Environmental Engineering Reference
In-Depth Information
4 Enzymes Involved in the Biodegradation of Azo Dyes
by Actinobacteria
Possible mechanisms of microbial removal of azo dyes include biosorption, bio-
accumulation, reduction and oxidation. However, bioremediation by biosorption
and bioaccumulation are slow and dyes are often not completely mineralized (Gadd
2009 ; Wang and Chen 2009 ). In this scenario, role of actinomycetes in the deg-
radation of azo dyes or toxic by-products through oxidation-reduction processes is
very important.
4.1 Reductive Degradation of Azo Dyes
Azo dyes are generally decomposed by microorganisms in a two-step process
(Dawkar et al. 2010 ; Khalid et al. 2010 ; Liu et al. 2011 ). In the
rst step, microbes
breakdown the azo bond (
N=N) of azo dyes with the help of azoreductase enzyme
in low oxygen condition. This process is also called decolorization, resulting in the
formation of colorless aromatic amines, which are mineralized in the oxidation
process. So far, a little is known about the azo dyes degradation pathways of
actinomycetes. Azoreductase enzyme has been reported in a few species of acti-
nomycetes. Usually, the azoreductase enzyme is not easily identi
ed in actino-
mycetes, due to its low concentration. However, the surfactant can be used to
improve the production of the enzyme in the microorganisms (Reese and Maguire
1969 ). For this purpose, Chengalroyen ( 2011 ) used two surfactants, Tween-80 and
Tween-20, to improve the production of azoreductase in Streptomyces. A closer
monitoring of different enzymes in the same host showed that Tween-80 might
result in the selective inhibition or enhancement of certain enzymes, suggesting that
a complex interaction is involved. The identi
cation of decolorizing strains in the
presence of Tween-20 suggests the release of superior stabilized-protein into culture
media (Chou et al. 2005 ). Chengalroyen ( 2011 ) also identi
ed azoreductase in
Streptomyces coelicolor by genome sequencing. This study provides an evidence
that the azoreductase is also widespread in Actinobacteria.
4.2 Oxidative Degradation of Azo Dyes
In the case of actinomycetes, oxidation process is the most dominant mechanism for
the degradation of azo dyes (Lu et al. 2013 ; Priyaragini et al. 2013 ). Usually,
peroxidase or lignin-degrading enzymes are involved in the degradation process;
either a single enzyme is being involved in the process or a group of enzymes act
synergistically. Dye degrading peroxidases are reported to degrade hydroxyl free
anthraquinone dyes (Sugano et al. 2006 ; Marchis et al. 2011 ). A combination of
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