of glucose can decrease the bioavailability, whereas higher concentrations promote the catabolism of atrazine.

The genes for enzymes AtzA-C have been found to be highly conserved in atrazine-degrading organisms worldwide. The prevalence of these genes could be due to the mass transfer of AtzA-C on a global scale. In Pseudomonas sp. ADP, the Atz genes are located noncontiguously on a plasmid with the genes for mercury catabolism. This plasmid is conjugatable to Gram negative bacteria in the laboratory and could lead to the worldwide distribution, in view of the extensive release of atrazine and mercury. AtzA-C genes have also been found in a Gram positive bacterium, but are chromosomally located. The insertion elements flanking each gene suggest that they are involved in the assembly of this specialized catabolic pathway. Two options exist for degradation of atrazine using microbes, bioaugmentation or biostimulation. Recent research suggests that microbial adaptation to atrazine has occurred in some fields where the herbicide is used repetitively, resulting in a decrease in herbicidal effectiveness. Like the herbicides trifluralin and alachlor, atrazine is susceptible to rapid transformation in the presence of reduced iron-bearing soil clays, such as ferruginous smectites. In natural environments, some iron-bearing minerals are reduced by specific bacteria in the absence of oxygen, thus the abiotic transformation of herbicides by reduced minerals is viewed as "microbially induced".

Controversy

According to Extension Toxicology Network in the U.S., "The oral median Lethal Dose or 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."

File:AtrazineUSAMap.png

Atrazine use in pounds per square mile by county. Atrazine is one of the most commonly used herbicides in the United States.

Atrazine was banned in the European Union (EU) in 2004 because of its persistent groundwater contamination. In the United States, however, atrazine is one of the most widely used herbicides, with 76 million pounds of it applied each year, in spite of the restriction that used to be imposed.^, It is probably the most commonly used herbicide in the world, and is used in about 80 countries worldwide. Its alleged endocrine disruptor effects, possible carcinogenic effect, and epidemiological connection to low sperm levels in men has led several researchers to call for banning it in the US.

Some studies suggest that even the concentrations meeting U.S. federal standards may be dangerous, with implications for human birth defects, low birth weights and menstrual problems. Findings from further studies released in early 2010 have tended to support the conclusion that even low doses can increase health risks, leading to calls for further testing and renewed EPA evaluation of atrazine's safety.

In 2006 the EPA concluded that the triazine herbicides posed "no harm that would result to the general U.S. population, infants, children or other...consumers."

EPA concluded in 2007 that atrazine does not adversely affect amphibian gonadal development based on a review of laboratory and field studies, including studies submitted by the registrant and studies published in the scientific literature. At this time, EPA believes that no additional testing is warranted to address this issue."

Effect on amphibians

Atrazine is a suspected teratogen, causing demasculinization in male northern leopard frog even at low concentrations, and an estrogen disruptor. A 2010 study found that atrazine rendered 75 percent of male frogs sterile and turned one in 10 into females.. A 2002 study found that exposure to atrazine caused male tadpoles to turn into hermaphrodites - frogs with both male and female sexual characteristics. But another study, requested by EPA and funded by Syngenta, was unable to reproduce these results.

Tyrone Hayes, Department of Integrative Biology, University of California, notes that all of the studies that failed to conclude that atrazine caused hermaphroditism were plagued by poor experimental controls and were funded by Syngenta, one of the companies that produce the chemical. The U.S. Environmental Protection Agency (EPA) and its independent Scientific Advisory Panel (SAP) examined all available studies on this topic — including Hayes' work — and concluded that there are "currently insufficient data" to determine if atrazine affects amphibian development. Hayes, formerly part of the SAP panel, resigned in 2000 to continue studies independently. The EPA and its SAP made recommendations concerning proper study design needed for further investigation into this issue. As required by the EPA, Syngenta conducted two experiments under Good Laboratory Practices (GLP) and inspection by the EPA and German regulatory authorities. The paper concluded "These studies demonstrate that long-term exposure of larval X. laevis to atrazine at concentrations ranging from 0.01 to 100 microg/l does not affect growth, larval development, or sexual differentiation." Another independent study in 2008 determined that "the failure of recent studies to find that atrazine feminizes X. laevis calls into question the herbicide's role in that decline." A report written in Environmental Science and Technology (May 15, 2008) cites the independent work of researchers in Japan, who were unable to replicate Hayes' work. "The scientists found no hermaphrodite frogs; no increase in aromatase as measured by aromatase mRNA induction; and no increase in vitellogenin, another marker of feminization."

In 2007, the EPA held another SAP on the topic and concluded "that atrazine does not adversely affect amphibian gonadal development based on a review of laboratory and field studies, including studies submitted by the registrant and studies published in the scientific literature." On its website in July 2009, the Agency said "At this time, EPA believes that no additional testing is warranted to address this issue."

A 2008 study reported that tadpoles developed deformed hearts and impaired kidneys and digestive systems when exposed to atrazine in their early stages of life. Tissue malformation may have been induced by ectopic programmed cell death, although a mechanism was not identified.

In 2009, University of Tennessee Department of Plant Sciences researchers found the combination of the herbicides mesotrione and atrazine can make sweet corn more nutritious. They found the herbicides directly up-regulate the carotenoid biosynthetic pathway in corn kernels, which is associated with the nutritional quality of sweet corn. Enhanced accumulation of lutein and zeaxanthin is important because dietary carotenoids function in suppressing aging eye diseases such as macular degeneration, now affecting 1.75 million older Americans.

In 2010, the Australian equivalent of the EPA, the Australian Pesticides and Veterinary Medicines Authority (APVMA), found the chemical safe to use:

The conclusion of the APVMA at that time, based on advice from DEWHA, was that atrazine is unlikely to have an adverse impact on frogs at existing levels of exposure. This advice was consistent with findings by the US EPA in 2007 (see below) that atrazine does not adversely effect amphibian gonadal development.

Further more, the APVMA responded to Hayes' 2010 published paper, Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis), by stating that his findings "do not provide sufficient evidence to justify a reconsideration of current regulations which are based on a very extensive dataset."

A 2010 study conducted by the U.S. Geological Survey observed substantial adverse reproductive effects on fish from atrazine exposure at concentrations below the USEPA water-quality guideline.

See also

• Endocrine disruptor
• Simazine

Further reading

• Syngenta's page about atrazine: atrazine.com
• Tyrone Hayes' page about his research on Atrazine: atrazinelovers.com
• Randall Amster, Silent Spring Has Sprung, Truthout, March 19, 2010.
• Synonyms and brand names
• "The Triazine Herbicides" Published February 2008, Elsevier, ISBN 978-0-444-51167-6

External links

• Atrazinelovers: an anti-atrazine website - maintained by Tyrone Hayes
• Atrazine News - an Atrazine specific news sitecs:Atrazin

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