Textile Dyes Degradation: A Microbial Approach for Biodegradation of Pollutants - Microbial Degradation of Synthetic Dyes in Wastewaters - page 190

and their role in environment protection. The chemical recalcitrance resulted from
the inability of microorganisms to degrade some industrially synthesized chemical
compounds.
White-rot fungi are unique among eukaryotes in their ability to cleave carbon-
carbon bonds in polycyclic aromatic hydrocarbons (PAHs). In fact, biological
degradation of these compounds was earlier considered as an exclusively bacterial
process (Gibson and Subramanian 1984 ). The breakdown of most organo-pollutants
by ligninolytic fungi is closely linked to ligninolytic metabolism. In this process,
degradation is stimulated by nutrient limitation, and it is generally believed that
enzymes, whose normal function is lignin degradation, also catalyze the highly non-
speci c xenobiotic oxidation. The biodegradation of these types of organo-pollu-

tants has been shown to be dependent on the lignin-degrading system of the
microorganism. Initial oxidation of several organo-pollutants has been reported to
be catalyzed by ligninases isolated from P. chrysosporium (Hammel et al. 1992 ).
The lignin-degrading system of this fungus includes a family of lignin peroxidases
or ligninases which catalyze the initial oxidative depolymerization of lignin.
Several mechanisms used by the fungi to degrade chemicals have recently been
elucidated. The

rst step in the degradation of many chemicals by white-rot fungi
often involves formation of highly reactive free radical intermediates. These free
radicals are formed any time; one electron is removed or added to the ground state of
a chemical. Such free radicals are very reactive and will rapidly give up or withdraw
an electron from another chemical. Free radical reaction often occurs as chains in
which many different radicals are generated subsequent to formation of initial radical
species and free radical reactions catalyzed by the peroxidases from white-rot fungi
also appear to be involved in the degradation of many pollutants. The free radical
process also provides some basis for the non-speci c nature of white rot fungi.

ssion of azo bonds by Pseudomonas
cepacia, followed by acetylation of the resulting amino benzenes. In a continuation
of the study, they also reported an oxidative pathway for degradation of amino
compounds which converted these compounds to amino hydroxy compounds.
These metabolites are subsequently metabolized through the Kreb

Idaka et al. ( 1987 ) described reductive

s cycle after
opening of ring. However, their degradation by Streptomyces spp. depends upon the
substitution pattern of the aromatic ring. Bacterial cytochrome P-450 is believed to
catalyze most of the reactions.

'

6 Role of Fungal Enzymes in Degradation

Production of laccase by white-rot fungus Phanerocheate chrysosporium, Neu-
rospora crassa and some other fungi has been extensively studied for removal of
pigments and phenol from liquid waste. The production of extracellular enzymes by
arti cially captured cells of fungi (Trichosporon cutaneum, Candida tropicals and

Phanerocheate chrysosporium) is possible. By using gel entrapment and adherence
to a matrix, encouraging results can be obtained for the wastewater treatment.