**4.1 Biodegradation of triazines**

In spite of the fact that triazine and phenylurea herbicides persist in the natural environment for a long time and do not undergo biodegradation easily there are some higher plants and microorganisms capable of metabolizing these compounds.

In tolerant plants triazines as well as phenylurea herbicides are readily metabolized. Plant metabolites include the hydroxy- and dealkylated derivatives of parental compounds. Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is metabolized in tolerant plants to hydroxyatrazine and amino acid conjugates, with further decompositon of hydoxyatrazine by degradation of the side-chains. The resulting amino acids on the ring are hydrolyzed and mineralized (i.e. degraded to CO2). In sensitive plants, unaltered atrazine accumulates, leading to chlorosis (a condition in which leaves produce insufficiant amounts of chlorophylls) and death. The similar degradation or action pathways apply for propazine (6-chloro-N2,N4-di-isopropyl-1,3,5-triazine-2,4-diamine) and simazine (6-chloro-N2,N4 diethyl-1,3,5-triazine-2,4-diamine). With chlorotoluron (3-(3-chloro-p-tolyl)1,1 dimethylurea), metabolites found in winter wheat include 3-chloro-p-toluidine,3- (3-chloro-4-methylphenyl)-1-methylurea and 1-(3-chloro-4-methylphenyl)urea (Tomlin, 2003).

Behki and Khan studied agricultural soils to which atrazine was applied for a long time. They isolated three bacteria strains (*Pseudomonas* family) capable of utilizing atrazine as the sole source of carbon (Behki & Khan, 1986). Those bacteria use the side-chain carbon, thus N-dealkylation resulting in desisopropylatrazine and desethylatrazine was observed. Two bacterial strains were able to cause the splitting of chlorine from atrazine as well as from the dealkylated metabolites. The same authors proved the capacity to degrade atrazine, propazine, and simazine in the bacteria of *Rhodococcus* species (Behki & Khan, 1994), the degradation rates being however lower than in *Pseudomonas* bacteria.

Not only bacteria but also other organisms such as soil fungal communities have been found to be able to attack and degrade triazines (Kodama et al., 2001).

A *Pseudomonas* bacterial strain was used to degrade atrazine by Wenk (Wenk et al., 1998). The rate of atrazine disappearance was shown to depend on the water content of the soil and on the number of inoculated bacteria; the time necessary for atrazine removal differed ranging from 1 to 25 days. A partial mineralisation of atrazine into CO2 was also observed.

Such results are in agreement with the findings of Crawford and his coworkers (Crawford et al., 2000), who concluded that the biodegradation rate is affected by the properties of soils and sediments, by agricultural cultivation practices and by the history of triazine application onto the particular soil.

Two genes responsible for s-triazine degradation have been found in four bacterial phyla (Jason Krutz et al., 2010).

#### **4.2 Biodegradation of phenylurea chlorotoluron**

Biotransformation of phenylurea herbicides by soil microorganisms (bacterial and fungi) has been reported by several authors (Badawi et al., 2009; Khadrani et al., 1999; Sørensen et al., 2003; Tixier et al., 2002). Bacteria degrade phenylurea herbicides by successive N-

In spite of the fact that triazine and phenylurea herbicides persist in the natural environment for a long time and do not undergo biodegradation easily there are some higher plants and

In tolerant plants triazines as well as phenylurea herbicides are readily metabolized. Plant metabolites include the hydroxy- and dealkylated derivatives of parental compounds. Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is metabolized in tolerant plants to hydroxyatrazine and amino acid conjugates, with further decompositon of hydoxyatrazine by degradation of the side-chains. The resulting amino acids on the ring are hydrolyzed and mineralized (i.e. degraded to CO2). In sensitive plants, unaltered atrazine accumulates, leading to chlorosis (a condition in which leaves produce insufficiant amounts of chlorophylls) and death. The similar degradation or action pathways apply for propazine (6-chloro-N2,N4-di-isopropyl-1,3,5-triazine-2,4-diamine) and simazine (6-chloro-N2,N4 diethyl-1,3,5-triazine-2,4-diamine). With chlorotoluron (3-(3-chloro-p-tolyl)1,1 dimethylurea), metabolites found in winter wheat include 3-chloro-p-toluidine,3- (3-chloro-

4-methylphenyl)-1-methylurea and 1-(3-chloro-4-methylphenyl)urea (Tomlin, 2003).

degradation rates being however lower than in *Pseudomonas* bacteria.

to be able to attack and degrade triazines (Kodama et al., 2001).

**4.2 Biodegradation of phenylurea chlorotoluron** 

onto the particular soil.

(Jason Krutz et al., 2010).

Behki and Khan studied agricultural soils to which atrazine was applied for a long time. They isolated three bacteria strains (*Pseudomonas* family) capable of utilizing atrazine as the sole source of carbon (Behki & Khan, 1986). Those bacteria use the side-chain carbon, thus N-dealkylation resulting in desisopropylatrazine and desethylatrazine was observed. Two bacterial strains were able to cause the splitting of chlorine from atrazine as well as from the dealkylated metabolites. The same authors proved the capacity to degrade atrazine, propazine, and simazine in the bacteria of *Rhodococcus* species (Behki & Khan, 1994), the

Not only bacteria but also other organisms such as soil fungal communities have been found

A *Pseudomonas* bacterial strain was used to degrade atrazine by Wenk (Wenk et al., 1998). The rate of atrazine disappearance was shown to depend on the water content of the soil and on the number of inoculated bacteria; the time necessary for atrazine removal differed ranging from 1 to 25 days. A partial mineralisation of atrazine into CO2 was also observed. Such results are in agreement with the findings of Crawford and his coworkers (Crawford et al., 2000), who concluded that the biodegradation rate is affected by the properties of soils and sediments, by agricultural cultivation practices and by the history of triazine application

Two genes responsible for s-triazine degradation have been found in four bacterial phyla

Biotransformation of phenylurea herbicides by soil microorganisms (bacterial and fungi) has been reported by several authors (Badawi et al., 2009; Khadrani et al., 1999; Sørensen et al., 2003; Tixier et al., 2002). Bacteria degrade phenylurea herbicides by successive N-

**4. Biodegradation of selected triazine and phenylurea herbicides** 

microorganisms capable of metabolizing these compounds.

**4.1 Biodegradation of triazines** 

dealkylation to substituted aniline products. Fungal pathways result in successive dealkylated metabolites as well as aniline derivatives, but Badawi (Badawi et al., 2009) reported the detection of a new major metabolite which (according to thin layer chromatography and nuclear magnetic resonance spectrometry) is a non-aromatic diol.

Biodegradation by some bacterial and fungal strains leads to the formation of very toxic substituted anilines which have even higher levels of LD50 - the dose required to kill half the members of a tested population after a specified test duration time (Tixier et al., 2000a; Tixier et al, 2009). The same applies to products of photochemical degradation (Tixier et al., 2000b).
