Microbial degradation of textile industry effluents: A review

During textile processing, different waste materials are produced at different stages including dyes and wastewater. These chemicals and wastewater are ultimately released into environment that negatively affects its biota because of detrimental changes brought into the surrounding by these unused and untreated materials. The major problems being imposed by textile industry include: excessive usage of water, control of its frequent discharge into environment and treatment of this effluent loaded waste water. Industries are struggling to find out some novel solutions for treatment of these wastes to diminish the environmental damage being caused by effluent discharge. In recent years, there is an increasing trend of using different kinds of microorganisms for degradation purpose of textile industries effluents. Because of low cost and long lasting effect of biological remediation of industrial wastes, there has been an increasing demand for new and novel biological solutions for industrial effluent management. In this context, this review summarize major textile dyes, different bioremediation ways by which textile dye effluents can be treated and microbes capable of degradation and decolorization that can be applied in order to develop cost effective methods for textile effluents treatment.


Introduction
Textile industry is considered one of the largest industry when it comes to amount of chemicals being used and thus it is the major contributor of chemical pollution in our environment. Complex chemicals and water is extensively used in textile industry during textile processing and untreated and unused leftover substances are released as waste water. This wastewater is heavily loaded with complex chemical compounds thus making this wastewater high in pH, color and lethal compounds. The mixing of this untreated wastewater into fresh water resources like ponds, rivers, streams, canals etc. is increasing water pollution ultimately affecting the aquatic biota [1, 2]. Textile industry effluents are a mixture of many polluting substances of complex chemical nature including organochlorine-based pesticides to heavy metals and these are mostly important parts of different types of dyes being used in the dyeing process of fabrics [3]. Effective treatment of fabric industry discharge has thus become a major ecological problem. Because it is very difficult to eradicate dyes from textile waste discharge, there must be some proper treatment processes for effluents to be released into the environment without affecting environmental health [4, 5]. In a broader sense, frequent encounter with textile dye effluents is a potential health threat and this has been reported to be direct or in direct cause of various health problems which include suppression of immunity system, autoimmune diseases, various complications of respiration, blood circulation, damage to central nervous system and neurobehavioral disorders, allergic reactions, different type of cancerous diseases, vomiting, copious diarrhea, tissue damage, eye skin disorders, eye infections, lung related problems are names to few [6]. Now-a-days, growing trend toward the practice of using microorganisms for environmentally friendly treatment of textile effluents has gain importance. Some microbes are reported to have potential to break down or absorb a large number of textile dyes effluents [7, 8].
In fact, some microorganisms which belong to different taxonomic groups such as different strains of bacteria, white rot fungi and algae also have ability to break down and decolorize various complex colored compounds that otherwise are difficult to remove from environment Unluckily most of these remain in the environment because of their high consistency against physical, chemical and microbial degradation and thus are continues threat to environment or to flora and fauna [31].

Textile industry dyes
Due to coloring properties, different type of dyes and pigments are widely used in textile industry as well as in other industries such as paper, plastics, leathers, and cosmetics industry. Large amount of water is used in textile processing that is being released untreated into the environment [32]. Dye effluents are one of the main pollutants carried to the environment, mainly by textiles industries. Classification of dyes depends upon their applications and chemical nature. There are two main components of a dye, first a group of atoms that causes dye color is known as chromophores and second are electron withdrawing or donating constituents of chromophores, called auxochromes hydroxyl (-OH) groups. One the basis of their chemical properties, auxochromes can fit into the following dye classes ; reactive, direct, acid, basic, disperse, pigment, vat, anionic, Sulphur, solvent and disperse dyes [34]. Azo dyes dominate with the maximum diversity of colors and are usually considered as largest class of dyes owing to their coloring properties [35]. Azo, anthraquinone and phthalocyanine are three most commonly used dyes that have high toxicity and also carcinogenic properties [11]. Because these are easy to manufacture, azo dye occupy approximately 80% of yearly making of commercial dyes throughout the world. Azo dyes (Table 1), as named on the basis of one or more azo bond (R1-N=N-R2) present in them, account for approximately 60-70% of all textile dyestuffs used in current era [36]. Because of their poor absorbability to the fibers and because of their constant chemical nature, their removal is very difficult from waste water by the usual treatments [37].     Table 6. Some of other algal species actively involved in degradation process are following Algal sp. References Chlorococcum sp.
[82] Spirogyra rhizopus [83] Role of fungi in degradation and decolorization of textile dyes (Mycoremediation) The practical of usage of fungi for achieving bioremediation of polluted soils and textile effluents is termed as mycoremediation. Different problematic substances such as petroleum hydrocarbon compounds, polychlorinated biphenyls, heavy metals (by biosorption), phenolic derived compounds, degradation resistant pesticides etc. have been reported to be degraded through mycoremediation.
These hazardous compounds are used by fungi as the nutrient source in this process of mycoremediation and changed them into simpler forms also causing their decolorization (  Different fungal species are also capable of reducing heavy metals from waste laden water some of which are listed in (Table 9). Trametes versicolor is an specie of interest because of its ability to decolorize different colored compounds released from textile and other industries [109, 110].

Fungal enzymes and biodegradation
Fungi are the good producer when it comes to enzyme production. Surface culture method is used to produce commercial fungal enzymes [111]. White rot fungi are very successful entities with extraordinary lignin degradation ability. Extracellular oxidoreductases enzymes such as laccases, peroxidases and oxidases are produced by WRF that perform biodegradation [112]. The main role of these enzymes is to degrade lignin but because these enzymes are not substrate-specific these are also capable to break different aromatic persistent compounds (Table 10) involved in causing environment related issues [113]. WRF are better dye-decolorizers as compared to prokaryotes. P. chrysosporium is a principle decolorizer compared to other microbes, whose decolorizing capability is due to lignin peroxidase (LiP) and manganese peroxidases (MnP) [114]. The survival of fungi depends upon their ability to adjust their metabolism according to changing level of carbon and nitrogen. Intracellular and extracellular enzymes are produced by fungi to achieve this metabolic activity and these further carry out nonspecific degradation of different resistant effluents such as aromatic hydrocarbons, organic waste, dye compounds etc. Nonsubstrate specific nature of fungal enzyme systems is the main reason behind their ability to disintegrate colored and metallic effluents [121]. Similar enzymes because of their low specificity for substrate (lignin) have also been employed successfully in the degradation of dyes. Only a few reports have shown the capability of Brown-rot fungi to remove various dyes [80].

Enzymatic degradation of textile effluents
Enzymes have several beneficial characteristics related to degradation. When it comes to the amount and composition of effluents, the most polluting effluent released from industrial sector is textile wastewater [122,123]. The disadvantages that we face while using microorganisms for degradation activities can be overcome easily by using enzymes as an alternate [124]. Enzymes have the capability to efficiently break down a target pollutant (Table 11) with no effect on the other constituents in the wastewater. Therefore, pollutants that are quite resistant toward disintegration can easily be degraded by enzymes. Hydrolases, dehalogenases, transferases and oxidoreductases are some of the most important enzyme classes actively involved in bioremediation processes. An example of enzyme active in removal of pollutants is Laccase [125] . Extracellular enzymes of ligninolytic fungi, white rot fungi are important for disintegration of dyestuff and also lignin [126]. It is advantageous to use WRF for degradative purposes then bacteria. The non-substrate specific nature of extracellular enzymes such as LiP, MnP, laccase and Mn-independent versatile peroxidases (VP) give white rot fungi this extraordinary efficiency to break down resistant aromatic color compounds [127]. Microbial enzymes have many desirable properties such as they give maximum possible yields, their genetic makeup can easily be manipulated, easy availability due to absence of seasonal fluctuations and high growth of microbes than enzymes produced by plants and animals. Microbial enzymes are also important in that they have high stability and are easy to produce. Several dyes have been reported to be decolorize by white rot fungi which shows that these entities are an important alternative for management of industrial dye containing effluents [128]. It was reported in another study that WRF breaks down Crystal Violet dye via Ndemethylation The main reaction of azo dyes disintegration by bacteria includes breakage of azo bonds by an azoreductase enzyme and an electron donor. Azo dyes containing textile waste water can be bio-treated effectively by developing treatment processes that uses azoreductase producing microbes. These azoreductase enzymes can catalyze the cleavage of azo compounds by reduction [136].
The most important bacterial enzymes involved in degradation activities are azoreductases (Table 12). Azoreductases cause breakage of azo bonds (-N = N-), producing aromatic amines which are further converted into CO2 and H2O under aerobic environments [137].

Yeast degradation ability
The large absorption of dyes, heavy metals, colorants makes the yeast a good microbe for bioremediation of azo dyes (

Degradation of textile effluent through biosorption
The absorption or uptake of chemicals by microbial mass is known as biosorption or more precisely it can also be described as " the capability of microbes to amass heavy metals from wastewater through metabolic or physiochemical paths of absorption"