Aerobic decolorization and degradation of azo dyes by suspended growing cells and immobilized cells of a newly isolated yeast Magnusiomyces ingens LH-F1
Introduction
In the recent years, more and more attention has been paid on the discharge of effluents containing synthetic dyes (Miranda et al., 2013). The release of these effluents into the environment is undesirable due to the serious environmental problems linked with the dyes and their breakdown products (Ozdemir et al., 2013). Among all kinds of commercial synthetic dyes, azo dyes are the largest class which shows a wide variety of colors and structures and represents up to 70% of the total textile dyestuffs used (Lang et al., 2013). Azo dyes belong to the class of aromatic and/or heterocyclic compounds which are characterized by the presence of one or more azo groups (NN), and their breakdown products are usually toxic and even carcinogenic to aquatic organisms (Pandey et al., 2007). Therefore, effective removal of azo dyes from wastewaters before discharge into the environment is of utmost practical importance.
Treatment alternatives including chemical, physical and biological processes have been studied for removing azo dyes from wastewaters (Crini, 2006). Among them, biological methods are commonly considered to be lower-cost, more effective and environmentally safe (Mohana et al., 2007). Most studies on azo dye biodegradation have focused on bacteria and fungi, in which bacteria were widely used for azo dyes decolorization due to their high activity, extensive distribution and strong adaptability (Pearce et al., 2003, dos Santos et al., 2007). However, decolorization byproducts such as aromatic amines could inhibit the activity of a large-scale of bacteria (Qu et al., 2010). By contrast, fungi can degrade complex organics through the catalysis by their extracellular ligninolytic enzymes including laccase, manganese peroxidase and lignin peroxidase (Gomi et al., 2011). Many fungal species such as Pleurotus ostreatus, Pichia sp., Penicillium sp., and Candida tropicalis, have been confirmed to decolorize azo dyes through adsorption and/or degradation (Kalmiş et al., 2008, Qu et al., 2010, Gou et al., 2009, Tan et al., 2013). Moreover, some fungi can partially or even completely mineralize azo dyes (Miranda et al., 2013, Qu et al., 2012, Tan et al., 2013). Compared with the dyes themselves, some decolorization intermediates such as aromatic amines and phenolics possessed higher toxicity and lower biodegradability. Fungi showed strong adaptability and efficient ability in removal of these aromatic compounds. For instance, the fungal strains belonging to Thamnidium elegans, Zygorhynchus moelleri and Yarrowia lipolytica were confirmed to efficiently degrade aromatic phenolics (Papanikolaou et al., 2008, Bellou et al., 2014). Therefore, exploitation and study of new fungal strains capable of degrading azo dyes efficiently are still necessary for field application.
Though the use of specific contaminant-degrading microorganisms in the wastewater treatment system can provide an effective way to enhance the degradation of toxic organic pollutants, high degradation rate of pollutants would not persist for a long time in wastewater treatment systems due to the loss of degrading microorganisms through being washed out from the system (Li et al., 2013). Immobilization of microorganisms has been suggested as a strategy for maintaining efficient degradation biomass in the systems, and different ways of immobilization have been developed which lead to improved effectiveness in wastewater treatment (Moreno-Garrido, 2008). Entrapment is one of the most commonly used methods for immobilizing single-celled microorganisms such as bacteria and yeast, which can effectively avoid the loss of microorganisms from treatment systems. In this way, the microbial cells are always immobilized in alginate beads or polyvinyl alcohol (PVA) gel pellets (Samuel et al., 2013, Martínez et al., 2013). Compared with other ways, though the entrapment methods may be complex to execute, they still offer advantages such as high cell densities, high stability, absence of cell washout and extended reaction times (Tuttolomondo et al., 2014).
In this study, a yeast strain LH-F1 capable of decolorizing various azo dyes under aerobic conditions was isolated, identified and characterized. Acid Red B was chosen as the model dye for further investigating the effects of different parameters on decolorization by suspended growing cells of strain LH-F1 and the cells immobilized by calcium alginate, respectively. Additionally, the possible degradation pathways were proposed according to the results of metabolites identification and related literatures, and the toxicity of the metabolites was assessed through phytotoxicity tests. As far as it is known, it is the first report of efficient decolorization and detoxification of azo dyes by both of the suspended growing cells and immobilized cells of a Magnusiomyces ingens strain.
Section snippets
Reagents
Azo dyes used in this study were purchased from Dye Synthesize Laboratory, Dalian University of Technology, Dalian, China. Biochemical reagents were purchased from TaKaRa Biotechnology Co., Ltd., Dalian, China. Other chemical reagents are analytical grade.
Microbial source and culture mediums
The microbial source used in this study was from the sea mud of a harbor industrial zone in Dalian, China. The culture medium (A) for acclimation and decolorization tests by growing cells of strain LH-F1 contains (g/L): K2HPO4 1.0, MgSO4⋅7H2O
Identification and decolorization characteristics of yeast strain LH-F1
A yeast strain which was capable of decolorizing various azo dyes (Table 1) and named as LH-F1 was isolated from the sea mud of a harbor industrial zone in Dalian, China. It was aerobic, spherical-shaped, and could even decolorize azo dyes such as Acid Red B (20 mg/L) on solid agar plate (data not shown). Colonies of strain LH-F1 was white, rough on the surface and irregular on the edge. To taxonomically identify the strain LH-F1, the 26S rDNA genes were amplified through PCR. The sequence of
Conclusions
The characteristics of M. ingens LH-F1 capable of aerobically decolorizing and degrading azo dyes were demonstrated in this study. The optimal conditions for decolorization of Acid Red B by both of suspended growing cells and immobilized cells of strain LH-F1 were investigated. The suspended and immobilized strain LH-F1 could completely decolorize 50–300 mg/L Acid Red B within 10–44 h and 4–7 h, and could tolerate up to 800 mg/L and 1200 mg/L of the dye, respectively. A possible degradation pathway
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (No. 51108223) and the Science and Technology Research Projects of Liaoning Provincial Education Department (No. L2013415).
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