Basic and applied aspects in the microbial degradation of azo dyes

Abstract

Azo dyes are the most important group of synthetic colorants. They are generally considered as xenobiotic compounds that are very recalcitrant against biodegradative processes. Nevertheless, during the last few years it has been demonstrated that several microorganisms are able, under certain environmental conditions, to transform azo dyes to non-colored products or even to completely mineralize them. Thus, various lignolytic fungi were shown to decolorize azo dyes using ligninases, manganese peroxidases or laccases. For some model dyes, the degradative pathways have been investigated and a true mineralization to carbon dioxide has been shown. The bacterial metabolism of azo dyes is initiated in most cases by a reductive cleavage of the azo bond, which results in the formation of (usually colorless) amines. These reductive processes have been described for some aerobic bacteria, which can grow with (rather simple) azo compounds. These specifically adapted microorganisms synthesize true azoreductases, which reductively cleave the azo group in the presence of molecular oxygen. Much more common is the reductive cleavage of azo dyes under anaerobic conditions. These reactions usually occur with rather low specific activities but are extremely unspecific with regard to the organisms involved and the dyes converted. In these unspecific anaerobic processes, low-molecular weight redox mediators (e.g. flavins or quinones) which are enzymatically reduced by the cells (or chemically by bulk reductants in the environment) are very often involved. These reduced mediator compounds reduce the azo group in a purely chemical reaction. The (sulfonated) amines that are formed in the course of these reactions may be degraded aerobically. Therefore, several (laboratory-scale) continuous anaerobic/aerobic processes for the treatment of wastewaters containing azo dyes have recently been described.

Reaction mechanism of azoreductases suggests convergent evolution with quinone oxidoreductases

“…Dyes that contain an azo bond (−N = N−) contribute significantly to world dye production (Stolz, 2001). Almost half of the dye used for dying fabrics does not bind to the fabric, and as a result, is lost in the effluent (Shore, 1995).…”

Aplicação conjunta de tratamento anaeróbio termofílico por lodo granular e de mediadores redox na remoção de cor de águas residuárias têxteis

“…O mecanismo de ação entre tais microrganismos é bem variado, envolvendo tanto mecanismos oxidativos como redutivos. Por exemplo, bactérias aeróbias somente removem cor quando possuem enzimas chamadas azo redutases, as quais são catalisadas na presença de oxigênio (Stolz, 2001). Tais enzimas foram descobertas em organismos do tipo Pseudomonas K22 e KF46, e Pagmentiphaga kullae K24 (Zimmermann et al, 1982;Blumel e Stolz, 2003).…”

Functional role of Trp-105 of Enterococcus faecalis azoreductase (AzoA) as resolved by structural and mutational analysis

“…Azo dyes are a major group of colorants used in textiles, plastics, pharmaceuticals, cosmetics and food products (Chen, 2006;Stolz, 2001). Many synthetic azo dyes can be converted to potentially carcinogenic aromatic amines after reduction (Platzek et al, 1999).…”