Microbial Degradation of Basic Dyes in Wastewaters - Microbial Degradation of Synthetic Dyes in Wastewaters - page 93

response surface methodology was applied to optimize the key variables for bio-
logical treatment of Malachite green by Cladospora sp. The optimization results
showed that maximum decolorization ef

ciency was achieved at the optimal con-
ditions of initial pH 8, initial dye concentration 10 mg l − 1 , algae amount 4 g and
reaction time 75 min (Khataee and Dehghan 2011 ). An orthogonal design (L 4 )2 3
was used to decolorize a dye industry ef uent by Aspergillus fumigatus XC6 under

optimum conditions of pH 3.0, nitrogen sources [0.2 % NH 4 Cl or (NH 4 ) 2 SO 4 ]or
carbon sources (1.0 % sucrose or potato starch) for 72 h (Jin et al. 2007 ). In another
study, a Taguchi design was adopted to determine the optimum conditions of
Pseudomonas sp. DY1 immobilization with Aspergillus oryzae for maximum
biodegradation of Malachite green, which was obtained at an initial pH 6.5, 37 C,

inoculation size of Pseudomonas sp. DY1 (dry cell mass) 0.01 g and of A. oryzae
(spore number) 1.0

°

10 9 . Decolorization and biodegradation of Malachite green by
immobilized pellets were 99.5 and 93.3 %, respectively, under optimum conditions
(Yang et al. 2011 ).

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4 Involvement of Oxidoreductive Enzymes in Degradation
Process

White-rot fungi (WRF) represent a diverse ecophysiological group comprising
mostly basidiomycetes and to a lesser extent, litter-decomposing fungi, having an
extracellular ligninolytic enzyme system capable of extensive depolymerization and
mineralization of a wide variety of recalcitrant compounds, such as xenobiotics,
lignin, and various types of dyes (Paszczynski and Crawford 1995 ). This feature is
based on the WRF s capacity to produce one or more extracellular lignin-modifying

enzymes (LME) that are substrate non-speci

'

c and also able to tolerate high con-
centrations of pollutants (Reddy 1995 ). The important LME produced by WRF are
oxidoreductases which include two types of peroxidases, lignin peroxidase (LiP, E.
C. 1.11.1.14) and manganese-dependent peroxidase (MnP, E.C. 1.11.1.13) and a
phenol oxidase (laccase, Lac, E.C. 1.10.3.2). These enzymes play an important role
in the dye degradation. The LME are also essential for lignin mineralization which
occurs through a combination of other processes involving other auxiliary enzymes
(by themselves unable to degrade lignin), such as glyoxal oxidase and superoxide
dismutase for intracellular production of H 2 O 2 , which acts as a co-substrate of LiP
and MnP, as well as glucose oxidase, aryl alcohol oxidase and cellobiose dehy-
drogenase which are involved in feedback circuits, linking the ligninolysis with
cellulose and hemicellulose degradation (Leonowicz et al. 1999 ).
Different WRF produce one or more of these LME, and based on the types of
enzymes produced by them, they are divided into four groups, namely LiP-MnP-
laccase-producing fungi; MnP-laccase-producing, LiP-MnP-producing and laccase-
producing fungi. WRF produce LME during their secondary metabolism since
lignin oxidation provides no net energy to the fungus; the synthesis and secretion of