14.7 Biodegradation of Dioxins and Dioxins-Like Compound
Previous literature suggests that during chlorine cycle, TCDD/F compounds undergo
biodegradation in environment. The aerobic bacteria from the genus Burkholderia,
Pseudomonas and Sphingomonas can degrade the lower chlorinated dioxins. Various studies have shed light on the co-metabolism of monochlorinated dioxins and
unsubstituted dioxin. This co-metabolism has been reported to be the primary
substrate. It has been documented that the degradation is initiated when the ring
adjacent to the ether oxygen is attacked by a unique angular dioxygenases. Furthermore, chlorinated dioxins can also be degraded co-metabolically by white-rot fungi
under aerobic conditions since it utilize extracellular lignin degrading peroxidases.
Degradation of dioxins is difficult partly because of its hydrophobic nature,
planner structure, strong absorption and adsorption to soil and sediment, which
results into limited bioavailability and pronounced resistance to attack by aerobic
and anaerobic microorganisms (Field and Sierra-Alvarez 2008). The resistance
to degradation by microorganisms could be due to lack of selective pressure related
to the concentration and availability of compound. Therefore, there has been a need
to find indigenous soil microflora exposed to the pollutant concentrations, which is
low in agricultural applications and high at industrial effluent and municipal sludge
sites (Thomas et al. 1996).
From the last decade, DF a relatively soft carbon source than chlorinated dioxins
has been used as a model compound to study the degradation of dioxin. DF (just like
dioxins) is a less bioavailable, resistant and hydrophobic molecule that has a planner
structure.
DF is a 2, 2-biphenylene oxide having two benzene rings joined with one ether
linkage. Some studies have reported that few strains of the bacteria such as Pseudomonas, i.e. HH69, CA10; Brevibacterium, Terrabacter, Sphingomonas,
Xanthomonas maltophilia and Serratia marcescens ISTDF2 l can lead to DF
degradation (Fortnagel et al. 1990; Strubel et al. 1991; Omori et al. 1997; Harms
et al. 1995; Wittich et al. 1992; Ishiguro et al. 2000; Sato et al. 1997; Jaiswal and
Thakur 2007). These studies have revealed that oxygenation at ether linkage (angular oxygenation) is common among all above-mentioned bacterial strains as initial
biotransformation step. 2, 2
0 , 3-trihydroxybiphenyl are formed via angular
dioxygenation of DF just after unstable phenolic hemiacetal as first stable intermediate metabolite (Fortnagel et al. 1990; Bunz and Cook 1993; Harms et al. 1995).
The cleavage in 2, 2
0 , 3-trihydroxybiphenyl leads to formation of 2-hydroxy-6-(2hydroxyphenyl)-6-oxo-2, 4-hexadienoic acid which is hydrolysed to form salicylic
acid and 2-hydroxypenta-2,4-dienoic acid. Hydrolysis of salicylic acid forms catechol and finally, cis, cis-muconate or 2-hydroxy muconic semialdehyde are formed
after dihydroxylation which is absorbed in tricarboxylic cycle (Fortnagel et al.
1990).
FA-HZ1 (Pseudomonas aeruginosa) isolated from landfill leachate is known to
be a DF degrading bacterium. Complete genome sequence of FA-HZ1 demonstrated
that it has only one circular chromosome and the complete genome has been
sequenced to investigate the molecular mechanism of DF degradation (Ali et al.
14 Metabolism of Dioxins and Dioxins-Like Compound, Its Regulation and. . .
297
Previous literature suggests that during chlorine cycle, TCDD/F compounds undergo
biodegradation in environment. The aerobic bacteria from the genus Burkholderia,
Pseudomonas and Sphingomonas can degrade the lower chlorinated dioxins. Various studies have shed light on the co-metabolism of monochlorinated dioxins and
unsubstituted dioxin. This co-metabolism has been reported to be the primary
substrate. It has been documented that the degradation is initiated when the ring
adjacent to the ether oxygen is attacked by a unique angular dioxygenases. Furthermore, chlorinated dioxins can also be degraded co-metabolically by white-rot fungi
under aerobic conditions since it utilize extracellular lignin degrading peroxidases.
Degradation of dioxins is difficult partly because of its hydrophobic nature,
planner structure, strong absorption and adsorption to soil and sediment, which
results into limited bioavailability and pronounced resistance to attack by aerobic
and anaerobic microorganisms (Field and Sierra-Alvarez 2008). The resistance
to degradation by microorganisms could be due to lack of selective pressure related
to the concentration and availability of compound. Therefore, there has been a need
to find indigenous soil microflora exposed to the pollutant concentrations, which is
low in agricultural applications and high at industrial effluent and municipal sludge
sites (Thomas et al. 1996).
From the last decade, DF a relatively soft carbon source than chlorinated dioxins
has been used as a model compound to study the degradation of dioxin. DF (just like
dioxins) is a less bioavailable, resistant and hydrophobic molecule that has a planner
structure.
DF is a 2, 2-biphenylene oxide having two benzene rings joined with one ether
linkage. Some studies have reported that few strains of the bacteria such as Pseudomonas, i.e. HH69, CA10; Brevibacterium, Terrabacter, Sphingomonas,
Xanthomonas maltophilia and Serratia marcescens ISTDF2 l can lead to DF
degradation (Fortnagel et al. 1990; Strubel et al. 1991; Omori et al. 1997; Harms
et al. 1995; Wittich et al. 1992; Ishiguro et al. 2000; Sato et al. 1997; Jaiswal and
Thakur 2007). These studies have revealed that oxygenation at ether linkage (angular oxygenation) is common among all above-mentioned bacterial strains as initial
biotransformation step. 2, 2
0 , 3-trihydroxybiphenyl are formed via angular
dioxygenation of DF just after unstable phenolic hemiacetal as first stable intermediate metabolite (Fortnagel et al. 1990; Bunz and Cook 1993; Harms et al. 1995).
The cleavage in 2, 2
0 , 3-trihydroxybiphenyl leads to formation of 2-hydroxy-6-(2hydroxyphenyl)-6-oxo-2, 4-hexadienoic acid which is hydrolysed to form salicylic
acid and 2-hydroxypenta-2,4-dienoic acid. Hydrolysis of salicylic acid forms catechol and finally, cis, cis-muconate or 2-hydroxy muconic semialdehyde are formed
after dihydroxylation which is absorbed in tricarboxylic cycle (Fortnagel et al.
1990).
FA-HZ1 (Pseudomonas aeruginosa) isolated from landfill leachate is known to
be a DF degrading bacterium. Complete genome sequence of FA-HZ1 demonstrated
that it has only one circular chromosome and the complete genome has been
sequenced to investigate the molecular mechanism of DF degradation (Ali et al.
14 Metabolism of Dioxins and Dioxins-Like Compound, Its Regulation and. . .
297