Abstract
A halotolerant yeast, Pichia occidentalis A2, was recently isolated that can decolorize various azo dyes. The azo dye decolorization performance of this strain was characterized, including the degradation pathway and detoxification effects of this yeast. Additionally, the effect of static magnetic field (SMF) on this decolorization process was investigated. Activities of key enzymes were analyzed to estimate the change of metabolic activity. Furthermore, possible mechanisms were analyzed through detecting differentially expressed genes between yeast A2 in the absence and presence of SMF. The results indicated that yeast A2 displayed the optimal decolorization performance when the concentrations (in g/L) of glucose, (NH4)2SO4, yeast extract, and NaCl were 4.0, 1.0, 0.1, and ≤30.0, respectively. Meanwhile, the optimal rotation speed, temperature, and pH were 160 rpm, 30°C, and 5.0, respectively. Acid Red B was decolorized and detoxified by yeast A2 through successive steps, including cleavage of the naphthalene–amidine bond, reductive deamination, oxidative deamination/desulfurization, open-loop of hydroxy-substituted naphthalene, and tricarboxylic acid cycle. The dye decolorization efficiency and halotolerance of yeast A2 were enhanced by 206.3 mT SMF. The activities of manganese peroxidase, and laccase were elevated 1.37- and 1.16-fold by 206.3 mT SMF, but lignin peroxidase activity showed little change. It was suggested from the transcriptome sequence that the enhanced halotolerance might be related to the upregulated genes encoding the enzymes or functional proteins related to intracellular synthesis and accumulation of glycerol.
Highlights
Hypersaline wastewater typically contains a variety of substances, including salts, organic matter, heavy materials, and radioactive materials (Pendashteh et al, 2011)
Enhancement of microbial tolerance to specific conditions such as low temperature and high osmotic pressure was observed through exposure to static magnetic field (SMF) (Niu et al, 2014; Shao et al, 2019). All these results suggest that the existing biological treatment processes can be further improved by applying external SMF
Yeast A2 was identified as Pichia occidentalis by 26S ribosomal DNA (rDNA) sequence analysis (Supplementary Figure S1B)
Summary
Hypersaline wastewater typically contains a variety of substances, including salts (at least 1%, m/v), organic matter, heavy materials, and radioactive materials (Pendashteh et al, 2011). Azo dyes are the most widely used type worldwide (Tony et al, 2009). Different bacteria and fungi are tried to decolorize azo compounds. Bacteria can degrade diverse organics and possess strong adaptability to complex environments. Decolorization intermediates (especially aromatic amines) cannot be used or further degraded by most bacteria due to a lack of the required metabolic enzymes (Qu et al, 2010). Azo dyes can be effectively decolorized by Basidiomycota and Ascomycota fungi through bioabsorption and/or biodegradation (Kalmis et al, 2008; Arora et al, 2011). Many fungi can detoxify azo dyes by decomposing the toxic intermediates using their non-specific oxidoreductive enzymes (Tan et al, 2013). It is expected that fungi could be effectively applied for the deep purification of recalcitrant organic pollutants
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