35
by rainfall, photochemical degradation by sunlight, microbiological degradation by
soil microorganisms, chemical degradation by soil constituents, and thermal degradation. S-triazines are toxic compounds and have been classified as possible human
carcinogens. Simetryn, one of the major methylthio-s-triazine herbicides used in
paddy fields, inhibits algal growth. Chlorinated triazine class of pesticides show
common neuroendocrine mechanism of toxicity resulting in both reproductive and
developmental concerns. The toxicity of these compounds has promoted research
for their degradation. Atrazine is the most studied chlorinated triazine herbicide for
toxicity and degradation.
2.1.2 Atrazine
Atrazine is one of the most widely used herbicides. Atrazine is very effective against
a wide range of weeds and less expensive when compared to the alternative products. Atrazine inhibits D-1 quinone binding involved in photosystem II. Atrazine
increases yield of about 6–50% based on crop. Despite the high agricultural yield,
there is a huge concern for the continued use of atrazine in several parts of the
world. For three decades from its discovery, microbial degradation was not observed
as the triazine ring contains no available electrons for aerobic biodegradation. For
microbial degradation, the only available energy source in atrazine is the ethyl and
isopropyl side chains attached to the triazine ring. However, atrazine was converted
by nonspecific monooxygenases to desethylatrazine and desisopropylatrazine.
Small amount of hydroxyatrazine was formed by chemical processes. Atrazine and
its metabolites are frequently detected in surface water and ground water at concentrations exceeding the safety levels. Atrazine is believed to cause endocrine disruption; carcinogenic effects including non-Hodgkin’s lymphoma, ovarian cancer,
colon cancer, leukaemia, multiple myeloma, reduced sperm quality in humans
(IARC 1999; Rusiecki et al. 2004), cancer, delayed reproductive development in
Table 2.1 (continued)
s-Triazine
herbicides
X
R 1
R 2
Soil
halflife
(days) Applications
WHO
classification
Trietazine
-Cl
-N(C 2 H 5 ) 2 -NHC 2 H 5
60
Potatoes,
legumes,
bananas, citrus,
coffee, maize,
sugarcane, tea,
tobacco
Slightly
hazardous
Terbuthylazine -Cl
-NHC 2 H 5
-NHC(CH 3 ) 3 22–60 Corn, sorghum,
grape
Slightly
hazardous
Where X, R 1 and R 2 = side chains of triazine ring (shown in Fig. 2.1)
2 Biodegradation and Bioremediation of S-Triazine Herbicides
by rainfall, photochemical degradation by sunlight, microbiological degradation by
soil microorganisms, chemical degradation by soil constituents, and thermal degradation. S-triazines are toxic compounds and have been classified as possible human
carcinogens. Simetryn, one of the major methylthio-s-triazine herbicides used in
paddy fields, inhibits algal growth. Chlorinated triazine class of pesticides show
common neuroendocrine mechanism of toxicity resulting in both reproductive and
developmental concerns. The toxicity of these compounds has promoted research
for their degradation. Atrazine is the most studied chlorinated triazine herbicide for
toxicity and degradation.
2.1.2 Atrazine
Atrazine is one of the most widely used herbicides. Atrazine is very effective against
a wide range of weeds and less expensive when compared to the alternative products. Atrazine inhibits D-1 quinone binding involved in photosystem II. Atrazine
increases yield of about 6–50% based on crop. Despite the high agricultural yield,
there is a huge concern for the continued use of atrazine in several parts of the
world. For three decades from its discovery, microbial degradation was not observed
as the triazine ring contains no available electrons for aerobic biodegradation. For
microbial degradation, the only available energy source in atrazine is the ethyl and
isopropyl side chains attached to the triazine ring. However, atrazine was converted
by nonspecific monooxygenases to desethylatrazine and desisopropylatrazine.
Small amount of hydroxyatrazine was formed by chemical processes. Atrazine and
its metabolites are frequently detected in surface water and ground water at concentrations exceeding the safety levels. Atrazine is believed to cause endocrine disruption; carcinogenic effects including non-Hodgkin’s lymphoma, ovarian cancer,
colon cancer, leukaemia, multiple myeloma, reduced sperm quality in humans
(IARC 1999; Rusiecki et al. 2004), cancer, delayed reproductive development in
Table 2.1 (continued)
s-Triazine
herbicides
X
R 1
R 2
Soil
halflife
(days) Applications
WHO
classification
Trietazine
-Cl
-N(C 2 H 5 ) 2 -NHC 2 H 5
60
Potatoes,
legumes,
bananas, citrus,
coffee, maize,
sugarcane, tea,
tobacco
Slightly
hazardous
Terbuthylazine -Cl
-NHC 2 H 5
-NHC(CH 3 ) 3 22–60 Corn, sorghum,
grape
Slightly
hazardous
Where X, R 1 and R 2 = side chains of triazine ring (shown in Fig. 2.1)
2 Biodegradation and Bioremediation of S-Triazine Herbicides