The bio-decolorization of methyl orange by S.putrefaciens CN32 and responding mechanism to salinity stress based on...

Salinity poses a significant obstacle to the bio-decolorization of azo dyes. However, the underlying responding mechanisms of bio-decolorization bacteria remain unclear. Shewanella putrefaciens CN32 (S.P CN32) can reduce azo dyes through special electron transfer pathways. Hence, this study used methyl orange (MO) as a representative of azo dyes to investigate azo dye decolorization by S.P CN32 and to explore the responding mechanism of S.P CN32 to salinity stress in the decolorization process. More than 95 % of MO was decolorized by S.P CN32 under the conditions of pH 6-10, MO concentration high than 850 mg/L, and salinity 0-2 %. Lactate was the optimal electron donor in MO decolorization process by S.P CN32. The complete decolorization time was lagged by 30 h under 2 % salinity. FTIR and LC-MS were utilized to identify metabolites and analysis possible metabolic pathways in MO decolorization process. The expression changes of genes involved in MO bio-decolorization and response to salinity stress were characterized by transcriptome analysis. The salinity inhibition on MO decolorization was linked to the down-regulation of azoreductase-associated electron transport pathways and electron transfer chains including Mtr pathway, NADH dehydrogenase, mequinones and heme synthesis. But the up-regulation of flavins synthesis could slightly remit the inhibition. When exposed to salinity, bacteria up-regulated genes expression associated with Na⁺/K⁺ transport, the transfer and biosynthesis of proline and glutamate. Most of genes related to energy production by lactate metabolism, TCA cycle and fatty acid metabolism also up-regulated, while most genes related to cell motility down-regulated. This is conducive to supply energy for adapting salinity stress to survive. This study demonstrated the potential role of S.P CN32 in MO decolorization process, and may provide some new insights into the bio-treatment of textile wastewater with high salinity from the perspective of the molecular mechanism.