Study of the effect of redox mediators on the decolorization of azo dyes by ascomycete yeasts

Abstract

Many studies have appeared in recent years describing the positive effect of redox mediators on the biological decolourisation of azo dyes impermeant to the cell membranes. The need for mediators arises from the intracellular location of the reducing species which is postulated to be NAD(P)H, except in some cases where a purely chemical reduction of the dye is likely to occur, e.g. by sulphide ion formed in anoxic environments by sufate-reducing microorganisms [1]. The decolourisation of several azo dyes by ascomycete yeasts does occur efficiently without the addition of any mediator [2]. However, since an increase in decolourising efficiency would be useful for a possible application of these microorganisms to the treatment of dye-containing effluents of the textile industry, we tested whether commonly used mediators – AQS and FAD – either alone or in conjugation with menadione, had any positive effect on the process. Under the conditions normally used in our decolourisation assays the observed results were, apparently, inconsistent. The development of an easy method to detect the presence of oxygen in the incubation medium allowed us to establish that the positive effect of menadione on the decolourising activity of yeast cells occured only under anoxic conditions. A possible explanation of this fact will be presented. Furthermore, the effect of mediators on the process can easily be predicted through the determination of the peak potential of the reduction wave for the different dyes and mediators by cyclic voltammetry.

[1] F. van der Zee “Anaerobic Azo Dye Reduction”. Full text dissertation from Wageningen University (2002).

[2] Patrícia A.Ramalho, H.Scholze, M.Helena Cardoso, M.Teresa Ramalho e A.M.Oliveira-Campos “Improved Conditions for the Degradation of Azo Dyes by Candida zeylanoides”. Enzyme and Microbial Technology 31 (2002) 848-854.