Degradation of Azo Dyes by White-Rot Fungi - Microbial Degradation of Synthetic Dyes in Wastewaters - page 312

Due to the similarity between the chemical structure of lignin and those of
several recalcitrant pollutants including synthetic dyes, the use of white-rot fungi
and/or their enzymes for the degradation of recalcitrant dyes has been widely used
(Paszczynski et al. 1991 ; Zhou and Zimmerman 1993 ).
Decolouration of azo dyes by the well known ligninolytic fungus Phanerochaete
chrysosporium was rst described in 1990 (Cripps et al. 1990 ). Since then,

P. chrysosporium has been subject of an intensive research for bioremediation
of recalcitrant compounds. Besides, it was found to be capable of decolourising
different types of azo dyes (Knapp et al. 1995 ; Moreira et al. 2000 ; Chagas and
Durrant 2001 ). The promising results obtained by P. chrysosporium have accel-
erated the search for other species of white-rot fungi with azo dye-degrading ability.
Thus, other white-rot fungi, such as Pleurotus ostreatus, Trametes versicolor,
Bjerkandera adusta and Thelephora sp., were also found to be able of decolourising
different azo dyes (Hein

ing et al. 1997 ; Cao 2000 ; Selvam et al. 2003 ).

2 White-Rot Fungi

White-rot fungi are the only micro-organisms able to degrade the whole wood
components i.e. cellulose, hemicellulose and lignin (Fig. 4 ). Their name derives
from the white appearance of the wood due to lignin removal when attacked by
these fungi (Pointing 2001 ).
Although some white-rot fungi are also Ascomycetes, but most of them are
Basidiomycetes taxonomically (Eaton and Hale 1993 ). White-rot caused by fungi
can be divided into simultaneous and selective lignin degradation types. Simulta-
neously, white-rot fungi degrade all wood cell wall polymers progressively, but
preferably they metabolise lignin and hemicellulose. The main characteristics of
white-rot fungi are presented in Table 1 (Mart nez et al. 2005 ).

In general, white-rot fungi are unable to use lignin as sole carbon source, but
they degrade it in order to gain access to polysaccharides (Sinsabaug and Liptak
1997 ; Yeo et al. 2007 ). The ability of white-rot fungi to degrade lignin is due to the
production of one or more extracellular non-speci

í

c lignin-modifying enzymes
(LMEs), which generate free radicals that randomly attack the lignin molecule,
breaking covalent bonds and releasing a range of mainly phenolic compounds.
Since radicals are highly reactive but short-lived molecules, they can reach only
closer lignocellulose material, forming erosion zones around the hyphae (Mester
and Field 1998 ; Carlile et al. 2001 ). There are two main types of LMEs: peroxi-
dases [lignin peroxidase (LiP) and manganese-dependent peroxidase (MnP)] and
phenoloxidases (laccases). Some fungi are able to produce both types of enzymes,
whereas others produce only one of them. In addition, white-rot fungi secrete low-
molecular weight mediators which increase the range of compounds that they can
degrade (Pointing 2001 ). A detailed description of LMEs and mediators are
included in Sect. 4 .