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 . Tiny amounts of these dyes can lead to aesthetic pollution . 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 . Some dyes can also lead to cancer when people are exposed to high levels . 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 . Physical treatment involves the use of techniques such as adsorption, filtration and reverse osmosis . Such treatment uses different materials such as charcoal and filters. Meanwhile, chemical treatment involves the use of chemicals to degrade these dyes . 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 , 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 . 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 .
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 .
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 . 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 .
On the other hand, peroxidases are heme-containing enzymes that require hydrogen peroxide to catalyze substrate conversions . These enzymes are present in almost all organisms. Some of the well-known peroxidases that are involved in dye degradation are chloroperoxidases , manganese peroxidases  and DyP-type peroxidases .
Meanwhile, azoreductases are dye-degrading enzymes that can cleave the azo bond (-N=N-) present in azo dyes (Fig. 2) . They are widely found in bacteria and fungi. Two types exist: flavin-dependent  and flavin-independent  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 .
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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Header image: iStock.com | Sergio Delle Vedove, LoraLiu; petri dish: D Tischler/MBT Lab
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