Microbial resources mediated bioremediation of persistent organic pollutants

Chapter 19

Microbial resources mediated bioremediation of persistent organic pollutants Siddharth Boudh1, Jay Shankar Singh1 and Preeti Chaturvedi2 1

Department of Environmental Microbiology, Babasaheb Bhimrao Ambedkar University, Lucknow, India, 2Aquatic Toxicology Laboratory,

Environmental Toxicology Division, CSIR—Indian Institute of Toxicology Research, Lucknow, India

Chapter Outline 19.1 Introduction 19.2 Characteristic and structural properties of persistent organic pollutants 19.3 Principle of microbial degradation of persistent organic pollutants 19.4 Bacterial mediated degradation of different types of persistent organic pollutants 19.4.1 Degradation of pesticides 19.4.2 Degradation of polycyclic aromatic hydrocarbons 19.4.3 Degradation of polychlorinated biphenyls 19.5 Fungi mediated degradation of persistent organic pollutants

19.1

283 284 284 289 289 290 290

19.5.1 Lignin peroxidase 19.5.2 Manganese peroxidase 19.5.3 Laccase 19.6 Algae mediated degradation of persistent organic pollutants 19.7 Actinomycetes mediated degradation of persistent organic pollutants 19.8 Conclusion and future challenges Acknowledgments References Further reading

291 291 291 291 292 293 293 293 294

290

Introduction

Persistent organic pollutants (POPs) are bioaccumulative, toxic chemicals persistent in nature that do not break down easily in the environment. The POPs are synthetic chemicals that can cause harmful effects on human health and animals. POPs are chemically stable and do not readily degrade in the environment. POPs are lipophilic (have an affinity for fats) and easily soluble in fat and biomagnified as they move up through the food chain. They can travel long distances on wind and water and can be found in regions far away from their points of origin or use. These compounds are semivolatile in nature and because of their physicochemical property they can occur either in vapor phase or can be absorbed in atmospheric particles. These properties help them in their wide ranging transportation in the atmosphere. POPs have become widespread environmental contaminants and a global problem, and due to rapid industrial development and human activities POPs have become lethal to mankind. Some major organic chemical compounds that cause contamination in environment are organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, and dibenzofurans (Ersekova et al., 2014). Various remediation technologies are being developed to treat these pollutants. These compounds stay intact within the environment massively for a long time period because of their nonbiodegradable nature, that is, they resist photolytic, chemical, and biological degradation. In addition, these compounds are of great concern because of long-range transport, persistent nature, and bioaccumulation tendency; once they enter into the food chain they get accumulated in the fatty tissue of the human body and pose a risk to cause adverse effects to the environment, wildlife, and human New and Future Developments in Microbial Biotechnology and Bioengineering. DOI: https://doi.org/10.1016/B978-0-12-818258-1.00019-4 © 2019 Elsevier B.V. All rights reserved.

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health (Kallenborn et al., 2012). Information on the concentration of POPs is increasing rapidly. Taken together the data and availability of the information on POPs have allowed an understanding of the environmental fate.

19.2

Characteristic and structural properties of persistent organic pollutants

Pure aldrin is white whereas technical grades are tan to dark brown. It is used for termite control. It is toxic to human but much less toxic to plants. Dieldrin use is restricted to control wood borers, termites, and textile pests. Dieldrin is highly toxic for fish but moderately toxic for warm blooded organisms. Endrin is white crystalline solid, used as insecticide and rodenticide. It is highly persistent in soil with a half-life of up to 12 years in some cases. It is highly toxic for fish, aquatic invertebrates, and phytoplankton. Pure chlordane is water insoluble has a melting point of 104 C. Chlordane is highly persistent in soil with an approximate half-life of 4 years. The chemical properties of chlordane are low water solubility, high stability, and semivolatility so it shows long-range transport, and has been detected in arctic air, water, and organisms. Pure heptachlor is a white crystalline solid that is insoluble in water. It is a persistent dermal insecticide with some fumigant action. It is nonphytotoxic at insecticidal concentrations and shows moderate toxicity for mammals. Dichlorodiphenyltrichloroethane (DDT) was first synthesized by Austrian chemist Othmar Zeidler, but its insecticidal properties were identified by Paul Muler in 1939. It is available in odorless, colorless crystalline, or white powder form and shows bioaccumulation in adipose tissues and biomagnification. It is mainly used on cotton crops; it is also used to control mosquito vectors of malaria in numerous countries. DDT is highly persistent in soil and its halflife extends up to 15 years, and to 7 days in air. DDT is highly toxic to fish and affects their behavior. DDT also affects temperature selection in fish. DDT is acutely toxic to birds by adversely affecting their reproduction. In humans DDT exhibits estrogen-resembling and probable carcinogenic properties. The International Agency for Research on Cancer (IARC) has classified DDT as a possible human carcinogen. Toxaphene is a waxy solid that is yellow to amber color with a chlorine/turpentine-like odor. It is most heavily used OCP. Toxaphene is highly toxic for fish and shows carcinogenic effect on rats and mice but it is nontoxic to plants. IARC has classified toxaphene and Mirex as a possible human carcinogen. Hexachlorobenzene (HCB) is a chlorinated monocyclic aromatic compound in which the benzene ring is fully substituted by chlorine. HCB is a white crystalline solid. It has been used as fungicide, in the formulation of military pyrotechnic products and tracer bullets, for impregnating paper, and as wood preservatives. It shows toxicity in aquatic animals and is a possible human carcinogen. PCBs and hexachlorocyclohexane are possible human carcinogens while polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are toxic for humans in some cases. There are several conventions held in different places on the globe to eliminate or restrict the production and use of POPs, such as the Stockholm Convention on Persistent Organic Pollutants, Convention on Long-Range Transboundary Air Pollution (CLRTAP), Rotterdam Convention on the prior informed consent procedure for certain hazardous chemicals and pesticides in international trade, and the Basel Convention on the control of transboundary movements of hazardous wastes and their disposal. Molecular structure of different types of POPs in the environment is given in Table 19.1. The Stockholm Convention is an international environmental treaty to eliminate or restrict the production and use of POPs so that the environment and human health stay protected. Initially, there were 12 POPs listed under Stockholm convention, which were grouped under Annex A, Annex B, Annex C (Table 19.2). The new list of 16 POPs, according to the Stockholm Convention on Persistent Organic Pollutants (POPs) June (2017), is shown in Table 19.3.

19.3

Principle of microbial degradation of persistent organic pollutants

Microbial degradation by the use of any kind of microorganism including bacteria, fungi, yeast, algae, and actinomycetes requires three vital components: an effective microbe, foodstuff, and nutrient. These three components are known as the “bioremediation triangle.” Microorganisms easily find their food from the contaminated environment as they consume pollutants or contaminants as a source of carbon, which provides the nutrients to grow and survive in such harsh environmental conditions. Microorganisms obtain their energy by breaking chemical bonds and transporting electrons away from the pollutant. This process of transporting electrons is termed as an oxidation reduction reaction, which changes the toxic chemical state of the pollutant. The pollutant that loses an electron is oxidized and the chemical that gains the electron is reduced. The energy released in this oxidation and reduction process is utilized by microorganisms along with carbon to increase the number of microbial cells for the degradation of pollutant more rapidly. Generally, microbes use molecular oxygen for accepting electrons but nitrate, sulfate, carbon dioxide, and iron also serve as

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TABLE 19.1 Structure of different types of POPs. Name

Molecular formula

Aldrin

Chlordane

Chlordecone

Dichlorodiphenyltrichloroethane (DDT)

Dieldrin

Endrin

Endosulfane

Heptachlor

Hexachlorobenzene

(Continued )

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TABLE 19.1 (Continued) Name

Molecular formula

Hexachlorocyclohexane

Hexabromobiphenyl

Mirex

Polychlorinated dibenzofurans (PCDFs)

Polychlorinated dibenzo-p-dioxins (PCDDs)

Pentachloroenzene

Perfluorooctanoates (PFOs)

Texaphene

TABLE 19.2 The POPs listed under Stockholm Convention. Annex A: Pesticides

Aldrin, chlordane, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, and polychlorinated biphenyl

Annex B: Industrial chemical

dichlorodiphenyltrichloroethane

Annex C: By-product

Polychlorinated dibenzo-p-dioxins, dibenzofurans, and hexachlorobenzene

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TABLE 19.3 Details of 16 new POPs. S. no.

Chemical

Annex

Specific exemptions/acceptable purposes

1.

Alpha hexachlorocyclohexane

A

Production: None Use: None

2.

Beta hexachlorocyclohexane

A

Production: None Use: None

3.

Chlordecone

A

Production: None Use: None

4.

Hexabromobiphenyl

A

Production: None Use: None

5.

Hexabromocyclododecane

A

Production: As allowed by the parties listed in the Register of specific exemptions. Use: Expanded polystyrene and extruded polystyrene in buildings in accordance with the provisions of the Part VII of Annex A

6.

Hexabromodiphenyl ether and heptabromodiphenyl ether (commercial octabromodiphenyl ether)

A

Production: None Use: Articles in accordance with the provisions of Part IV of Annex A

7.

Hexachlorobutadiene

A and C

Production: None Use: None

8.

Lindane

A

Production: None Use: Human health pharmaceutical for control of head lice and scabies as second-line treatment

9.

Pentachlorobenzene

A and C

Production: None Use: None

10.

Pentachlorophenol and its salts and esters

A

Production: As allowed for the parties listed in the Register in accordance with the provisions of Part VIII of Annex A Use: Pentachlorophenol for utility poles and cross-arms in accordance with the provisions of Part VIII of Annex A

11.

Perfluorooctane sulfonic acid (PFOS), its salts and perfluorooctane sulfonyl fluoride (PFOSF)

A

Production: For the use below Use: Acceptable purposes and specific exemptions in accordance with Part III of Annex B

12.

Polychlorinated naphthalenes

A and C

Production: For the use below Use: Production of polyfluorinated naphthalenes, including octafluoronaphthalene

13.

Technical endosulfan and its related isomers

A

Production: As allowed for the parties listed in the Register Use: Crop pest complexes as listed in accordance with the provisions of Part VI of Annex A

14.

Tetrabromodiphenyl ether and pentabromodiphenyl ether (commercial pentabromodiphenyl ether)

A

Production: None Use: Articles in accordance with the provisions of Part V of Annex A

15.

Decabromodiphenyl ether (Commercial mixture, c-DecaBDE)

A

Production: As allowed for the parties listed in the Register Use: Vehicles, aircraft, textile, additives in plastic housings, etc., polyurethane foam for building insulation, in accordance with Part IX of Annex A

16.

Short-chain chlorinated paraffins (SCCPs)

A

Production: As allowed for the parties listed in the Register Use: Additives in transmission belts, rubber conveyor belts, leather, lubricant, additives, tubes for outdoor decoration, bulbs, paints, adhesives, metal, processing, plasticizers

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Microbial degradation of POPs

effective electron acceptors. On the basis of the removal or degradation of a pollutant from the contaminated environment, there are basically two important systems, which are as follows. In situ remediation practices are applied for the treatment of pollutants at the contaminated site where they are found. This technique was originally developed as a cheap, more reliable, and standard alternative method to clean up water bodies and soil sediments contaminated with organic and inorganic pollutants such as hydrocarbons, pesticides, chemical solvents, and heavy metals (Boudh and Singh, 2019). It involves an aerating system to stimulate microorganisms for the degradation of an organic contaminant in the unsaturated zone, that is called as “bioventing” or in saturated soils and groundwater. It is a highly superior method for the management of contaminated environment and enhances chemostatic abilities and catabolic activities of microorganisms. The chemotaxis increases the bioavailability of pollutants under the influence of chemical gradient to the microbial cell and thus plays a crucial role in the transformation of the chemical pollutant (Jain et al., 2005). It is a biological process that involves the removal of contaminated soil and water from the contaminated site, which is then treated at another place to enhance microbial degradation of organic pollutant with help of indigenous microbial population and supplying nutrients. It requires the bioreactor and nutrients to speed up the breakdown of organic pollutants into their by-products by the process of biostimulation. The continuous entrance and subsequent activities of POPs form industrial and agricultural sources into the environment, unceasingly deteriorating the quality of environmental flora and fauna. Due to high exposure and induced toxicity, POPs have become the major contaminants in the list of priority pollutants. Over the past few years, various conventional treatment technologies including incineration, land filling, solvent extraction, chemical reduction, solidification alkali metal reduction, and stabilization have been utilized for the removal and degradation of POPs from industrial wastes and soil sources (Perelo, 2010). But, due to the expensive cost and maintenance of sophisticated instruments, and requirement of a high range of chemicals and manpower for the eradication of POPs, such traditional methods are not very effective or reliable for complete degradation or removal of such pollutants regarding their complexity and chemical toxicity in terms of generating an enormous amount of sludge and secondary pollutants, as well as their lethal effects on the working and local area pollution (Chandra and Kumar, 2015). Hence, to overcome these problems of traditional technologies, microbial degradation technology is emerging as a sustainable and cost-effective tool in the field of bioremediation, which explored the new ways and potential of microorganisms (including bacteria, fungi, algae, and actinomycetes) for the significant remediation of POPs from the environment (Fig. 19.1). Microbial technologies have a number of advantages over chemical technologies due to their nontoxicity and safety from an environmental point of view. Microorganisms are able to utilize minerals, hydrocarbons, lindane, pesticides, and metals by converting or changing their physical and chemical nature into simpler and less or nontoxic products and to use for their growth, nutrition, and metabolic activities (Boudh and Singh, 2019; Gadd, 2010). Microbial communities have the ability to biotransform highly toxic and hazardous states of organic pollutants into a nontoxic product by developing different kinds of resistant mechanism and enzymatic reactions (Ortiz-Hernandez et al., 2013). Different types of microbial enzymes like laccase, lignin peroxidases (LiP), manganese peroxidases (MnP), hydrolases, glutathione (GSH) S-transferases (GST), and oxidoreductases produced from bacterial, algal, and fungal sources have been well reported for the detoxification and degradation/removal of POP by catalyzing various enzymatic reaction of

Bacterial

Mycobacterium, Pseudomonas, Bacillus, Alcaligenes, Burkholderia, Escherichia, Azotobacter, etc.

Fungi

Pleurotus ostreatus, Pestalotiopsis, Trametes versicolor, Lentinulae dodeshas, Aspergillus, etc.

Algae

Caepidiumantarticum, Dermarestia, Focus, Ascophyllumnodosum, etc.

Actinomycetes

Arthrobacter, Brevibacterium, Clavibacter, Corynebacterium, Mycobacterium, Rhodococcus, etc.

FIGURE 19.1 Microbes involved in degradation of POPs.

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oxidation of aromatic compounds, hydrolysis and ring cleavage, reduction of nitro and amino groups, dechlorination, and addition of functional groups to convert their chemical structure (Ramakrishnan et al., 2011). Thus bioremediation is the most advantageous and worthwhile process by which microorganisms are stimulated to quickly reduce or remove hazardous organic pollutants and contaminants to environmentally safe levels in soil sediments and other matrices.

19.4

Bacterial mediated degradation of different types of persistent organic pollutants

Bacteria are one of the most diverse and largest representatives of a taxonomic unit that is ubiquitously distributed into the environment and plays an important role in the field of bioremediation for the degradation and detoxification of POPs and xenobiotics compounds from the contaminated site in an in situ and ex situ manner. Microbial remediation of POPs by the use of bacterial species is a viable biodegradation technology to transform and mineralize harmful offensive organic pollutants into inoffensive metabolites. It is a feasible remedial practice that exploits the metabolic flexibility and adaptability of a microorganism to survive under such a contaminated environment and degrade hazardous pollutants. The biodegradation process through microbes can take place either aerobically or anaerobically and degrade pollutants into their final products of biomass, carbon dioxide, and methane. Aerobic degradation process involves a variety of aerobic bacteria such as Pseudomonas, Bacillus, Micrococcus, and Escherichia, which are able to grow in the presence of oxygen and utilize molecular oxygen as a terminal electron acceptor and cosubstrate for the conversion or biotransformation of the pollutant into of carbon dioxide. The anaerobic process is completed in the absence of oxygen with help of anaerobic bacterial strains of Desulfovibrio, Methanococcus, Methanospirillum, Desulfotomaculum, etc., which react with the functional group of organic pollutants and withdraw the electron from it and cause breakdown or degradation of such pollutant leading to the formation of methane and inorganic salts. Anaerobic microbial degradation is more useful for transformation or mineralization of tetrachloro-ethylene, chlorinate phenols, and nitro-substituted aromatics in comparison to the aerobic process. Anaerobic degradation also involves methanotrophic and methanogenic bacteria due to the formation of methane gas as by-product of anaerobic digestion. Methanotrophs are able to utilize this methane for their metabolic activity and this simultaneously results in the formation of carbon dioxide as a final end product (Jha et al., 2011). During methanogenesis, these microbes oxidize methane gas by the use of oxygen and convert it into carbon dioxide. Methanotrophs also produce methane monooxygenase enzyme, which can react with methane and degrade an extensive range of chlorinated hydrocarbons. In addition to methanotrophs, aerobic and anaerobic degradation processes also utilize nitrifying and denitrifying, sulfate reducers, and ferric ion reducers bacteria act as an electron acceptor that favors the complete mineralization of pollutant depending on their electron acceptor availability (Diaz, 2004). Many bacteria have been recognized and reported for their degradative potential to utilize many organic pollutants including PAHs, PCBs, DDT, etc.

19.4.1

Degradation of pesticides

Pesticides are extensively used in agricultural practices and continuously accumulate into soil systems, which persists for years due to their problematic degradation (Boudh and Singh, 2019). Generally, aldrin, dieldrin, DDT, benzene hexachloride (BHC), endrin, carbamates, organochlorines, etc. are used, which are highly toxic and harmful for nontargeted organisms. Bacterial reduction of these pesticides represents a safe and natural way for environmental management. A variety of bacterial strains have been reported from different habitats such as soil, wastewater, activated sludge, and animal feces that can easily metabolize DDT and convert it into dichlorodiphenyldichloroethane (DDD) under both aerobic and anaerobic conditions (Lai and Saxena, 1982). Most pesticides are degraded via enzymatic pathway due to the chemical complexity of pesticides. Enzymatic degradation of pesticides involved two-stage metabolism processes. There are mainly three types of enzymes responsible for the proper degradation and detoxification of pesticides. In first stage metabolism are hydrolases, esterases, and mixed function oxidases (MFO), whereas, second stage metabolism requires GST. Hydrolases commonly work in the absence of redox cofactors. Esterases catalyze the hydrolysis of carboxyl esters, amides and phosphate esters, etc. MFO catalyzes both oxidation and reduction reactions by incorporating one oxygen molecule into the substrate while the other one is reduced to water. MFO comprises two membrane protein enzymes: cytochrome P450 and NADPH-dependent cytochrome P450 reductase. The Alcaligenes eutrophus A5 was found capable for the aerobic degradation of DDT (Nadeau et al., 1994). This bacterium possess dioxygenase enzyme that can cause oxidation of DDT at ortho and meta position resulting in an unstable intermediate dihydrodiol compound. This compound further degraded into two hydroxylated compounds by dehydrogenase and finally through meta-cleavage results in a terminal end product of 4-CBA (chlorobenzoate).

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The OCPs are highly toxic and carcinogenic and their extensive use in the agricultural practices create a dangerous threat to the environment. The Alcaligenes faecalis JBW4 was identified and reported (Kong et al., 2013) for the effective degradation of potential for endosulfan residue of organochlorine.

19.4.2

Degradation of polycyclic aromatic hydrocarbons

PAHs are ubiquitous, stable recalcitrant pollutants in the environment with high hydrophobicity and low solubility and therefore are not easily eliminated from the atmosphere (Brassington et al., 2007). A large number of bacteria that can easily metabolize PAHs completely into carbon dioxide and other metabolic intermediates have been identified (Pseudomonas, Mycobacterium, Alcaligenes) (Harayama, 1997). Several low molecular weight PAHs (naphthalene and anthracene) are easily degraded by the soil microorganisms. Streptomyces sp. ERI-CPDA-1 isolated from oil contaminated soil was able to degrade 99.14% naphthalene, 17.5% phenanthrene, and 98.25% diesel oil in just 7 days (Balachandran et al., 2012). Brevibacterium sp. DPO1361 and Pseudomonas sp. F274 is reported for the effective reduction of fluorine via monooxygenation resulting in the formation of phthalate followed by converting into protocatechuate. Enzymatic degradation of phenanthrene was observed by Pseudomonas mendocina with help of two active enzymes: one is PAH dioxygenase and the other is catechol-2-3 oxygenase (Tian et al., 2002). Dioxygenases enzymes are mainly responsible for the degradation of PAH by oxidizing its benzene ring and from cis-dihydrodiols. These dihydrodiols are further catalyzed by dehydrogenase to form dihydroxylated intermediates that can finally be metabolized into carbon dioxide and water through catechols. Sometime complex bi- and tricyclic PAHs are degraded by the oxidation of multicomponent enzyme system of naphthalene dioxygenase that contained NADH oxidoreductase, ferredoxin, and an oxygenase component. Biotransformation of naphthalene was easily carried out by bacteria as it is the most simple and soluble form of PAH catalyzed by naphthalene dioxygenase (Goyal and Zylstra, 1997), and attacks the aromatic ring to form cis-(1R, 2S) dihydroxy-1,2-dihydronaphthalene. This intermediate is simultaneously dehydrogenated to 1,2-dihydronaphthalene via cis-dehydrogenase and subsequently metabolized to salicylate hydroxyl-2H-chromene-2-carboxylic acid, cis-o-hydroxybenzyl pyruvate, and 2-hydroxybenzaldehyde. The salicylate is subsequently converted to catechol through decarboxylation followed by ring fission in meta- and ortho-pathways.

19.4.3

Degradation of polychlorinated biphenyls

PCBs are hazardous pollutants and extremely toxic due to their long-term persistence. They include chlorinated organic pollutants such as pentachlorophenol (PCP), tributyltin, tetrachlorophenol, and atrazine. These PCB compounds can be degraded by several aerobic and anaerobic bacterial strains such as Pseudomonas, Bacillus, Rhodococcus, Burkholderia (Ahmed and Focht, 1973). These bacteria transform PCBs into their metabolic products by utilizing biphenyl as a sole carbon source for their growth and nutrition. PCBs are mostly degraded by microbial biphenyl dioxygenase via metacleavage pathway, which results in two major intermediate compounds: trichloroacetic acid (TCA) and CBA. Various types of biphenyl (Bph) dioxygenase (BphA, BphB, BphC, and BphD) are reported for the catabolic transformation of PCBs via biphenyl upper (Bph) pathway and lower pathway. Bph dioxygenase is a three-component system enzyme having an electron transport chain (that catalyzes the transportation of electron from NADH to terminal dioxygenase) ferredoxin and ferredoxin reductase (Furukawa et al., 2004). Pseudomonas pseudo alcaligenes KF707 was investigated for the efficient degradation potential of PCBs with reduced accumulation of CBA (Ohmori et al., 2011). It follows an upper pathway enzymatic system. The degradation of PCB starts with the formation of 2,3-dihydro-2,3-diol by action of 2,3 dioxygenase (BphA) by introducing molecular oxygen into biphenyl molecule followed by the action of dihydrodiol dehydrogenase (BphB) leading to the dehydrogenation of 2,3-dihydro-2,3-diol into 2,3-dihydroxybiphenyl. This 2,3-dihydroxybiphenyl is cleaved by 2,3-dihydroxybiphenyl 1,2 dioxygenase (BphC) at meta position and results in the formation of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid. This intermediate is catalyzed by 2-hydroxy-6-oxo-6-phenylhexa-2,4-dieneoate hydrolase (BphD) and finally results in an end product of chlorinated benzoic acid (Ohmori et al., 2011).

19.5

Fungi mediated degradation of persistent organic pollutants

Fungi are the most diverse and primitive saprophytic, eukaryotic organisms having the ability to grow on different organic and inorganic substrates present in the environment. They play an important role in the decomposition of efficient decomposers of lignin and cellulose. Lignin and cellulose are long chain organic pollutants that are difficult to degrade. The use of fungi for biodegradation of such long chain organic compounds has intensified in recent years.

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Other recalcitrant organic pollutants including phenols, PCBs, petroleum, PAHs, oils, wood, textile dyes, pesticides, and leather can also be converted into harmless products by use of fungi. The process of utilization of fungal species for the degradation and biotransformation or remediation of toxic pollutants is known as mycoremediation or mycodegradation. The increasing use of such remediation technology provides a new opportunity for the biodegradative pathway to decontaminate the environment. Mycodegradation is recognized as very useful, cost-effective, and eco-friendly way for the removal of POPs. White rot fungi are most probably used for degradation and reduction of such pollutants by using a wide array of intracellular as well as extracellular enzymes includes laccases, lipases, LiP, MnP, versatile peroxidase, and sometimes cytochrome P450 (Bhattacharya et al., 2012). These enzymes have high oxidative potential and ability to oxidize bulky substrates into a valuable product through their flexible machinery conjoining with secondary metabolites. The white rot fungi have the significant competency to reduce complex polysaccharide, lignin, PAHs, and PCBs by involving enzymatic biotransformation from soil sediments and wastewaters into a new simpler form. Phanerochaete chrysosporium is a member of the white rot fungi and is one of the best strains that has been extensively reported for the enzymatic reduction of fluorine and benzopyrene into quinone through secreting LiP and MnP. Several other fungal strains like Pleurotus ostreatus, Pestalotiopsis, Trametes versicolor, Lentinula edodes, Aspergillus, and certain molds can greatly extend the process of remediation of insecticide endosulfan, imazalil, thiophanate, ortho-phenyl phenol, atrazine, chlorpyrifos, starch, pectins, fats, and keratin even more profitably via oxidative and reductive processes of ligninolytic enzymes (Chandra and Chowdhary, 2015). Ligninolytic enzymes play a crucial role in the delignification of lignocellulosic waste materials, degradation of toxic pollutants, and removal of xenobiotic substances, wastewater treatment and decolorization of dyes, and conversion of high molecular weight coal fraction to low molecular weight coal fractions (Yadav and Yadav, 2015).

19.5.1

Lignin peroxidase

It is an extracellular heme peroxide of white rot basidiomycetes belongs to the family of oxidoreductases and its various isoforms are known to exist from other fungal sp. also. Aromatic rings are oxidized by LiP enzymes, which are moderately activated via electron donating substitutes. It is mainly responsible for the oxidative cleavage of Cα Cβ bonds in nonphenolic aromatic substrates to form highly reactive aryl cation radicals (Ruiz-Duen˜as et al., 2008).

19.5.2

Manganese peroxidase

MnP is a glycoprotein dependent H2O2 oxidize Mn (II) to Mn (III) and catalyzes the degradation of phenolic substrates with the formation of phenoxy radicals (Wong, 2009). Mn (III) acts as a chelating agent with organic acids having high redox potential and oxidizes phenolic compounds.

19.5.3

Laccase

Laccases are multinuclear, multicopper containing enzymes that catalyze the oxidation of various organic substrates including phenol with the simultaneous reduction of molecular oxygen to water (Alcade, 2007). They are not only produced from white rot fungi, but also from the members belong to the family Basidiomycetes, Ascomycetes, and Deuteromycetes. Laccases have been used for various applications in organic synthesis for example in the oxidation of functional groups, the coupling of phenol and steroids and medical agents, that is, anesthetics, antiinflammatory, antibiotics, etc. (Chandra and Chowdhary, 2015). Rather than these enzymes, there are several other extracellular oxidoreductases and auxiliary oxidases like glucose oxidase, glyoxal oxidase, aryl alcohol oxidase, pyranose 2-oxidase, and cellobiose/quinone oxidoreductase that may also contribute into the biodegradation of organic pollutants (Wong, 2009).

19.6

Algae mediated degradation of persistent organic pollutants

The utilization of micro- and macroalgae for the treatment of POPs, including recalcitrants, xenobiotics and pesticides, phenolics, and heavy metal compounds from wastewater and soil is known is phycoremediation. Phycoremediation is gaining greater importance and attention from the environmental perspective due to its excellence in photosynthesis, energy metabolism, and survival capability under harsh stress condition of extremely toxic pollutants. In marine and aquatic ecosystem algae, cyanobacteria, and protozoan species are effectively involved in the degradation process of pesticides, polyaromatic compounds, and hydrocarbons. Various algal species like Rhodococcus, Caepidium antarcticum, Dermarestia sp., Fucus sp., and Ascophyllum nodosum are investigated and reported for efficient degradation of

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New and Future Developments in Microbial Biotechnology and Bioengineering

hydrocarbons by producing certain hydrocarbon-degrading enzymes (Chekroun et al., 2014). Generally, hydrocarbons, lindane, pesticides, organ chlorines, organophosphates, carbonates, nitro phenols, cyclic aromatic, and xenobiotic compounds severely affect the environment by their widespread use in agricultural and industrial management practices (Boudh and Singh, 2019). These compounds dangerously produce hazardous health effects in all living beings due to their toxic persistent nature. Therefore phycoremediation is considered as the most desirable technology for the removal and detoxification of harmful products into a harmless product through bioaccumulation strategies or by doing CO2 fixation. Degradation of 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene was investigated by (Cruz-Uribe et al., 2007) by the use of three different algal species Acrosiphonia coalita (green alga), Porphyra yezoensis (red alga), and Portieria hornemannii and observed that all the three species are capable to reduce explosive toxic TNT compound to 2-amino,4,6 2,4-Dinitrotoluene (DNT) and 4-amino, 2,6, dinitrotoluene under axenic conditions. Algae and cyanobacteria utilize organic pollutants as their nutritive and growth source. An investigation of gene expression of algae Chlamydomonas reinhardtii under the higher concentration of TNT was reported by Patel et al. (2004). According to their investigation, B158 responsive genes were differentially expressed in the presence of TNT stress, resulting in the upregulation of the genes responsible for energy metabolism and photosynthetic ability. The most common feature of algae and cyanobacteria is their photosynthetic quality because they contain chlorophyll pigment machinery. This photosynthetic ability plays an important role in the degradation of organic pollutants. The mechanism of degradation of any organic or xenobiotic pollutant can be processed in three phase route by algae and other photosynthetic organisms (Torres et al., 2008). Phase I process involves the oxidation, reduction, and hydrolysis reaction by addition of reactive functional groups, which transforms lipophilic recalcitrants or xenobiotics into watersoluble compound and NADPH with the help of Cyt P450 and Cytb5. In Phase II large and polar groups like GSH and glucuronic acid are added to xenobiotics and finally in Phase III followed by Phase I and II, xenobiotic compounds are compartmentalized into cell walls or vacuoles (Torres et al., 2008). Like, bacterial and fungal organisms, algal species also produce some valuable enzymes, which are very useful for eradication of POPs (Chekroun et al., 2014).

19.7

Actinomycetes mediated degradation of persistent organic pollutants

Actinomycetes belong to the phylum of Gram-positive bacteria having great economic importance in respect to agriculture and forestland due to their valuable involvement in the soil ecosystem. These microorganisms have a prodigious potential for the degradation and detoxification of a variety of organic pollutants like pesticides, hydrocarbons, and aromatic compounds. Biodegradation of contaminated pesticides like organophosphates, acetanilide, organochlorines, carbamates, sulfonylurease, and s-triazines is very possible with the use of actinomycetes (De Schrijver and De Mot, 1999). Actinomycetes also play an important role for the degradation of complex polymers and help in the bioremediation of terrestrial and aquatic soil ecosystem through the recycling of contaminated toxic organic pollutants (Goodfellow and Williams, 1983). Actinomycetes strains produce a variety of extracellular peroxidase enzymes, cellulose, and hemicellulose degrading enzymes, which are found to most suitable for the solubilization of lignin and its related complex polysaccharide compounds into the simple form of sugars (mono- and disaccharides) by using hydrolases (Monson et al., 2001). Currently, petroleum hydrocarbons are becoming a major environmental soil contaminant, which raises serious concerns regarding their extensive use in industries and households in the form of energy and chemical substances. Some useful research investigations explored the importance of Streptomyces flora as a good bioagent for the biotransformation and biodegradation of chlorinated hydrocarbons (Barabas et al., 2001). Actinomycetes microorganisms related to the genus Arthrobacter, Brevibacterium, Clavibacter, Corynebacterium, Mycobacterium, Rhodococcus, Streptomyces, etc. have been reported for their extreme potential for the remediation of pesticides through cometabolism. These microbes could utilize substrates of pesticides as a source of energy for their growth and metabolizing into a nutrient source of carbon, phosphorus, nitrogen, and sulfur (De Schrijver and De Mot, 1999). Polychlorinated phenols, DDT, aldrin, and s-triazines have been found to detoxified and metabolized into their intermediates under aerobic as well as an anaerobic environment with help of actinomycetes. Most PCPs are degraded by removing chlorine substituents and cleaving ring whereas mono- and dichlorophenols are degraded only after ring cleavage. Cytochrome P450s are imperative enzymes that are mainly responsible for biodegradation mechanism of actinomycetes. These enzymes are related to monooxygenases and having broad substrate specificity in actinomycetes catalyzes the oxidation of substrate for the remediation purpose of pollutants. Mycobacterium chlorophenolicum PCP-1 is reported for degradation of PCP by sequential dechlorination catalyzed by microbial enzyme action (Apajalahti and

Microbial resources mediated bioremediation of persistent organic pollutants Chapter | 19

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Salkinoja-Salonen, 1987). It can easily cultivate at higher concentration PCP because it could consume PCP as its sole carbon source for its growth and energy. Dechlorination starts with para- and ortho-hydroxylation of PCP catalyzed by membrane-bound P450 and cytoplasmic hydroquinone dehalogenase enzymes yielding small intermediates and finally degraded into maleic acid (Uotila et al., 1991). Biodegradation of DDT to DDD and Pentachloronitrobenzene (PCNB) to pentachloro aniline was also reported by Nocardia and Streptomyces strains. However, actinomycetes are a considerable source of treatment for POPs, but, a keen advancement is needed for their better exploitation in the field of bioremediation for the complete mineralization and detoxification of harmful organic chemicals.

19.8

Conclusion and future challenges

Contamination by POPs is widespread because POPs are chemically stable, bioaccumulate, and circulate globally by means of atmosphere, oceans, and other pathways. This article is about the bioremediation of POPs by microbial resources and future research for the next upcoming years. By upgrading the basic source inventories, national and international supplemented broad area coverage of POPs concentration and its bioremediation reliability improvements may be beneficial databases for POPs and its bioavailability from groundwater, soil, and sediments. Expert research communities who have studied more traditional POPs can shift their studies to newer classes of POPs, possibly more polar persistent compound metabolites, and their biodegradation, toxicity mechanism, trace gas/ particles, and other contaminants, which cause a serious threat to the entire globe. Hence more detailed studies could be done that can help in understanding the important aspects about controlling this global problem of POPs and development in quantitative assessment can be undertaken in the exposure and air exchange of the POPs. Further improvement is required and it is very challenging to the researchers to develop new techniques in identification and quantification of new POPs. Government bodies need to make stricter rules and regulations on releasing and exposing the harmful contaminants into the environment. Anthropogenic activities have caused huge pollution to the environment. The POPs and organic pollutants such as PAHs, pesticides, industrial chemicals, and by-products like PCDFs are toxic to the environment and represent a threat to humans and wildlife because they bioaccumulate in adipose tissue as they are soluble in fat and insoluble in water and persistent in nature, making POPs extremely dangerous. Bioremediation is a costeffective, safe, and remarkable alternative to traditional physicochemical practices for the remediation of these POPs at a contaminated site, as it can selectively degrade the pollutants without harming the site or its indigenous flora and fauna. However, despite being an effective solution to the contaminated environmental problems bioremediation technologies have limited applications due to the substrate and environmental variability and limited biodegradative potential of microorganism. Fundamental to these concerns is the management of POPs (POPs) including various other harmful chemicals that are lethal to humans and wildlife. Therefore the health problems, environmental concerns, and various techniques for biodegradation of POPs need to be addressed before their extensive implementation.

Acknowledgments The authors thank the Head, Department of Environmental Microbiology, Babasaheb Bhimrao Ambedkar University, Lucknow, for providing facilities and encouragements. Siddharth Boudh is thankful to UGC, New Delhi, for financial support from Rajiv Gandhi National Fellowship (Award Letter No. F.1-17.1/2013-14/RGNF-2013-14-SC-UTT-37387).

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Further reading Boudh, S., Tiwari, S., Singh, J.S., 2017. Microbial-mediated lindane bioremediation. In: Singh, J.S., Seneviratne, G. (Eds.), Agro-Environmental Sustainability. Springer, The Netherlands, pp. 213 233.