Biodegradation and Detoxification of Azo Dyes by a Newly Isolated Halotolerant Yeast Candida tropicalis SYF-1

Abstract

Abstract

A halotolerant yeast was recently isolated and identified through 26S rDNA and internal transcribed spacer sequencing analysis. Its capacity of decolorizing various azo dyes was investigated, and the dye that was decolorized the fastest was chosen as the target compound for further study. Effect of parameters, including concentrations of nutrients (external carbon, nitrogen, and vitamin mixture), salinity (NaCl concentration), environmental factors (rotation speed, temperature, and pH), and the concentration of the target dye, on decolorization and cell growth of the yeast was investigated. A possible decolorization pathway of the target dye was proposed depending on detection of breakdown byproducts and related literatures. In addition, detoxification effectiveness of the target dye was also estimated through the Microtox test. Results showed that the yeast named SYF-1 was identified as Candida tropicalis and could decolorize six different azo dyes, among which Acid Red B (ARB) was decolorized the fastest, mainly through biodegradation. Growing cells of the yeast SYF-1 displayed the highest decolorization efficiency, with 4.0 g/L glucose, 0.6 g/L ammonium sulfate, 0.08 g/L yeast extract, ≤30 g/L NaCl, rotation speed ≥160 revolutions per minute, temperature 30°C, and pH 7.0–8.0. Both decolorization and cell growth processes were inhibited by higher concentrations of ARB. A degradation pathway of ARB by the yeast SYF-1 was proposed as that consisting of reduction, monooxygenase-oxidization, reductive/oxidative deamination, oxidative desulfurization, and ring-cleavage processes. Moreover, the acute toxicity level of 50 mg/L ARB decreased from original high toxicity to nontoxicity after treatment for 12 h.