Combined intra- and extracellular reduction involved in the anaerobic biodecolorization of cationic azo dye by Shewanella oneidensis MR-1

Microbial reduction decolorization is a promising strategy for cationic azo dye pollution remediation, but the reduction mechanism is unclear yet. In this work, the anaerobic reduction decolorization mechanism of cationic red X-GRL (X-GRL) by Shewanella oneidensis MR-1 (MR-1) was investigated from both intracellular and extracellular aspects. The exogenous additional riboflavin treatment test was used to analyze the extracellular reduction mechanism of X-GRL, and the actual role of riboflavin during the reduction of X-GRL was identified by three-dimensional fluorescence analysis for the first time. The proteinase K and the electron competitor treatment tests were used to analyze the intracellular reduction mechanism of X-GRL. Moreover, the effect of external environment on the reduction mechanism of X-GRL was elucidated by the decolorization performance of MR-1 wild type and its mutants, ΔomcA/mtrC, ΔmtrA, ΔmtrB and ΔcymA, under different external pH conditions. The results indicated that X-GRL could be decolorized by MR-1 in both extracellular and intracellular spaces. The extracellular decolorization of X-GRL could be caused by Mtr respiratory pathway or the indirect reduction of riboflavin, while the intracellular decolorization might occur due to the intracellular reduction depending on CymA pathway and a NADH-dependent reduction catalyzed by intracellular azoreductases. Furthermore, the proportion of extracellular decolorization decreased, whereas that of intracellular decolorization increased as the environmental pH rose.