Influence Of Static Magnetic Field On Newly Isolated Halotolerant Yeasts Capable Of Degrading Azo Dyes And Mechanism Study

Azo dyes are the most commonly used commercial types.They present serious threat to the environment due to their recalcitrance and toxicity.Biological processes are the most widely applied treatments.Microorganisms including bacteria,fungi and algae can degrade azo dyes.Among them,fungi can degrade many organics which can not be decomposed by bacteria,through producing non-specific enzymes,thus attract more and more attention.Meanwhile,many azo-dye-containing wastewaters always contain high concentration of salt,which will inhibit the efficiency of bioprocesses.Therefore,it is important to exploit effective halotolerant microbes to cope with treatment of hypersaline wastewaters.In addition,many researchers found that magnetic field could further improve growth,metabolic activity and tolerance to extreme conditions of microbes,thereby to further improve biological efficiency with insignificant increase of cost.Based on the above background,isolation,identification and systematic characterization of halotolerant fungal strains which were capable of efficiently degrade azo dyes were performed in the present study.Then the possibility of further enhanced treatment efficiency and stability by using static magnetic field(SMF)was discussed.Furthermore,possible mechanisms were investigated through comparative transcriptomics method.Firstly,two high-efficient azo-degrading halotolerant yeasts A1 and A2 were isolated from sea mud,which were identified as Candida tropicalis and Pichia occidentalis through26S r DNA sequencing method.Effects of different parameters on dye decolorization and cell growth of the two yeasts were investigated.The optimal conditions for the yeast A1 were:glucose concentration was 4.0 g/L,(NH₄)₂SO₄ concentration was 0.8 g/L,yeast extract concentration was 0.1 g/L,Na Cl concentration was?30 g/L,rotation speed was?160 r/min,temperature was 35?and p H was 7.0;those for the yeast A2 were:glucose concentration was 4.0 g/L,(NH₄)₂SO₄ concentration was 0.4 g/L,yeast extract concentration was 0.1 g/L,Na Cl concentration was?30 g/L,rotation speed was?160 r/min,temperature was 30?and p H was 5.0.Azo dye of higher concentration may inhibit the growth of both two yeasts.Possible degrading pathways of the target dye(Acid Red B,ARB)by the two yeasts were proposed and detoxification(acute toxicity)effect was further studied.The results showed that the possible degrading pathways for two yeasts were the same,which both started from the breakdown of azo bond through the catalysis of reductases and/or oxidoreductases,resulting in dye decolorization.Then the dye ARB was further degraded through a series steps including deamination,desulfurization,mono-oxidative open-loop of hydroxy-substituted naphthalene or benzene and final TCA cycle.Acute toxicity of ARB significantly decreased from high-toxic level to low-toxic and micro-toxic levels after treatment by the yeasts A1 and A2 for 24 h,respectively.At last,possible influencing mechanisms of SMF on the yeasts A1 and A2 were discussed through comparative transcriptomics method.Based on the result of macroscopic experiments,the optimal SMF intensity for the yeasts A1 and A2 were 24.6 m T and 206.3 m T,with which two yeasts displayed the highest metabolic activity.SMF with the corresponding optimal intensity also improved halotolerance of two yeasts.Increase of treatment efficiency was attributed to the increase of key enzymes'activity.Up-regulation of genes responsible for carbohydrate metabolism,biological redox processes,intracellular production of glycerol and cell wall constituents'regulation was detected,which was related to the enhancement of decolorization efficiency and halotolerance of the two yeasts.