Textile Dyes Degradation: A Microbial Approach for Biodegradation of Pollutants - Microbial Degradation of Synthetic Dyes in Wastewaters - page 194

in uences their degradability by pure cultures and enzymes produced. Variations

regarding dye decolorization could be possible due to the complex chemical
structure of dyes, e.g. diazo dyes have more complex structure than mono-azo dyes.
With recent

ndings, it has also been demonstrated that chemical structure of dyes
affects their decolorization process (Nozaki et al. 2008 ). Asgher et al. ( 2007 )
evaluated the dye decolorization ef ciency of mixed microbial consortia from

wastewater treatment plants of different textile units and concluded that the
microbial consortium is a robust process for the bioremediation of textile dye
ef

uents. The nature of substituents on the aromatic ring has been shown to

in uence the enzymatic oxidation. Electron donating methyl and methoxy sub-

stituents enhanced the enzymatic degradation of azo phenols, while electron
withdrawing chloro, uoro and nitro substituents inhibited oxidation by a laccase

from Pyricularia oryzae and MnP from P. chrysosporium (Chivukula and Rega-
nathan 1995 ; Pasti-Grigsby et al. 1992 ). Some studies have demonstrated that
shaking conditions do not support decolorization. The reason for no decolorization
at shaking condition might be due to the competition of oxygen and dye for the
reduced electron carriers under aerobic condition. Similar studies were carried out
by Parshetti et al. ( 2006 ) and they reported that Malachite green was completely
decolorized under static condition within 5 h by a bacterium Kocuria rosea MTCC
1532. The nitrogen concentration in the culture medium also in

uences the growth
of fungi. However, decolorization of some dyes, e.g. Drimaren brilliant blue by
fungi was not found dependent on the initial nitrogen concentration (Machado et al.
2006 ). The fungi T. villosa and P. sanguineus belong to a group of fungi whose
ligninolytic systems are not regulated by nitrogen concentration, and so was the
case for Pleurotus ostreatus and Ceriporiopsis subvermispora also (Leatham and
Kirk 1983 ; Niku-Paavola et al. 1990 ; Ruttimann-Johnson et al. 1993 ). In fact, the
rates of pollutant degradation are proportional to the concentration of the chemical.
The biodegradation of other commercially important classes of textile dyes, have
also been addressed. Bakshi et al. ( 1999 ) found enhanced biodecoloration of syn-
thetic commercial textile dyes by P. chrysosporium by improving Kirk s medium

with respect to buffer, C:N ratio, Mg 2+ and Zn 2+ , temperature shifts, agitation, and
sun

'

ower oil addition.

9 Toxicity of Dyes and Their Effects on Fungi

Serious concern about textile dyes and intermediate compounds was rst raised due

to its toxicity and carcinogenicity that can cause damage to human health and
environment (Banat et al. 1996 ). This is mainly due to the fact that many dyes are
manufactured from known carcinogens, such as benzidine, naphthalene and other
aromatic compounds (Nascimento et al. 2011 ). Many workers have studied the
effects of dyes on microorganisms. Among them, Stearn and Stearn ( 1924 ), Dion
and Lord ( 1944 ), Aiquel and Herraro ( 1948 ) have shown that basic dyes are toxic to
some fungi, e.g. Fusarium culmorum and some Aspergilli. Further, similar studies