Keywords

4.1 Introduction

Environmental pollution by various pollutants poses irresistible and irreparable deterioration of air, water, and soil. Major sources of pollutants are industrial effluents, mining activities, sewage sludge, inadequate use of fertilizers, pesticides, and insecticides, etc. (Peng et al. 2008; Bagul et al. 2015; Varjani 2017). All these pollutants can be divided into two major groups, (i) organic and (ii) inorganic, which can cause adverse effect on flora, fauna, and human health (Varjani et al. 2015). Moreover, some of toxic pollutants , viz., heavy metals, polyaromatic compounds, pesticides, and radionuclides, are non-biodegradable in nature which renders hazardous impact on humans and environment, globally (Berreck et al. 1992; Peng et al. 2008; Abdel-Shafy and Mansour 2016). Hence, there is need to develop treatments that can minimize or even eliminate such pollutants from environment (Varjani 2017). There are a number of physicochemical and biological processes that are commonly employed to remove pollutants from industrial wastewaters before their discharge in the environment (Fomina and Gadd 2014; Varjani and Upasani 2016). Bioremediation is the use of biological interventions of biodiversity for mitigation (and wherever possible, complete elimination) of the noxious effects caused by environmental pollutants in a given site (Peng et al. 2008; Varjani 2017). These technologies have become attractive alternatives to conventional cleanup technologies due to relatively low capital costs and their inherently aesthetic nature (Prasad 2011; Varjani et al. 2015). The aim of bioremediation is the application of biosystems such as microbes and higher organisms like plants (phytoremediation) to reduce the potential toxicity of chemical contaminants in the environment by degrading, transforming, and immobilizing these undesirable compounds (Ezeonu et al. 2012; Peng et al. 2008; Varjani 2016). Fungi prove to have high potential in the degradation of high-molecular-weight compounds and therefore are used widely to remediate environmental pollution. With the adaptability of fungi, mycoremediation could be an alternate way to ensure good cleaning efficiency during the winter when necessary growth conditions for plant-based systems are lacking (Esterhuizen-Londt et al. 2016). Therefore, remediation with fungi can be suitable method to improve environment quality and sustainability.

The purpose of this chapter is to provide knowledge about major environmental pollutants and use of fungi to treat these contaminants present in environment. The chapter also gives brief introduction to fungi and its classes.

4.2 Environmental Pollution

Rapid urbanization and intensified industrialization all over the world have posed a major risk to the environment (Prasad 2011). Environment (French word Environ = surrounding) includes biotic components like plant, animals, microbes, etc. and abiotic components like air, water, soil, light, etc. (Goel 2006; Mullai 2012). Environmental pollution is defined as negative or undesirable change in environment, which has detrimental effect on it (Varjani 2017). Natural environment consists of four interlinking systems, namely, the atmosphere, the hydrosphere, the lithosphere, and the biosphere. These four systems are in constant change, and such changes are affected by human activities and vice versa (Mullai 2012). Figure 4.1 shows the three major kinds of pollution that occur in the environment.

Fig. 4.1
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Types of environmental pollution

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Environmental pollution is classified in various groups. For instance, pollution of air is termed as atmospheric pollution; the pollution of hydrosphere or water is termed as water pollution. Pollution due to disposal of wastewater is termed as industrial effluent pollution. Similarly, indiscriminate dispersal of domestic sewage is called domestic effluent pollution . In addition to these, pollution of lithosphere or land is called soil pollution. For instance, pesticide residue contributes toward soil pollution. Urban areas are blessed with the menace of noise, which at times becomes intolerable and is called noise pollution (Khopkar 2005).

4.2.1 Major Environmental Pollutants

Pollutants are difficult to concisely define. They exist in several forms including solids, liquids, vapors, gases, ions, and mixtures of these primary states of matter. Pollutants may persist in environment for a short time (e.g., short-lived reactive chemical species with lifetime less than 1 s) or for several years (e.g., very small particles and nonreactive gases). Tables 4.1, 4.2, and 4.3 give a detailed account of major air, water, and soil pollutants, their sources, and effects.

Table 4.1 Types of air pollutants , their sources, and effects (According to Flagan and Seinfeld 1988; Sivasakthivel and Reddy 2011)
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Table 4.2 Types of water pollutants, their sources, and effects (According to Goel 2006; Bagul et al. 2015)
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Table 4.3 Types of soil pollutants , their sources, and effects (According to Ashraf et al. 2014)
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4.3 Classification of Fungi

Fungus is a large group of organisms that includes yeasts and molds as well as more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, protists, and bacteria. Figure 4.2 describes the four major classes of fungi.

Fig. 4.2
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Fungal classification

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Fungi are not able to ingest their food like animals do, nor can they manufacture their own food, the way plants do. Instead, fungi feed by absorption of nutrients from environment around them. They accomplish this by growing through and within substrate on which they are feeding. The hyphae secrete digestive enzymes which break down substrate, making it easier for fungus to absorb nutrients which substrate contains. With the growth of industry, there has been a considerable increase in discharge of industrial waste to environment, chiefly soil and water, which has led to accumulation of heavy metals, especially in urban areas (Dixit et al. 2015).

4.3.1 Ascomycetes

This phylum contains a large number of species. They commonly develop by sexual reproduction, but asexual reproduction is also common. The spores of these fungi formed inside the saclike structure called an ascus. The cells of ascomycete hyphae may contain many nuclei with septate hyphae, e.g., morels (Morchella sp.) (Ravan and Johnson 2002).

4.3.2 Zygomycetes

The zygomycetes (phylum Zygomycota) lack septa in their hyphae except when they form sporangia or gametangia. They are mostly microscopic in nature. This kind of fungi developed sexually as well as asexually. They are multinucleate and lack septa. Asexual reproduction occurs more frequently than sexual reproduction, e.g., Rhizopus (black bread mold) (Ravan and Johnson 2002).

4.3.3 Basidiomycetes

All members of the Basidiomycota produce their spores on a characteristic cell called basidium and reproduce by sexual means. Asexual reproduction occurs occasionally. A basidiomycete mycelium made up of monokaryotic hyphae is called primary mycelium, e.g., Agaricus (mushroom) (Ravan and Johnson 2002).

4.3.4 Deuteromycetes

Members of this family do not reproduce sexually . Most are thought to be derived from Ascomycota. A well-known penicillin antibiotic is derived from genera Penicillium of this phylum. A number of imperfect fungi occur widely on food. Fusarium species growing on spoiled food produce highly toxic substances such as trichothecenes, e.g., Penicillium and Aspergillus (Ravan and Johnson 2002).

Apart from this, fungi form two key mutualistic symbiotic associations, viz., lichen and mycorrhizae. Mycorrhizae are used to remediate different environmental pollutants from soil as they have association with plant roots (Ravan and Johnson 2002).

4.4 Bioremediation

“Remediate” means to solve a problem, and “bioremediate” environment means to use biological systems to solve an environmental problem such as contaminated soil or groundwater (Chibuike 2013). Bioremediation is the use of living microorganisms /plants to degrade environmental pollutants or to prevent pollution (Barr and Aust 1994; Varjani 2017). In other words, it is a technology for removing pollutants from the environment, thus restoring original natural surroundings and preventing further pollution (Thakur 2014). The goal of bioremediation is employment of biosystems such as microbes and higher organisms like plants (phytoremediation) and animals to reduce potential toxicity of chemical contaminants in environment by degrading, transforming, and immobilizing these undesirable compounds (Sasikumar and Papinazath 2003).

4.5 Mycoremediation

Mycoremediation is the use of fungi to degrade or remediate pollutants (Esterhuizen-Londt et al. 2016). A wide array of materials can be degraded or deteriorated by fungi, and extensive research on mycoremediation shows that it represents a clean method to treat soil and water without formation of metabolites which are dangerous to the environment and human health. Fungi have been recognized to degrade various compounds/materials (Berreck et al. 1992; Gaikwad and Sonawane 2012; Abdel-Shafy and Mansour 2016). Polyethylene , with a molecular weight of 4000–28,000, is bioremediated by cultivation of Penicillium simplicissimum YK (Yamada-Onodera et al. 2001). Saccharomyces cerevisiae was used for removal of heavy metals like lead and cadmium from contaminated soil and revealed 65–79% biosorption of heavy metals within 30 days (Damodaran et al. 2011). Mucor hiemalis was shown to be an effective fungus for bioremediation of pharmaceutical xenobiotics, e.g., acetaminophen (Esterhuizen-Londt et al. 2016). Fungi are also useful in bioremediation of hydrocarbon pollutants (Norton 2012; Varjani 2017). Mycorrhiza-assisted remediation not only ensures the removal of soil pollutants but also improves the structure of soil and helps in plant nutrient acquisition. Mycorrhizal fungi also detoxify the organic and inorganic toxic substances (Chibuike 2013). Ulfig et al. (2006) reviewed the ability of keratinolytic fungi to remove hydrocarbons from different media. Fungi are especially well suited to PAH degradation relative to other bacterial decomposers (Peng et al. 2008).

4.5.1 Fungi: An Environmental Indicator

Fungi are useful to indicate various types of environmental contaminants; however, they are unable to map the pollution. Thelephora caryophyllea accumulates metals in the soil (Maurice and Lagerkvist 2000). The pathogenic and allergic spores of Aspergillus, Rhizopus, and Alternaria are indicative of air pollution (Gaikwad and Sonwane 2012). The ectotrophic symbiosis, i.e., mycorrhiza-forming fungi and mycorrhizal roots, is highly responsive to air pollution and can be used as bioindicators (Fellner 1990).

Yeasts are used in various tests for determination of mutagenic or carcinogenic action. Due to limited permeability, yeast cells exhibit lower sensitivity to mutagens or carcinogens than do bacteria. The general permeability of Saccharomyces cerevisiae cells can be enhanced by mutation, and on this basis, a more sensitive test has been developed to study environmental pollution (Terziyska et al. 2000). Recent advances in knowledge of multicolored fluorescent proteins from yeasts and fungi have opened a door regarding the sensing systems used for environmental pollutants (Singh 2006).

4.5.2 Fungi: Remediation of Pollutants

In any ecosystem , fungi are among the major decomposers of plant polymers such as cellulose, hemicellulose, and lignin (Pletsch et al. 1999; Christian et al. 2005). With the adaptability of fungi, mycoremediation could be an alternate way to ensure greater cleaning efficiency during the winter when the necessary growth conditions for plant-based systems are lacking (Steffen et al. 2007). Remediation using fungi, especially mycoremediation of soils, has been demonstrated in several cases (Shah et al. 1992; Yamada-Onodera et al. 2001; Hamman 2004; Siddiquee et al. 2015; Esterhuizen-Londt et al. 2016). Fungi have proven to modify soil permeability and ion exchange capacity and detoxify contaminated soil. Edible and/or medicinal fungi also play a role as natural environmental remediators (Pletsch et al. 1999), as do aquatic fungi (Kshirsagar 2013). Fungi are usually slow in growth and often require substrates for co-metabolism. The mycelial growth habit of these organisms is responsible for rapid colonization of substrates (Siddiquee et al. 2015). Myco-transformation is a term used to describe biotransformation of pollutants in simpler molecules by using fungi. Brown rot and white rot are categories of fungi that produce different suites of digestive enzymes that have each shown potential for mycoremediation (Shah et al. 1992; Yamada-Onodera et al. 2001; Christian et al. 2005; Kshirsagar 2013). It has been proposed that fungi might be deployed in biodegradation of sites that are polluted by complex mixtures of PAH, for example, from creosote, coal tar, and crude oil (Adenipekun and Lawal 2012).

4.5.3 Role of White-Rot and Brown-Rot Fungi in Bioremediation

White-rot fungi produce digestive enzymes that preferentially degrade lignin, a component of wood that is broadly similar in molecular structure to petroleum hydrocarbons (Shah et al. 1992; Kshirsagar 2013; Varjani 2017). Bumpus et al. (1985) proposed the use of this fungus in bioremediation as Phanerochaete chrysosporium have extracellular oxidative ligninolytic enzymes that have the ability to degrade toxic or insoluble compounds more efficiently than other fungi or microorganisms . In addition to P. chrysosporium, several other white-rot fungi (e.g., Pleurotus ostreatus, Trametes versicolor, Bjerkandera adusta, Lentinula edodes, Irpex lacteus) are known to degrade these compounds (Bumpus et al. 1985; Shah et al. 1992; Hamman 2004; Adenipekun and Lawal 2012; Siddiquee et al. 2015). It has been estimated that approximately 30% of the literature on fungal bioremediation is concerned with white-rot fungi (Singh 2006; Rhodes 2014). As stated by Christian et al. (2005), white-rot fungi secrete enzymes such as lignin peroxidases, manganese peroxidases, and laccases, and they are able to mineralize a wide range of highly recalcitrant organo-pollutants that are similar in structure to lignin. As white-rot fungi grow by hyphal extension, they can reach pollutants in soil easily than other organisms . Soils may also be decontaminated from crude oil, with requirement that lignocellulosic substrates (e.g., sawdust straw and corncob) are also provided, to support growth of fungal species in soil (Lang et al. 1995). Brown-rot fungi degrade cellulose in the cell wall, leaving lignin as a typically brownish deposit. Wetzstein et al. (1997) have reported degradation of enrofloxacin (a fluoroquinolone antibacterial drug) used in veterinary medicine using brown-rot fungus named Gloeophyllum striatum.

4.5.4 Techniques Used in Mycoremediation

Lamar and White (2001) advocated a four-phase strategy for implementation of mycoremediation . This includes (i) bench-scale treatability, (ii) on-site pilot testing, (iii) production of inoculums, and (iv) full-scale application.

Substrates such as wood chips, wheat straw, peat, corncobs, sawdust, a nutrient-fortified mixture of grain and sawdust, bark, rice, annual plant stems and wood, fish oil, alfalfa, spent mushroom compost, sugarcane bagasse, coffee pulp, sugar beet pulp, okra, canola meal, cyclodextrins, and surfactants can be used in inoculum production both off-site or on-site or as mixed with contaminated soils to improve the processes of degradation (Singh 2006; Rhodes 2014). It is critical to attain the correct nitrogen/carbon ratio in substrates used so to avoid any impeding effect on efficiency of fungi in bioremediation process. Fungal inocula coated with alginate, gelatin, agarose, carrageenan, chitosan, etc., in form of pellets, may offer a better outcome than with inocula produced using bulk substrates (Rhodes 2014).

4.5.4.1 Mycofiltration

The idea of mycofiltration was germinated in the year 1970 by Paul Stamets during his study at Evergreen State College, Olympia, Washington. He found that fungi with their threadlike mycelium have the capability of absorbing tobacco smoke, ink, and water. The garden giant mushrooms were also used in mycofiltration to clean up the flow of fecal coliform-contaminated water (Stamets 2011). In mycofiltration, the mycelia are used as a filter to remove toxic materials and microorganisms from water in soil. This ecologically rational biotechnology is a promising technique for enhancing the management of storm water and agricultural runoff. The approach of adding cultivated fungi to surface water management practices was invented by Stamets in the late 1980s (Stamets 2005).

A mycofilter is basically a hessian sack filled with wet straw, wood chip, and mycelium (non-fruiting part of fungi). They look like slightly moldy sandbags and do the important work of cleaning. Likewise mycofiltration is also used to remove E. coli from storm water and results highlight challenges of using traditional microbial indicator methods, such as enzyme-linked chromogenic media, to assess capacity for eco-technologies like mycofiltration to remove pathogens from polluted waters (Taylor et al. 2015). Some species of mushroom are also known to act as mycofilters and remove toxins from polluted waters. Figure 4.3 describes steps for preparation of mycofilters used in contaminated river/streams.

Fig. 4.3
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Illustration of the steps involved in preparation of mycofilter

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4.5.5 Fungi and PAHs (Polyaromatic Hydrocarbons)

PAHs are building blocks of life and they are very common on planet earth. However, accumulation and chemical alteration of these PAHs are following a pattern now dominated by actions of humans (Fernandez-Luqueño et al. 2011; Norton 2012). PAHs form when carbon materials are burned incompletely. Furthermore, large amounts of PAHs are extracted, refined, and transported which contaminate environment. Major anthropogenic sources of PAHs include residential heating, coal gasification and liquefying plants, carbon black, coal-tar pitch and asphalt production, coke and aluminum production, catalytic cracking towers, and related activities in petroleum refineries and motor vehicle exhaust (Peng et al. 2008; Fernandez-Luqueño et al. 2011; Abdel-Shafy and Mansour 2016). Because of hydrophobic nature of PAHs, they can easily accumulate in fatty tissue and spread throughout the food chain (Christian et al. 2005; Fernandez-Luqueño et al. 2011; Varjani 2017). Most harmful among PAHs are those with more than four rings; they are often mutagenic and carcinogenic which emphasizes the importance of their removal from environment (Steffen et al. 2007; Varjani et al. 2015). Biodegradation of PAHs has been studied for more than 20 years, and already in the 1980s, a basidiomycete, the white-rot fungus Phanerochaete chrysosporium, was shown to be capable of degrading PAHs (Steffen et al. 2007).

Fungi can degrade high-molecular-weight PAHs when compared to bacteria which can degrade relatively smaller molecules, and therefore they are well suited to PAH degradation (Peng et al. 2008). They also function well in nonaqueous environments where hydrophobic PAHs accumulate; a majority of other microbial degradation occurs in aqueous phase . Also, they can function in very low-oxygen conditions that occur in heavily PAH-contaminated zones (Fernandez-Luqueño et al. 2011). There are more than 50 fungal species or groups that can degrade various PAHs effectively as reviewed by Fernandez-Luqueño et al. (2011). A wide variety of fungi have evolved effective mechanisms to attack specific PAHs (Abdel-Shafy and Mansour 2016). One reason for this ability lies in the similarity between lignin, a long aromatic family of molecules that is present in wood, and PAHs (McCrady 1991; Shah et al. 1992; Kshirsagar 2013). Lignin is found in all vascular plants, mostly between cells but also within the cells and in the cell walls. It makes vegetables firm and crunchy (McCrady 1991; Shah et al. 1992). It functions to regulate the transport of liquid in living plants, and it enables trees to grow taller and compete for sunshine. It is very resistant to degradation being held together with strong chemical bonds; it also appears to have a lot of internal H bonds. It is bonded in complex and various ways to carbohydrates (hemicelluloses) in wood (McCrady 1991). White-rot fungi are well known for their ability to ultimately transform lignin to CO2 in nature through a process called mineralization ; fungi are equipped with certain extracellular enzymes, such as phenol oxidases (laccase) and peroxidases, which are capable of oxidizing lignin and other compounds through a formal abstraction of one electron (Mai et al. 2004; Varjani 2016).

Steffen et al. (2007) have reported that oyster mushroom , Pleurotus ostreatus, can degrade 80–95% of all PAHs present in soil. Many species of white-rot fungi have been studied extensively and are ubiquitous PAH degraders. Two main pathways are identified for these basidiomycetes; first is the cyctochrome 450 system, much like the system in mammal livers that break down large molecules into metabolites; however, many of these metabolites are toxic themselves. The second one, i.e., lignin extracellular degradation pathway, is preferable because the metabolites are fully broken down into carbon dioxide. A mixture of bacteria and white-rot fungi could complete the degradation of PAH most effectively as the fungi break down the largest molecules of PAHs into low-molecular-weight compound and bacteria can then act on those molecules (Peng et al. 2008). Fungi can degrade polyvinyl alcohol (PVA) as well. According to research work carried out by Jecu et al. (2010), Aspergillus niger can substantially degrade the PVA , starch, and glycerol. They examined degradation under scanning electron microscope and observed significant changes in polymer surface aspects depending on medium culture composition, the presence of supplementary carbon source facilitating microbial growth and degradation process.

There is a scope for the use of fungi in decomposing in situ intractable, persistent, and highly toxic pollutants, including TNT (2,4,6-trinitrotoluene) (Stamets 2005). By inoculating a plot of soil contaminated using diesel oil, with mycelia from oyster mushrooms (Pleurotus ostreatus), it was found that after 4 weeks, 95% PAHs had been converted to nontoxic compounds. It seems that the naturally present community of microbes acts in concert with fungi to decompose contaminants, finally to CO2 and H2O (Rhodes 2014). Fungi have an astonishing potential to clean up contaminated environments.

4.6 Conclusion

Fungi are efficient in degradation of high-molecular-weight compound into simpler compound via enzyme activity. The research in this field has shown that there is a fungus to degrade every type of environmental pollutant. Mycoremediation has ability to transform contaminated wasteland into a diversified ecosystem. However, application of this technology is difficult; skilled and trained personnel with proper knowledge of the subject are required to do the work of fungal remedy. Furthermore, there is a need to carry out research on potentiality of mushroom and also to exploit other species of fungi for degradation of pollutants.