Reductive biological treatment of textile effluents

Abstract

Our group is undertaking an investigation on the potential application of ascomycete yeasts to the decolourisation of azo dyes. Two of these strains, Candida zeylanoides (UM2) and Issatchenkia occidentalis (UM41), were isolated from contaminated soil and have been shown to mediate dye decolourisation through reductive cleavage of the azo bond. The rates of colour loss in the presence of yeast cells are independent of their previous exposure to the dye, suggesting that the decolourising activity, under the conditions tested, is constitutive. The process requires intact cells and an external carbon and energy source and depends on pH, temperature and dissolved oxygen. Interestingly, anaerobic conditions do not allow decolourisation. The kinetic study of the cells decolourising activity demonstrated that such activity has a maximum in the late exponential growth phase. Although glucose is the standard carbon and energy source we have also observed decolourisation by cells growing at the expense of ethanol. Decolourisation rates are considerably dependent on the dye structure. Of considerable practical interest is the observation that some of the amines produced by azo dye reduction can be used as carbon and nitrogen sources by the yeast. In order to get further insight on the yeast decolourising activity we have prepared some mutants of a laboratory strain of Saccharomyces cerevisiae and performed inhibition studies. The experimental evidence suggests that major part of the decolourising activity of intact yeast cells is due to a very well characterized plasma membrane redox system.