Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 252

used for biodegradation of textile wastewater. A common mechanism, in which the
heme groups are the main constituents, catalyzes the reactions in the presence of
hydrogen peroxide (Jamal et al. 2012 ; Zucca et al. 2012 ). The decolorization of
dyes by white-rot fungi was

rst time reported by Glenn and Gold ( 1983 ) and been
used by others to rapidly assess the biodegradative potential of a diverse group of
white-rot fungi (Gao et al. 2006 ). Now, white-rot fungus has become a model for
degradation research (Pazarlioglu et al. 2010 ).
The modi

ed lignin has been used as a dispersing agent in synthesizing many
textile dyes, Lignin peroxidase (LiP), also known as ligninase, was rst discovered

for partial decolorization of methylated lignin in vitro (Tien and Kirk 1983 ). The
use of lignin degrading fungus has attracted increasing scienti c interest, as these

organisms are able to degrade a wide range of recalcitrant organic pollutants
because they do not require pre-conditioning before the degradation of pollutants
and the enzyme secretion depends on nutrient limitation rather than presence of
pollutant (Wesenberg et al. 2003 ). The extracellular enzyme system from fungus
also enables them to tolerate high concentration of pollutants.
Manganese peroxidase (MnP) from the lignin-degrading fungus Phanerochaete
chrysosporium can oxidize a variety of organic compounds including dyes, but only
in the presence of Mn(II). MnP catalysis occurs in a series of irreversible oxidation-
reduction (redox) reactions which follow a ping-pong mechanism. MnP catalyzes
the oxidation of several phenols and aromatic amine dyes (Mielgo et al. 2003 ).
There are also reports of azo dye degradation by MnP from Bjerkandera adusta and
Pleurotus eryngii. The reactions with the dyes were characterized by their apparent
K m values ranging from 4 to 16 M and speci c activities ranging from 3.2 to

μ

10.9 U/mg (Hein ing et al. 1997 ). Recently, it has been reported that Bjerkandera

sp. BOS55 (ATCC 90940) was able to decolorize high dye concentration (up to
1,500 mg l − 1 ) within 10 min reaction time. This is not because of change in pH, but
has a speci

c oxidation capacity as high as 10 mg dye degraded per unit of MnP
consumed which caused more than 90 % decolorization (Mielgo et al. 2003 ).

4.4 Laccases

Laccases are copper containing enzymes which catalyze the oxidation of several
aromatic substances (phenols) with the preferable reduction of oxygen to water
(Robles et al. 2000 ). Phenols are oxidized by laccases to generate phenoxy radicals,
which couple to form oligomeric and polymeric products. The laccases produced by
the white-rot fungus during its secondary metabolic stage of growth are known to
oxidize a wide variety of organic compounds. Laccase decolorizes some textile
dyes without direct cleavage of the azo bond through a highly non-speci

c free
radical mechanism, thereby avoiding the formation of toxic aromatic amines.
Because of its wide range of substrates laccases can be used for biodegradation of
textile dyes. Industrial dye decolorization by laccases from ligninolytic fungi
(Pleurotus ostreatus, Pleurotus ostreiformis, Trametes versicolor, etc.) has been