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Have you ever looked around you and paid attention to the colorful products such as clothes, cosmetics, and foods that you purchase? If you wonder where all these colors come from, then you might want to know more about azo dyes.

Dyes and their effects in the environment

Azo dyes have become an important part of different industries [1,2], allowing consumers to choose different colors for the product of interest. Whether you prefer yellow, black, or red, these dyes can display any of these colors. However, they can have a significant impact on health and the environment [3]. Tiny amounts of these dyes can lead to aesthetic pollution [4]. They can also affect biological activities in the aquatic environment when they decrease the level of oxygen needed by aquatic animals and plants to survive [5]. Some dyes can also lead to cancer when people are exposed to high levels [6]. Although not all azo dyes are harmful, careful assessment and examination of these dyes must be done before they can be used on different products.

Several solutions such as physical and chemical treatments have been proposed to remove or degrade these dyes in the environment [7]. Physical treatment involves the use of techniques such as adsorption, filtration and reverse osmosis [8]. Such treatment uses different materials such as charcoal and filters. Meanwhile, chemical treatment involves the use of chemicals to degrade these dyes [8]. There are several issues that arise from both treatments. First, the costs of the materials/equipment needed might be impractical on an industrial scale. In addition, the use of materials such as charcoal and filters would lead to other environmental problems such as waste disposal. Some dyes are also resistant to chemical treatment and therefore the chemical will not have any effect. Another consequence of chemical treatment is the generation of toxic intermediates that can worsen the environmental problem. Since physical and chemical treatments have several drawbacks [9], finding an alternative that is eco-friendly is a must.

Microbes to the rescue

Microorganisms are everywhere. These are small organisms that can easily adapt to their environmental surroundings, so some have evolved that harbor enzymes or metabolites that are useful or relevant to the health sector, for example antibiotics, or to the food sector, for instance probiotics. This just shows the impact that microbes have now in our daily lives. Hence, this leads scientists to explore more what these microorganisms can do. The use of microorganisms to degrade environmental pollutants such as hydrocarbons or heavy metals has also been documented in several studies [10,11]. This specialized area in the environmental sciences is called bioremediation.

Bioremediation explores the ability of microorganisms to transform or destroy pollutants such as azo dyes into less harmful compounds [12]. Different microorganisms like bacteria, fungi, and algae are being extensively studied to decolorize and degrade azo dyes. The use of bioremediation for the treatment of azo dyes has several advantages over the physical and chemical treatments available, such as being environmental-friendly, being more cost-efficient, producing less sludge and producing less or non-toxic by-products [13,14]. Without a doubt, the use of bioremediation is a more environmental-friendly option to combat the problem.

Fig. 1 Growth of Arthrobacter sp. in a medium supplemented with (A) brilliant black and (B) methyl red to screen possible dye degraders isolated from soil.

The ability of microbes to decolorize azo dyes (Fig.1) is a complex mechanism that can vary from one organism to another. Each organism has its own way how to do this. Yeasts such as Saccharomyces cerevisiae and Candida albicans have been mainly studied for their biosorption properties [8,15,16]. In this mechanism, the dyes accumulate in the cell wall and act as an adsorbent. Meanwhile, filamentous fungi such as Phanerochaete chrysosporium and Trametes versicolor harbor intracellular and extracellular enzymes that can degrade different compounds including complex dyes [17, 18, 19]. Bacteria, on the other hand, can either act as an adsorbent or produce enzymes such as azoreductases or dye peroxidases to degrade azo dyes. Some microorganisms can even combine these two mechanisms [20].

Enzymes in action

Enzymes are more effective in degrading the various kinds of hazardous azo wastes. Microbes harbor several such enzymes, for example laccases, peroxidases and azoreductases [21].

Fig. 2 From screening to action – the process of looking for potential dye degraders and studying their enzyme activity. (A) Potential dye degraders are often grown on culture media such as in agar plates or liquid medium containing the dye of interest (B) Purification of dye-degrading enzymes such as this azoreductase by chromatography, with the yellow color indicating a flavin cofactor (C) Mechanism of azoreductases against the azo dye methyl red

Laccases are enzymes from the multi-copper oxidase protein family [21]. These enzymes are attractive for bioremediation application as they degrade different pollutants such as azo dyes, quinones, anilines and phenols [21, 22]. They do not require any additional co-factors such as vitamins. Most of the known laccases are of fungal or plant origin [23].

On the other hand, peroxidases are heme-containing enzymes that require hydrogen peroxide to catalyze substrate conversions [21]. These enzymes are present in almost all organisms. Some of the well-known peroxidases that are involved in dye degradation are chloroperoxidases [24], manganese peroxidases [25] and DyP-type peroxidases [26].

Meanwhile, azoreductases are dye-degrading enzymes that can cleave the azo bond (-N=N-) present in azo dyes (Fig. 2) [27]. They are widely found in bacteria and fungi. Two types exist: flavin-dependent [28] and flavin-independent [29] azoreductases. They also require a co-substrate NAD(P)H and are perhaps the most characterized enzymes out of the dye-degrading enzymes with great importance for designing bio-based wastewater treatment for the removal of azo dyes [30].

Microbes for future

Industrial revolution has surely made our lives easier. However, the trade-off has great repercussions on our environment. As we face several problems with regards to hazardous industrial wastes such as dye-contaminated wastewaters, we must strive to find green alternatives or solutions to combat these problems. Undoubtedly, mining more into microbes that degrade dyes in wastewaters can be beneficial for human population and the environment. We must find ways or methods to harness this potential for degrading dyes and even for other hazardous substances. Bioremediation can be one of the key solutions to the accelerating environmental threat and contamination.


Category: Environmental Biotechnology | Bioremediation

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[2] Gao Y, Li C, Shen J, Yin H, An X, Jin H. Effect of food azo dye tartrazine on learning and memory functions in mice and rats, and the possible mechanisms involved. J Food Sci. 2011;76(6):T125-T129. DOI: 10.1111/j.1750-3841.2011.02267.x
[3] Chung KT. Azo dyes and human health: a review. J Environ Sci Health C. 2016;34(4):233-261. DOI: 10.1080/10590501.2016.1236602
[4] Hanis KKA, Nasri ARM, Farahiyah WKW, Rabani MYM. Bacterial Degradation of Azo Dye Congo Red by Bacillus sp. J Phys: Conf Ser. 2020;1529(2):022048. IOP Publishing. DOI: 10.1088/1742-6596/1529/2/022048
[5] Hu TL, Wu SC. Assessment of the effect of azo dye RP2B on the growth of a nitrogen fixing cyanobacterium–Anabaena sp. Bioresour technol. 2001;77(1):93-95. DOI: 10.1016/s0960-8524(00)00124-3
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[8] Saratale RG, Saratale GD, Chang JS, Govindwar SP. Bacterial decolorization and degradation of azo dyes: a review. J Taiwan Inst Chem Eng. 2011;42(1):138-157. DOI: 10.1016/j.jtice.2010.06.006
[9] Robinson T, McMullan G, Marchant R, Nigam, P. Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresour technol. 2001;77(3):247-255. DOI: 10.1016/S0960-8524(00)00080-8
[10] Varjani, SJ. Microbial degradation of petroleum hydrocarbons. Bioresour technol. 2017;223:277-286. DOI: 10.1016/j.biortech.2016.10.037
[11] Colin VL, Villegas LB, Abate CM. Indigenous microorganisms as potential bioremediators for environments contaminated with heavy metals. International Biodeterioration & Biodegradation. 2012;69: 28-37. DOI: 10.1016/j.ibiod.2011.12.001
[12] Vidali M. Bioremediation. an overview. Pure and applied chemistry. 2001;73(7):1163-1172. DOI: 10.1351/pac200173071163
[13] Azubuike CC, Chikere CB, Okpokwasili GC. Bioremediation techniques-classification based on site of application: principles, advantages, limitations and prospects. World J Microbiol Biotechnol. 2016;32(11):1-18. DOI: 10.1007/s11274-016-2137-x
[14] Shah MP. Chapter 6 - Bioremediation of Azo Dye. Eds: Shah MP, Rodriguez-Couto S. In Microbial wastewater treatment. Elsevier. 2019:103-126. DOI: 10.1016/B978-0-12-816809-7.00006-3
[15] Jadhav JP, Parshetti GK, Kalme S, Govindwar SP. Decolourization of azo dye methyl red by Saccharomyces cerevisiae MTCC 463. Chemosphere. 2007;68(2):394-400. DOI: 10.1016/j.chemosphere.2006.12.087
[16] Vitor V, Corso CR. Decolorization of textile dye by Candida albicans isolated from industrial effluents. J Ind Microbiol Biotechnol. 2008;35(11):1353-1357. DOI: 10.1007/s10295-008-0435-5
[17] Paszczynski A, Crawford RL. Degradation of azo compounds by ligninase from Phanerochaete chrysosporium: involvement of veratryl alcohol. Biochem Biophys Res Commun. 1991;178(3):1056-1063. DOI: 10.1016/0006-291x(91)90999-n
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[22] Chivukula M, Renganathan V. Phenolic azo dye oxidation by laccase from Pyricularia oryzae. Applied and Environmental Microbiology. 1995;61(12):4374-4377. Chivukula M, Renganathan V. Phenolic azo dye oxidation by laccase from Pyricularia oryzae. Appl Environ Microbiol. 1995;61(12):4374-4377.
[23] Frasconi M, Favero G, Boer H, Koivula A, Mazzei F. Kinetic and biochemical properties of high and low redox potential laccases from fungal and plant origin. Biochim Biophys Acta. 2010;1804(4):899-908. DOI: 10.1016/j.bbapap.2009.12.018
[24] Li X, Zhang J, Jiang Y, Hu,M, Li S, Zhai Q. Highly efficient biodecolorization/degradation of Congo red and alizarin yellow R by chloroperoxidase from Caldariomyces fumago: catalytic mechanism and degradation pathway. Ind Eng Chem Res. 2013;52(38):13572-13579. DOI: 10.1021/ie4007563
[25] Harazono K, Watanabe Y, Nakamura K. Decolorization of azo dye by the white-rot basidiomycete Phanerochaete sordida and by its manganese peroxidase. J Biosci Bioeng. 2003;95(5):455-459. DOI: 10.1016/s1389-1723(03)80044-0
[26] Rajhans G, Sen SK, Barik A, Raut S. Elucidation of fungal dye‐decolourizing peroxidase (DyP) and ligninolytic enzyme activities in decolourization and mineralization of azo dyes. J Appl Microbiol. 2020;129(6):1633-1643. DOI: 10.1111/jam.14731
[27] Misal SA, Gawai, KR. Azoreductase: a key player of xenobiotic metabolism. Bioresour Bioprocess. 2018;5(17):1-9. DOI: 10.1186/s40643-018-0206-8
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[29] Kumaran S, Ngo ACR, Schultes FPJ. Tischler, Draft genome sequence of Kocuria indica DP-K7, a methyl red degrading actinobacterium. 3 Biotech. 2020;10(4): 175. DOI: 10.1007/s13205-020-2136-3, Erratum in: 3 Biotech. 2021 Sep;11(9):417. DOI: 10.1007/s13205-021-02895-5
[30] Dong H, Guo T, Zhang W, Ying H, Wang P, Wang Y, Chen Y. Biochemical characterization of a novel azoreductase from Streptomyces sp.: Application in eco-friendly decolorization of azo dye wastewater. Int J Biol Macromol. 2019;140:1037-1046. DOI: 10.1016/j.ijbiomac.2019.08.196

Date of publication: 29-Nov-2021

Facts, background information, dossiers

  • environmental biotechnology
  • bioeconomy
  • bioremediation
  • biosorption
  • azo dyes
  • azo compounds
  • environmental pollutants
  • microorganisms
  • microbes
  • enzymes
  • azoreductases
  • laccases
  • peroxidases

More about Ruhr-Universität Bochum

  • Authors

    Prof. Dr. Dirk Tischler

    Dirk Tischler, born in 1982, studied applied natural science at the TU Bergakademie Freiberg (TUBAF), Germany, and completed his doctoral studies on styrene monooxygenases under supervision of Prof. Dr. Michael Schlömann in 2012. He continued as junior group leader in the field of industria ... more

    Selvapravin Kumaran

    Selvapravin Kumaran, born in 1996, studied Applied Microbiology at Indira Gandhi college of arts and science, Puducherry, India (2014–2017) and completed his master's degree in Biochemistry at Ruhr-Universität Bochum with a specialization in azo dye degrading soil bacteria and its genome in ... more

    Anna Christina R. Ngo

    Anna Christina R. Ngo, born in 1990, studied Microbiology at the University of Santo Tomas, Manila, Philippines, where she received her bachelor’s and master’s degree. She did the research for her master’s thesis with a focus on bioremediation, under the supervision of Dr. Gina R. Dedeles a ... more

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