Bacterial Enzymes and Multi-enzymatic Systems for Cleaning-up Dyes from the Environment - Microbial Degradation of Synthetic Dyes in Wastewaters - page 27

environmental hazardous chemical additives (Forgacs et al. 2004 ; Chen 2006 ).
Biological treatment technologies are attractive alternatives to the traditional
physicochemical methods, as they are low-cost, environmentally friendly and can
selectively provide a complete degradation of organic pollutants without collateral
destruction of either the site

ora or fauna (Anjaneyulu et al. 2005 ; Chen 2006 ;
Husain 2006 ; Rodriguez Couto 2009a ). It has been demonstrated that microor-
ganisms are able to degrade synthetic dyes to non-colored compounds or even
mineralize them completely under certain environmental conditions (dos Santos
et al. 2007 ; Saratale et al. 2011 ;Sol

' s

s et al. 2012 ; Chengalroyen and Dabbs 2013 ;
Khan et al. 2013 ). However, the fact that most of dye pollutants persist for long
periods in the environment indicates the natural inadequacy of microbial activity to
deal with these xenobiotic compounds. Biological systems need to exhibit not only
a high catalytic versatility towards the degradation of a complex mixture of
structurally different dyes, but also a superior robustness against the toxic effects of
the dyes and their products, in addition to the salts, detergents, surfactants, and
metals present in the dye-containing ef

í

uents, often at extreme pHs or high tem-
peratures (Anjaneyulu et al. 2005 ; Chen 2006 ). Considering these requirements,
there is currently no simple solution for the biological treatment of dye-containing
ef

uents.
Enzymatic processes are particularly sought for the treatment of dye-containing
efuents, mainly because of their specicity and relatively ease of engineering
towards improved robustness; enzymes only “ attack ” the dye molecules, while

valuable dyeing additives or bers are kept intact and can potentially be re-used

(Kandelbauer and Guebitz 2005 ). Likewise, new recycling technologies will allow
a huge reduction in water consumption in the textile nishing industry. Although

dye molecules display high structural diversity, they are only degraded by a few
enzymes that share common mechanistic features as they all catalyze redox reac-
tions and, exhibit relatively wide substrate speci

cities. The most important dye
degrading enzymes are: azoreductases, laccases and peroxidases (Kandelbauer and
Guebitz 2005 ). Azoreductases are oxidoreductases, which are particularly effective
in the degradation of azo dyes through reduction of the azo linkage, the chromo-
phoric group of azo dyes (Kandelbauer and Guebitz 2005 ; Rodriguez Couto
2009b ). The majority of characterized azoreductases are FMN or FAD dependent
enzymes that require the addition of NAD(P)H as electron donors for the reduction
of azo dyes releasing aromatic amines as products (Stolz 2001 ; Deller et al. 2008 ).
Laccases are multi-copper oxidases that couple the one-electron oxidation of four
substrate molecules to the four electron reductive cleavage of the O

O bond of
dioxygen to water. These enzymes have a great potential in various biotechno-
logical processes mainly due to their high non-speci

-

c oxidation capacity, the lack
of requirement for cofactors, and the use of the readily available molecular oxygen
as an electron acceptor (Stoj and Kosman 2005 ; Morozova et al. 2007 ; Haritash and
Kaushik 2009 ; Mikolasch and Schauer 2009 ). These include the detoxi

cation of

industrial ef uents (Rodriguez Couto and Toca Herrera 2006 ), mostly from the

paper and pulp, textile and petrochemical industries, and bioremediation to clean up
herbicides, pesticides and certain explosives in soil (Morozova et al. 2007 ; Haritash