Decolorization Of Dyes By Microorganisms

Synthetic azo, anthraquinone, triarylmethane, and other dyes are extensively used for textile dyeing, paper printing, and color photography. A necessary criterion for the application of these dyes is that they resist photodegradation and washing processes. In addition, these dyes are also resistant to microbial degradation and are not easily removed from wastes during conventional biological treatment (Meyer 1981; Shaul et al. 1991). The removal of synthetic dyes from waste-waters is mainly achieved by elimination of the dye stuffs using adsorption, chemical oxidation, and flocculation.

Azo linkages and aromatic sulfo groups do not occur naturally and they resist oxidative degradation. Partial degradation has been observed for anthraquinone and azo dyes during aerobic activated sludge treatment, but substantial removal of color from the wastewater was attributed to adsorption of the intact dye onto the sludge (Athanasopoulos 1991). The dyestuffs are generally more susceptible to anaerobic sludge degradation. The anaerobic reduction of azo dyes to colorless products by bacteria has been reported (Meyer 1981). The initial step in this transformation is the reductive fission of the azo group, resulting in the formation and accumulation of toxic and possibly carcinogenic aromatic amines. The rapid decolorization of a number of textile dyes and effluents by an immobilized anaerobic consortium had also been demonstrated (Nigam et al. 1996; Oxspring et al. 1996).

The ability of P. chrysosporium to decolorize polymeric dyes, such as Poly B-411, Poly R-481, and Poly Y-606, has been correlated with its lignin degrading system (Glenn and Gold 1983; Gold et al. 1983; 1988). Strains of P. ostreatus have also been shown to effectively decolorize Poly B-411 (Platt et al. 1985). P. pulmonarius increased Poly B decolorization, as well as enhanced laccase production. Laccase is often produced by Pleurotus sp. as part of their ligninolytic system but dye decolorization by this fungus appears to be unrelated due to increased laccase secretion (Masaphy and Levanon 1992).

The utilization of the white-rot fungi and their nonspecific ligninolytic enzyme system for the biodegradation of other industrial dyes has found increased interest, since it has also become known that P. chrysosporiurm is able to degrade and decolorize a range of azo, sulfonated dyes, heterocyclic, and triphenylmethane dyes (Bumpus and Brock 1988; Cripps et al. 1990; Paszczynski et al. 1992; Spadaro et al. 1992) (Table 1). The triphenylmethane dye, crystal violet, has been decolorized by P. chrysosporium. Three metabolites were identified and the dye was N-demethylated by purified lignin peroxidase providing further evidence that the ligninolytic system is at least involved in oxidative biodegradation (Bumpus and Brock 1988). Pararosaniline and cresol violet and cresol red were also decolorized.

A number of other azo, sulfonated azo, triphenylmethane, and heterocyclic dyes can also be degraded by various strains of P. chrysosporium (Table 1). This further demonstrates the usefulness of the ligninolytic system in biodegrading and decolorizing wastewaters contaminated with dyes from textile and other industries (Cripps et al. 1990). Fluorescein acid (Gogna et al. 1992) and thiazine dyes (Kling and Neto 1991) have also been degraded by P. chrysosporium or lignin peroxidase. Many studies have used P. chrysosporium, although a H2O2-dependent enzymatic decolorization by P. ostreatus has also been shown (Vyas and Molitoris 1995). Knapp et al. (1995) suggests that more work on the effect of the decolorizing capacity of other members of the Basidiomycetes is warranted.

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