Bacterial Degradation of Textile Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 253

reported (Rodr guez et al. 1999 ). Recently, laccase was isolated from the white-rot

fungus Lenzites gibbosa and expressed in Pichia pastoris. This recombinant laccase
ef

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ciently decolorized alizarin red, neutral red, Congo red and crystal violet,
without the addition of any redox mediators. This study strongly suggests that the
decolorization capacity of recombinant laccase could be useful as biocatalyst for the
treatment of dye-containing ef uents (Zheng et al. 2014 ).

5 Mechanism and Pathways of Degradation

Biodegradation refers to chemical decomposition of organic substances by living
organisms. In the decomposition or degradation, microbial enzyme systems act as
biocatalysts to break down the original dye structure. There are several reports that
microorganisms including bacteria, fungi, and yeasts can degrade textile dyes
(Kolekar et al. 2008 ; Zucca et al. 2012 ). The reduction of azo linkage mediated is
the initial step by the enzyme, azoreductase in the biodegradation of azo dyes
(Levine 1991 ; Chung et al. 1992 ). The mineralization of dyes is usually achieved
with two stage anaerobic/aerobic treatments. In this process, several environmental
and physiological factors can in

uence the microbial activity and consequently the
efcacy and effectiveness of complete biodegradation. The biodegradation process
is depending on many factors, like concentration and structure of dyes, temperature,
pH and the microorganisms involved in it. Considering all these factors, the deg-
radation of dyes produced many small molecules which become very dif cult to be

degraded by the organism. These small molecules are aromatic amines and may
cause more toxic and carcinogenic effect on the

ora and fauna of water bodies,
where aromatic amine containing waste water is getting discarded. In this scenario,
it is very important to analyze the degradation of the dye as well as their inter-
mediates, especially aromatic amines (Forss and Welander 2009 ). The study of the
biodegradation products of synthetic dyes is very important in order to know about
the environmental fate of these pollutants. Telke et al. ( 2010 ) reported the degra-
dation of Congo red (1.0 g l − 1 ) within 12 60 h duration by an isolated bacterium

Pseudomonas sp. SU-EBT. As shown in Fig. 4 , the complete mineralization of azo
dyes without the release of aromatic amines has also been observed in Reactive
Blue 59 degradation by Alishewanella sp. KMK6, where the single microorganism
was able to degrade 2.5 g l − 1 dye within 6 h (Kolekar and Kodam 2012 ).
It is well known that several enzymes are present in the periplasmic and cyto-
solic fractions of the microorganisms where azoreductase cleaves an azo bond by
reductive cleavage. The coordinated action of these enzymes give rise to low
molecular weight molecules from complex dyes. The tracking of the various
enzyme activities evidenced an involvement of various oxidative as well as
reductive enzymes during the decolorization process. Induction of various enzymes
during decolorization gives an additional insight of decolorization mechanism and
also supports the active role of microorganisms in the biodegradation process
(Phugare et al. 2010 ), mainly through oxidative enzymes, such as cytochrome

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