Atrazine

From Freepedia

Atrazine, 2-chloro-4-(ethylamine)-6-(isopropylamine)-s-triazine, is a 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 quinine-binding protein in photosystem II, inhibiting electron transport. According to the Environmental Protection Agency (EPA) the US used 363 million kg of Atrazine from 1980 to 1990. The herbicide is classified as a class C carcinogen which has been shown to cause chromosomal damage in hamster ovary cells. The half-life of atrazine in soil is 15 to 100 days. Alternative derivatives of atrazine as well as the breakdown products could be carcinogenic as well. Atrazine and its derivatives are used in many industrial processes as well, including use in dyes and explosives. Hydroxyatrazine is unregulated and no negative effect is known. Despite recommendations for controlled and managed atrazine applications, the use will probably continue to compromise soil and groundwater|(ground)water]] worldwide (Ralebitso TK, et al) (Wackett LP, et al).

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.

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. 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 degredation. This path can occur by a single Pseudomonas species or by a number of bacteria (Zeng Y, et al) (Wackett LP, et al).

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 (Ralebitso TK, et al).

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 (Cai B, et al). 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 (Wackett LP, et al). Two options exist for degredation of atrazine using microbes: bioaugmentation or biostimulation (Wackett LP, et al).

References

Cai B, Han Y, Liu B, Ren Y, Jiang S. (2003). Isolation and characterization of an atrazine-degrading bacteriam from inductral wastewater in China. Lett Appl Microbiol. 36:272-276.

Ralebitso TK, Senior E, van Verseveld HW. (2002). Microbial aspects of atrazine degradation in natural environments. B iodegradation. 13:11-19.

Wackett LP, Sadowsky MJ, Martinez B. (2002). Biodegradation of atraxine and related s-triazine compounds: from enzymes to field studies. Appl Microbiol Biotechnol. 58:39-45.

Zeng Y, Sweeney CL, Stephens S, Kotharu P. (2004). Atrazine Pathway Map. Wackett LP. Biodegredation Database. Online. http://umbbd.ahc.umn.edu/atr/atr_map.html Unformatierten Text hier einfügen