Advances in Microbe-assisted Phytoremediation of Polluted Sites

By Ying Ma

Advances in Microbe-assisted Phytoremediation of Polluted Sites provides a comprehensive overview of the use of phytoremediation to decontaminate polluted land through microbial enhanced phytoremediation, including the use of plants with respect to ecological and environmental science. The book discusses the potential of microbial-assisted phytoremediation of the contaminant, including heavy metals, pesticides, polyaromatic hydrocarbons, etc., with case studies as examples. Key subjects covered include plant-microbe interaction in contaminated ecosystems, microbe-augmented phytoremediation for improved ecosystem services, and success stories on microbe-assisted phytoremediation of contaminated sites.

With increasing demand for land-space for social, industrial and agricultural use, the theoretical millions of hectares of contaminated sites around the world are a resource sorely needed that currently cannot be utilized. Decontamination of this land using ecologically-sound methods is paramount not only to land use, but in the prevention of toxic substances deteriorating local ecosystems by reducing productivity and contaminating the food chain – which can eventually aggregate in food chains and pose the potential risk of non-curable diseases to humans such as cancer.

• Provides novel information on the potential for microbial inoculants to be used in phytoremediation
• Discusses principles and mechanisms of plant-microbe interaction for enhanced phytoremediation with improved soil health
• Investigates phytoremediation solutions for a multitude of contaminants, including heavy metals, fly ash, petroleum, arsenic, TPH, mining effluents, fluoride, lead and other major pollutants

• Language: English
• Publisher: Elsevier Science
• Release date: Aug 3, 2022
• ISBN: 9780128235300

Book preview

Part 1

Overview of microbe-assisted phytoremediation

1. Microbe-assisted phytoremediation of environmental contaminants 3

2. Microbial augmented phytoremediation with improved ecosystems services 27

3. Role of genetic engineering in microbe-assisted phytoremediation of polluted sites in the era of climate 63

4. Phytoremediation potential of genetically modified plants 85

Chapter 1

Microbe-assisted phytoremediation of environmental contaminants

Anuradha Devia, Luiz Fernando Romanholo Ferreirab,c, Ganesh Dattatraya Sarataled, Sikandar I. Mullae, Nandkishor Moref, Ram Naresh Bharagavaa

aLaboratory of Bioremediation and Metagenomics Research (LBMR), Department of Environmental Microbiology, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow, U.P., India

bWaste and Effluent Treatment Laboratory, Institute of Technology and Research (ITP), Tiradentes University, Farolândia, Aracaju-SE, Brazil

cGraduate Program in Process Engineering, Tiradentes University (UNIT), Farolândia, Aracaju-Sergipe, Brazil

dDepartment of Food Science and Biotechnology, Dongguk University-Seoul, Ilsandong-gu, Goyang-si, Gyeonggi-do, Republic of Korea

eDepartment of Biochemistry, School of Applied Sciences, REVA University, Bangalore, India

fDepartment of Environmental Sciences, Babasaheb Bhimrao Ambedkar University (A Central University), Lucknow, U.P., India

Chapter Outline

1.1 Introduction 4

1.2 Environmental contaminants: Types, nature, and sources 5

1.3 Impact of environmental contaminants on the environment and human health 7

1.4 Plant-microbe association/interaction and its role in phytoremediation of environmental contaminants 8

1.4.1 Phytoremediation of organic and inorganic contaminants 8

1.4.2 Phytoremediation of wastewater 14

1.4.3 Role of constructed wetlands in treatment of wastewaters 15

1.5 Mechanisms involved in the phytoremediation of environmental contaminants 16

1.5.1 Phytostabilization 16

1.5.2 Phytovolatilization 17

1.5.3 Phytodegradation 17

1.5.4 Phytoaccumulation 17

1.5.5 Phytoextraction 18

1.5.6 Rhizoremediation 18

1.6 Economic importance of microbe assisted phytoremediation of environmental contaminants 21

1.7 Conclusion 21

1.1 Introduction

Nowadays, the concern of environmental contamination has increased due to the high production demand of rapid population growth, which prompts the generation of plenty of waste and environmental contaminants. The toxic heavy metals, greenhouse gases, oil spillages, nonbiodegradable materials, unutilized fertilizers, pesticides, and other hazardous materials are the main source of environmental contamination (Kishor et al., 2020). Endocrine disruptors (EDs), pharmaceuticals, pesticides, hormones, toxins and industrial wastewaters also contribute majority of environmental contamination (Rasheed, Bilal, Nabeel, Adeel, & Iqbal, 2019). The long-term susceptivity to these environmental contaminants might have counter health effects like organ dysfunction, cancer, physical, psychological, neurological disorders, and compromised immunity (Godduhn & Duffy, 2003). Thus subsequently, the remediation of contaminated area is mandatory in order to retain the area and to reduce the entrance of toxins into the food chain (Hooda, 2007). Contaminants from water and soil can be remediated by various methods, but most of these are expensive, labor-consuming, and require on-site renovation through chemical or physical methods (Kishor, Bharagava, & Saxena, 2018). Due to these drawbacks, the scientists had developed some eco-friendly technologies using microorganism and plants or with combination of both for the elimination of toxins from contaminated soil and water (Glick, 2003).

Bioremediation is a method which practices microbes (bacteria, yeast algae, and fungi) used for the remediation of contaminated soil and water. In this innovation, the proliferation of native consortia (microbial) is endorsed aimed at chosen movement by monitoring abiotic and biotic conditions for contaminated sites (Weyens et al., 2015). Microbes are recognized to be principal microbe groups form symbiotic associations with plants as shown in Fig. 1.1. Microorganisms play a major role in nitrogen, fixation phosphate solubilization and mobilization, nutrient management, probiotics and biotic elicitors, biodegradation agents. Various, fungi, algae and bacterial strains have been explored for the management of hazardous organic and inorganic waste including heavy metals produced by various industries (Basit, Shah, Ullah, Muntha, & Mohamed, 2021). Microorganisms are responsible for the health of the plant, crop production, and phytoremediation for knocking out pollution load in environmental monitoring. The important perception of microbe assisted phytoremediation is to effectively remediate contaminants from the environment using both plant and microbes. The technique has great potential in the tropics, as it favours plant growth and enhancing microbial activity due to climatic conditions (Liu, Yang, Liang, Xiao, & Fang, 2020).

Fig. 1.1 Microbe and plant interaction for phytoremediation.No permission required

The degradation of organic contaminants using microbes and plants is termed phytodegradation. Also, microbes are used for increasing the efficiency of organic contaminant degradation in the root zone and are termed as rhizodegradation (Basit et al., 2021). For example, mycorrhiza (a beneficial association between a fungus and the roots of vascular plants) are effective recalcitrant polymers recyclers of lignin. By the application of filamentous or unicellular fungi reduction of harmful wastes from the environment is performed. Some microbe and plant species can be used for heavy metal remediation.

Plant Ricinus communis cultivated in the presence of Pseudomonas sp. M6 was found tolerant to nickel, Eichhornia crassipes cultivated in presence of Pycnoclavella diminuta found tolerant to resistant to chromium, Cajanus cajan and Proteus vulgaris KNP3 found tolerant to copper (Rai, Kim, Lee, & Lee, 2020). Fungi (AMF) Glomus mosseae and plant species Vetiveria zizanioides found tolerant to iron, copper, cadmium, lead, and zinc (Kafil, Boroomand Nasab, Moazed, & Bhatnagar, 2019). Jampasri et al. (2020) reported that the tolerance of C. odorata and M. luteus to moderate concentrations of lead and fuel oil made them extremely good applicants of bacteria-assisted phytoremediation of lead-fuel oil cocontaminated soils (Sharma, 2021).

It is perceived that plant-microbe interaction plays a significant aspect during remediation by degrading, detoxifying or sequestrating the contaminants by promoting plant growth (Weyens et al., 2015). It was reported that microbe assisted phytoremediation is a cost-effective, ecofriendly, nonintrusive, esthetically pleasing, and informally recognized sustainable expertise meant for the remediation of contaminated soil or water areas (Alkorta & Garbisu, 2001). The microbe assisted phytoremediation of contaminants involves phytoextraction, phytovolatilization phytostabilization, and rhizofiltration (Glick, 2003). The purpose of this chapter is to confer the probable and limitations of remediation by microbe-assisted phytoremediation of environmental contaminants including wastewater.

1.2 Environmental contaminants: Types, nature, and sources

Environmental contaminants are chemical, physical, biological or radiological substances that have adverse effects on air, water, soil, and living organisms. Due to the industrialization and overuse of chemicals, our environment has become contaminated with various types of contaminants. The source of contaminants may be point or nonpoint. However, the common sources of contaminants are industrial activities and accident, oil spillage, mining, ammunitions, fossil fuels and war agents (Kishor et al., 2020). The contaminants may be natural or xenobiotic in nature. The common environmental contaminants are PAHs, heavy metals, pesticides, organic, and inorganic solvents. The discharge of these contaminants leads to environment contamination which leads to human health problems. The contaminants generally may be classified into two categories: inorganic and organic (Kishor et al., 2018).

Natural processes like continental dust, volcanic eruptions and anthropogenic activities as combustion of phosphate fertilizers, mining, fossil fuel combustion, metal industries lead to release of environmental contaminants and heavy metal accumulation in environment (Parmar, Dave, Sudhir, Panchal, & Subramanian, 2013). While, metals are the natural elements of the earth’s crust, their increasing concentration is harmful to ecological and human receptors (Kishor et al., 2020). Most of compounds and their elemental metals are enormously constant in the atmosphere. This metal-containing particulate matter could be distributed to significant distances by water also by wind. Organic contaminants are carbon-based compounds that are resilient to natural degradation conditions therefore, stay in the environment throughout significant stretches of time like pesticides, industrial chemicals, PAHs, POPs (Dzantor & Beauchamp, 2002). Because of high stability besides distant transportation, these contaminants contaminate water, air and the soil ecosystem where they are bioaccumulated by plants and living organisms. The first apparent revealed was in 1970 when polar bears were found to bioaccumulated pesticides in their fat tissues (Arslan, Imran, Khan, & Afzal, 2017). In human, the long exposure and accumulation of these contaminants can be prompt to metabolic disorders, cardiovascular diseases, physical health illness, and severe damage to fetus growth (Thakur & Pathania, 2020).

The characteristics and nature of industrial effluent relies upon the type of industry, raw materials used, processes applied, and product quality (Saxena & Bharagava, 2017). Different industries regularly release high-strength wastewaters, which is categorized by increased level of total suspended solids (TSSs), chemical oxygen demand (COD), biochemical oxygen demand (BOD), and total dissolved solids (TDSs) various organic/inorganic contaminants (Saxena & Bharagava, 2017). Phenolics are the most common contaminants related to industrial wastewater, herbicides, and pesticides, which utilized in manufacturing chemicals like alkylphenols, xylenols, cresols, aniline, phenolics, pesticides, resins, dyes, explosives, and other compounds (Kishor et al., 2018).

Persistent organic pollutants are carbon-based chemicals having different physical and chemical properties, which are resistant to environmental degradation through chemical, biological, and photolytic processes because of their persistence for long time in the environment (Thakur & Pathania, 2020). Endocrine disruptors are used in industries as plasticizers, plastic resins (cellulosic/polyurethane and polymers polyvinyl resins). "PCBs are hydrophobic, low vapor pressure, less water-soluble and highly soluble in organic solvents, fats, and oils. PCBs are resistant to acids, bases, oxidation, hydrolysis, and temperature change and are widely used in industries. They can produce enormously poisonous chemicals such as dibenzodioxins and dibenzofurans through partial oxidation. PCBs can penetrate skin, polyvinyl chloride (PVC), and latex and show toxic effects (Thakur & Pathania, 2020). Azo dyes are synthetic recalcitrant dyes used nearly in every industry such as pharmaceutical, food, tannery, textile, acrylic, plastics, and cosmetics.

Petroleum hydrocarbons are the source of refinery industrial wastewaters. The most well-known petroleum hydrocarbons incorporate aliphatic, branched, and cycloaliphatic alkanes as well as monocyclic and PAHs, which comprise of naphthalene, anthracene, fluorene, phenanthrene, pyrene, fluoranthene, benzo[a]anthracene, and benzo[a]pyrene (Rabbani et al., 2021). Pesticides, herbicides, fungicides and insecticides are utilized in agricultural practices to unwanted plants, control pests, fungi and crop destroying insects respectively, which optimize crop productivity (Sarma, Nava, & Prasad, 2019). Pesticides like dichlorodiphenyltrichloroethane (DDT), aldrin, endrin, heptachlor, chlordane, mirex, hexachlorobenzene, dieldrin, and toxaphene, fungicides such as hexachlorobenzene, pentachlorophenol, and benzothiazole. Herbicides like 2,4-D, atrazine, picloram, chlorophenyl compounds insecticide such as inorganic contaminants wastewater containing cadmium utilized in rechargeable batteries, special alloy production, plastic stabilizer and also present in tobacco smoke (Thakur & Pathania, 2020).

Industries like metallurgical, refractory brick, chemical, tannery, dyes, and wood preservation are the significant consumers of chromium (Cr). Lead (Pb) the major contaminant of soil and water mostly occurs from human derived activities, smelting and mining, industrial, batteries, gasoline, pesticides, paints, and explosives. The major sources of Hg exposure include its use in thermometers, dental amalgams, sphygmomanometers, fossil fuels emissions, barometers, incandescent lights, ritualistic practices using mercury, batteries and the incineration of medical waste (Bharagava, Saxena, Mulla, & Patel, 2018).

1.3 Impact of environmental contaminants on the environment and human health

Environmental contaminants like POPs are most widespread organic contaminants (Kishor et al., 2021). These contaminants resist chemical, biological, and photodegradation and remain in the environment for a very extensive time. Polychlorinated dibenzo-p-dioxins (PCDDs), PAHs, PCBs, polychlorinated dibenzofurans (PCDFs), DDT, hexachlorocyclohexane (HCH), chlordane, toxaphene (organic pesticides), dioxins, and antimicrobials are also stable and toxic to human, animals, and aquatic flora and fauna (Roop Kishor et al., 2020). Some temporary effects of PAHs revelation to human causes are eye irritation, skin irritation, nausea, vomiting, diarrhea, and confusion (Kishor et al., 2021). The continuing revelation of PAHs causes deoxyribonucleic acid (DNA) mutations, leukemia, developmental malformations, decreased immune function, cataracts, asthma like symptoms, oxidative stress, skin, reproductive defects, bladder, brain, bone, and scrotal cancer (Kishor et al., 2020).

A long prenatal exposure to neurological and immunodeficiency drugs causes many disorders, brain, pancreatic, breast, prostate cancer etc. (Alharbi, Basheer, Khattab, & Ali, 2018). Acetylcholinesterase enzyme which is accountable for the hydrolysis of acetylcholine serves to stop the excitation of nerve after transmission of nerve impulse is also inhibited by DDT. PCBs, polybrominated diphenyl ethers (PBDEs) even in smaller quantity can impair the development of brain in youngers causing lifelong behavioral impairment (Rai et al., 2020). Reports also show that there is also mental and physical retardation of development in children who are exposed to persistent organic contaminants (POPs) (Alharbi et al., 2018). PCBs exposure can cause neurological disorder as hyperactivity, attention deficits, reduced memory and have adverse effect on immune system. Fish, birds around the water bodies containing POPs result behavioral abnormalities, when consumed by humans with suppressed immune systems who more susceptible to these contaminants. These organic contaminants are disturbing ecological balances among flora and fauna altering the metabolic activities and entering in food chain. Climate change, smog, global warming, acid rains, barren soil lands are some environmental issues (Kishor et al., 2020).

Heavy metal contamination adversely effects human health through different exposure routes. Heavy metals are hazardous, nonbiodegradable and lethal contrasting organic contaminants but these heavy metals might strictly restrict in the degradation and reduction of organic substances. Numerous harmful human activities stated above discharge toxic metals and metalloids cause soil contamination by extreme accumulation or depositions. Several heavy metals are carcinogenic to humans and animals and can cause DNA damage probably because of their mutagenic ability (Sivaram, Logeshwaran, Lockington, Naidu, & Megharaj, 2019). The clean-up of contaminated sites is extensively important in order to reclaim the contaminated area and to diminish the entry of toxins into the food chain. Naturally atmosphere gets contaminated through indirect or direct revelation of heavy metals released in aquatic bodies which reduces soil fertility and water quality respectively which ultimately affects plants and aquatic life and further human health. Various toxic environmental contaminants with their sources of emittance having effect on environment and human health are listed in tabular form (Table 1.1).

Table 1.1

From Saxena, G. and Bharagava, R. N. (2017). Organic and inorganic pollutants in industrial wastes: Ecotoxicological effects, health hazards, and bioremediation approaches. In Environmental Pollutants and their Bioremediation Approaches (pp. 23–56). CRC Press. https://doi.org/10.1201/b22171

1.4 Plant-microbe association/interaction and its role in phytoremediation of environmental contaminants

The interaction of microbial communities with plants has an important role in the physiological aspect and also plant growth, this interaction assist inhibition of phytopathogens, promotion of detoxification enhancement of nutrient availability, release of growth-promoting molecules, improvement of stress tolerance by induction of systematic acquired host resistance (Oliveira et al., 2015). This microbe-plant association helps in elimination of contaminants from the soil and water through increasing interested rhizospheric microbial population that stimulates metabolic activity (Kuiper, Lagendijk, Bloemberg, & Lugtenberg, 2004). Microbe assisted phytoremediation is a green and cost-effective emerging approach to deal with environment contaminants which can transform, degrade, assimilate, metabolize, or detoxify organic and inorganic contaminants from soil or water (Moreira, Lima, Ribeiro, & Guilhermino, 2006).

1.4.1 Phytoremediation of organic and inorganic contaminants

Phytoremediation can be practiced for remediation of inorganic and organic contaminants present both in liquid or solid state in soil or water. Phytoremediation of contaminants includes the following steps: uptake, translocation, transformation, compartmentalization, and sometimes mineralization (Jha, Misra, & Sharma, 2017). Inorganic contaminants could be reserved up as of the soil and immobilized elated to the plant shoot (phytoextraction) or through the roots (Phytoimmobilization). Mostly the bioavailability of metals in soil is relatively low, plants have very efficient metal uptake classifications via transporter molecules like copper (Cu) transporter 1and protein zinc-regulated transporter protein. Furthermore, plants are proficient of secreting metal chelating molecules like [siderophores, organic acids (citrate, malate)] and biosurfactants like (rhamnolipids around soil), and also press out proton (H+) to acidify the soil through roots results mobilization of soil bound metals. In opposition to organic contaminants heavy metals cannot be biodegraded by the plant but can be transformed from one oxidation state or organic complex to another oxidation state or organic compound (Truu, Truu, Espenberg, Nõlvak, & Juhanson, 2015). Accordingly, heavy metals have a propensity to accrue in the plant. Plants ranging as of (annual herbs to perennial shrubs) and trees nearly 450 hyperaccumulator are identified for example tobacco, maize, pennycress, mustard, sunflower brake fern, Russian thistle, python tree, rattle bush, willow, poplar have been known to accumulate and detoxify extremely high amount of metal ions, such as in (cobalt (Co), nickel (Ni), lead (Pb), cadmium (Cd), zinc (Zn), and manganese (Mn) (Badr, Fawzy, Khairia, & Al-Qahtani, 2012).

Organic contaminants such as chlorinated solvents and (PAHs) could be reserved up and immobilized through plant roots and transpired by the shoot. On the other hand, inorganic contaminants are metabolized by plants (phytodegradation). This metabolism of organic contaminants by plants has three phases: transformation, conjugation and compartmentalization. In the transformation phase, chemical modification of contaminants (hydrolysis and oxidation reduction) takes place then transformed into water soluble and more polar form via enzymes (cytochrome carboxylesterases or P450) (Rai et al., 2020). In the conjugation the endogenic molecules it transformed contaminant become less toxic to plants (glutathione S-transferases and glycosyltransferases). Now the compartmentalization phase in which contaminants are transported to the various compartments (storage inside the vacuole or integration in the cell wall) of the cell but in some cases could be excreted from the cell. Whereas, there is a principle difference between plant and microorganism in their metabolism of contaminants, plants did not hold broad catabolic pathways aimed at degradation and mineralization mostly contaminants not being utilized as a source of carbon (C), nitrogen (N) and energy in the process of remediation (Truu et al., 2015).

In the remediation technique sometimes more toxic by-products may be produced unlike to the preliminary pollutant like trichloroethylene (TCE) into trichloroethane, metabolites release by volatile contaminants by evapotranspiration into the environment has been reported (Arslan et al., 2017). Few contaminants like PCBs, PAHs, linear halogenated hydrocarbons and nitroaromatics can be entirely mineralized by plants such as poplar, alfalfa, willow, and different grass varieties (Thakur & Pathania, 2020). Within plant cells regrettably, there is no natural transporter for organic contaminants. Instead, their passive uptake arises considering the artificial origin of organics.

The altering chemical nature of xenobiotics (hydrophobic or hydrophilic) influences the uptake in plant cells. The hemicellulose of the cell wall and lipid bilayers of the plasma membrane can bind with hydrophobic organic contaminants (Rai et al., 2020). These contaminants can be further translocated to plant aerial parts along with the transpiration stream channels, as predicted through an index which is the transpiration stream concentration factor (TSCF). Afterward, the organic toxicants like PCBs, PAHs, and petroleum hydrocarbons could be mineralized through rhizospheric microbes associated with specific plant species as accumulators. Furthermore, the role of enzymatic transformation through nitro reductases, dehalogenases, laccases, peroxidases which is extremely vital in converting hazardous xenobiotics into relatively less toxic chemical substances (Cherian & Oliveira, 2005).

1.4.2 Phytoremediation of wastewater

Discharge and transportation of untreated or partially treated industrial water, leaching of pesticides and rest of fertilizers are the important aspects that produce high levels of contaminants that can contaminate the soil and surface water systems (Hegazy, Abdel-Ghani, & El-Chaghaby, 2011). Research on remediation of contaminants is great deal. Alternative to conventional method is microbial phytoremediation which are relatively reasonable and environmentally as well as ecologically advantageous. The various type of contaminants found in wastewater releasing from different industries, phenols related with distilleries, paper mills and pulp, textiles, oil refineries, coal mines, wood preservation, preparation of numerous chemicals such as xylenols, alkylphenols, phenolic, aniline, resins, explosives, dyes, pesticides (Kishor et al., 2021). Endocrine-disrupting chemicals used in industries (as plasticizers), in the industrial manufacturing of plastic resins like polyurethane polymers and polyvinyl resins.

Chlorinated phenols released laterally with wastewaters from paper-pulp mills, tanneries, distilleries. Dyes are source of leather, textile, paint, cosmetics, pharmaceutical, acrylic, plastics (Kishor et al., 2020). Cadmium (Cd) is used as a plastic stabilizer, rechargeable battery, alloy production, pigments, coatings platings, etc. Chromium industries like metallurgic, chemical, leather, bricks and petroleum hydrocarbons from refinery industrial wastewaters. The remediation of these contaminants from the contaminated matrix is done through the plant associated microbes. PGPR might be helpful in phytoremediation like this can suppress plant pathogens, resistant to abiotic stress, lower the toxicity by biosorption and bioaccumulation in bacterial cells have enormously high ratio of surface to volume area, also encourage plant growth by secreting several hormones, antibiotics, organic acids. Endophytes which are bacteria that reside in the inner tissues of host plants are also capable to tolerate high metal concentration and hence lower the phytotoxicity to remediating plants and help in promoting growth by various means and increase the phytoremediation effectiveness (Ma, Prasad, Rajkumar, & Freitas, 2011). Arbuscular mycorrhizal fungi (AMF) that colonize plant roots are also reported to defend their host plants against heavy metal toxicity by their mobilization from soil and result in phytoremediation (Khan et al., 2014). Bioenergy can be produced by the plant biomass burning and land restoration can be achieved for sustainable agricultural development (Khan et al., 2014).

1.4.3 Role of constructed wetlands in treatment of wastewaters

The constructed wetlands for remediation of contaminated soils and waters through integration of microbes and plants has been increased, because of its cost-effective and impressive working substitutes to traditional technologies for the removal of extensive variety of contaminants from various type of industrial, contaminated groundwater and municipal wastewaters (Kishor et al., 2021). This method is being utilized to treat numerous types of organic contaminants like petroleum hydrocarbons, pesticides, explosives, chlorinated solvents, and inorganic contaminants like radionuclides, metals and metals with metalloids (Truu et al., 2015).

In constructed wetlands, the substrates are typically waterlogged; the supply of oxygen to wetland substrates is much slower than to dry land substrates, because oxygen diffuses through water 10,000 times slower than through air creates low in or devoid of oxygen (Rabbani et al., 2021). These anaerobic conditions require special adaptations in microbial community and plants depending on the chemical constituents of the substrate a vast range of terminal electron acceptors other than oxygen are used for respiration by these organisms the terminal electron acceptor is used is depends on their availability and their substrate (Otte & Jacob, 2006). This can be understood by considering example of water lodging of a well-aerated and dry soil, in these circumstances the redox reactions occur because of the respiration activity of microbes subsequent a thermodynamic sequence, from more oxidized to more reduced. In deficient of oxygen (O2), nitrate (NO3 −) is reduced to nitrogen (N2) or nitrous oxide (N2O). This process further leads to the removal of nitrogen from the substrate and formation of gases, this process is so-called denitrification. After total nitrate utilization, ferric iron (Fe³+) may be reduced to ferrous iron (Fe²+). After removal of most ferric irons the sulfate (SO4²−) might be reduced to sulfide (S2), and after sulfate utilization carbon dioxide (CO2) might be reduced to methane (CH4) (Otte & Jacob, 2006).

This is a highly complex and exclusive process, denitrification won’t occur in the existence of oxygen, iron reduction won’t occur in the presence of nitrate, this is because the reduction of ferric to ferrous iron is thermodynamically unfavorable in the existence of nitrate. The wetland substrates hold a vast variation of organic and inorganic redox sensitive compounds. The reaction depends upon the thermodynamic characteristics of the system and on the diversity of microbe capable for oxidizable substrates (Otte & Jacob, 2006). Though the biogeochemical measures related in transformation of the organic chemicals of vegetated treatment wetlands are so far seldom assessed most likely attributable to the intricate and synergistic nature of ongoing processes. The viability of wetlands for applications in microbe assisted phytoremediation has much scope for additional advancement of wetlands (Truu et al., 2015).

In a complex treatment wetland structure few elimination pathways of organic compounds like photochemical oxidation, volatilization, sorption, sedimentation and biodegradation may occur at the same time while plants might contribute either by directly accumulation of contaminants (Jha et al., 2017). Phytovolatilization and metabolic alterations or through generating circumstances favoring contaminant removal inside the treatment systems. Likewise, including as appropriate surface for biofilm anchorage, advancing the change and growth of diverse microorganisms inside the structures by secreting root exudates, impelling and deliver oxygen (O2) to the deeper layers of wetland media, holding suspended solids particles and insulating against low temperature. The general significance of this method differs, depending on the inorganic or organic contaminant is to be treated, the treatment wetland types these conditions may be free-water, subsurface flow, horizontal flow or vertical flow, soil matrix, type of vegetation, operational design, wastewater loading rate and retention time (Almuktar, Abed, & Scholz, 2018).

Though, the effect of plant existence and the capability of particular microbial species to progress the elimination productivity for the compound. Because of numerous different variables such as structure of rhizosphere microbial communities, the properties of the wastewater and environment such as temperature, availability of electron acceptors also operational conditions like hydraulic retention time, loading mode, specific surface area might all demonstration in show (Truu et al., 2015). For instance, in case of surface flow constructed wetland planted or unplanted mesocosm were not pointedly diverse in their capacities to remediate contaminants like pharmaceuticals. There are few examples of studies where plants potentially used constructive wetlands for remediation of contaminants first one is Phragmites australis which has probable to extract and accumulate Cr from tannery effluent. Typha latifolia a broad-leaf plant has been recognized for Cu and Cd removal from industrial effluent. Also, in a research lab-scale experiment heavy metal (Cd (II), Hg (II), Cr (VI) and Pb (II)) have been remediated (Truu et al., 2015).

1.5 Mechanisms involved in the phytoremediation of environmental contaminants

Phytoremediation of environmental contaminants from the polluted sites commonly occurs over any one or more than one process of the subsequent working such as: phytoaccumulation, phytostabilization, phytodegradation, and phytovolatilization (Muthusaravanan et al., 2018).

1.5.1 Phytostabilization

Phytostabilization is an interaction where plant species are applied to immobilize contaminants specifically metals at contaminated sites by accumulating contaminants with the help of roots through root hairs (adsorption onto root surface, or precipitation within the rhizosphere of certain plant species). The plant-related microbes in phytostabilization helps in promoting the plant growth and also their metal tolerance capacity that restrict the metal absorption and mobilization to higher vegetative parts by diminish the metal bioavailability in the rhizosphere itself (Bolan, Park, Robinson, Naidu, & Huh, 2011). The plant-associated microbes developed various distinct working to immobilize or to inactivate the metal adsorption in plants. Sometimes, they use hydraulic control to prevent leachate migration which can be accomplished through the large amount of water transpired by plants.

The anaerobic, decreased situations of the mass substrate might prompt to the development of inexplicable metal sulfides, while the aerobic, oxidized situations of the plant rhizospheres and the surface layer induce immobilization because of adsorption on and coprecipitation with iron (Fe) and manganese oxide (MnO) and phosphate ion (PO4³−). The plants likewise contribute by providing organic matter. Contaminants uptake by the roots and reimbursed to the substrate. While, these methods prefer immobilization, there is obvious problem that likewise plants other organisms, may mobilize contaminants (Otte & Jacob, 2006).

1.5.2 Phytovolatilization

Phytovolatilization includes the contaminants take-up through the plants, developing and liberating them in the lesser toxic form into the atmosphere, some of the metals such as selenium (Se), mercury (Hg) that could be converted to their volatile form dimethyl selenide and mercuric oxide and afterward volatilized into the atmosphere. The converted volatile compounds dimethyl selenide and mercuric oxide released into atmosphere are deadly to the biota. Mostly plants can volatilize dimethyl selenide and mercuric oxide however additional toxic compounds present with selenium can inhibit the process these compounds include sulfate and boron. Boron and high salinity can extensively decease in most of the plants (Muthusaravanan et al., 2018). Consequence to this, plant might be cultivated in crop rotations so the phytovolatilization of selenium-contaminated soils brings about biomass that would be able to be utilized as an enhancement of supplement for feeds of livestock.

1.5.3 Phytodegradation

It is also mentioned as phytotransformation, this is for exceptionally hazardous organic contaminants from the contaminated water soil could be engaged up and transformed to fewer hazardous forms through plants (Kishor et al., 2021). Breakdown of contaminants encompassing the plants through metabolic processes and the interruption of contaminants around the plants through the consequence by catalyzing enzymes formed by the plants to accelerate degradation Trinitrotoluene is most dangerous explosive many plant species are found that are able to breakdown Trinitrotoluene in their tissues (Kishor et al., 2021).

1.5.4 Phytoaccumulation

It is the process where plants absorb the contaminants from contaminated sites along with other nutrients and water required for their growth. The absorbed contaminants are not destroyed but get accumulated in shoots, leaves and other plant parts (Muthusaravanan et al., 2018). This strategy is broadly used for utilized for metallic and radionuclide wastes. This process has incredible breadth for marketing of this technology due to low-cost abs sustainability it is apt for environmental problems associated with metal-contaminated sites mu (Muthusaravanan et al., 2018).

1.5.5 Phytoextraction

Phytoextraction is the expulsion of contaminants from soil and water. Former is effective for uptake and harvest when there must be high amount of pollutant present in contaminated water comparative to the uptake by the plants (Otte & Jacob, 2006). In case of contaminated soil, it is not clearly defined the quantity of contaminants are much higher than that taken up by the plants, since the quantity separated are generally not sufficient to diminish the levels of contamination considering with significant time.

1.5.6 Rhizoremediation

It is an economical and sustainable technology that relies upon rhizospheric microorganism’s degradability and phytoremediation capacity of a plant. Symbiotic interactions in the rhizosphere, ectomycorrhizal fungi under the ground system of plants where numerous root associated bacteria assist in elimination of contaminants through the metabolic activity and secretion of enzymes and proteins from the root system of plant (Rai et al., 2020).

Rhizoremediation is the involvement of specific plants and their rhizospheric microbes for the degradation of contaminants (Hoang et al., 2021) by indirect stimulation of catalytic activities of microbes through root exudates of plants (Lacalle, Gómez-Sagasti, Artetxe, Garbisu, & Becerril, 2018). The rhizosphere is a soil zone around the roots about 1–2 mm influenced by plant activities (Brink, 2016). Many studies are available on rhizospheric degradation, here a brief description of important rhizosphere characteristics for countering organic contaminant degradation is been provided (Kai, Effmert, & Piechulla, 2016). The principle behind rhizoremediation could be described as plant roots providing a nutrient and carbon-rich environment wherein microbial activity is stimulated and in return, some of the microorganisms facilitate the degradation of contaminants (Newman et al., 2016).

The plant roots can release 6 to 21% of photosynthetically fixed carbon into the rhizosphere (rhizodeposition) (Chowdhury, Farrell, & Bolan, 2014; Hu et al., 2019). The plant roots can also secrete different organic compounds like amino acids, organic acids, sugars, phenolic compounds, polysaccharides, and humic compounds which are called exudates (Herz et al., 2018). The quantity and composition may vary depending on the plant species, external biotic and abiotic factors, health and age of the plant (Dietz et al., 2019). Rhizodeposition can be classified into four categories based on their modes of production passive exudates, lysates (from senescent tissues and roots), mucilages (from the root tip and epidermal cells), and secondary plant metabolites (Martin, George, Price, Ryan, & Tibbett, 2014). These compounds can be used as energy carbon sources by microbes. The root exudates attract microbes (chemotaxis) leading to their proliferation and subsequent metabolism (Huang et al., 2014). The formation of reactive oxygen species (ROS) by respiration of root cells, leads to oxidatively stressful environment in rhizosphere (Segura & Ramos, 2013). Then the competent microorganisms are able to use plant root-derived compounds also, could tolerate the toxicity of ROS (Segura & Ramos, 2013).

Genomics advances have immensely progressed the approach to explore black box for innovative perceptions around the usage of rhizoremediation. These rhizospheric microbes possess catalytic tools to covert recalcitrant contaminants like PAHs, PCBs, solvents, chlorinated aliphatic compounds and metal immobilization exclusion (Thijs & Vangronsveld, 2015).

1.5.6.1 Plant growth promoting rhizobacteria (PGPR)

Utilization of PGPR for expansion in crop yield is effectively proven. PGPR had the capability to produce plant growth-promoting metabolites like siderophores, phytohormones. Also, can increase soil nutrient availability by nitrogen fixation, organic compound mineralization and phosphate solubilization. Regarding the contamination, the mechanism of plant growth promotion by these PGPR can be direct or indirect which enables nutrient regulation and hormonal regulation in plants, that results the resistance against plant pathogens (Mishra, Mishra, Arshi, Agarwal, & Dwivedi, 2019). The direct mechanism includes atmospheric nitrogen fixation, synthesis of siderophore for iron chelation, solubilization of phosphorus, supplying siderophore-iron complex for the plants so that they can produce different phytohormones like cytokinins, gibberellins, and auxins. The indirect mechanism includes controlling of phytopathogens by the producing antibiotics, depletion of iron in the soil. This will ultimately stimulate the plant growth (Mishra et al., 2019). On the basis of interaction with host plant the PGPRs are of two types: (a) Symbiotic rhizobacteria (intracellular PGPR) invade and infest the interior part of the plant cell to survive like nodule forming bacteria. (b) Free-living rhizobacteria (extracellular PGPR) reside outside the plant like Azotobacter, Pseudomonas, Bacillus, and Burkholderia (Babalola & Akindolire, 1984). PGPRs can enhance plants growth and tolerance by various means which can serve for further phytoremediation process like rhizoremediation, phytoaccumulation etc. The microbe assisted phytoremediation on different contaminated sites are illustrated in Table 1.2.

Table 1.2

In context to contaminants the availability of metal in soil is closely tied to the soil properties and the metabolites that are released by PGPR (siderophores, organ acids, and plant growth regulators) (Marques, Rangel, & Castro, 2009). PGPR directly results on plant growth dynamics or indirectly through chelation, acidification and of heavy metals immobilization in the rhizosphere. This plant-microbe association could act as remediator by further emerging phytoremediation. Soil microorganisms have substantial part in building up the soil structure, productiveness and in contamination remediation (Chaudhry, Blom-Zandstra, Gupta, & Joner, 2005). The way in to their expediency is their close mutual relationship and optimistic effect on plant development and metabolism. PGPR-related phytoremediation under field conditions is important, since of the statistics that until now this technique is only feasible is lab scale.

In a study, Abdelkrim et al. (2020) assessed the impact of PGPRs inoculation in a field study and found that in situ inoculation resulted in significant increase in shoot length, dry weight of roots and numbers of nodules. Abdelkrim et al. Research executed by inoculating field with Rhizobium leguminosarum (M5) + Bacillus simplex + Luteibacter sp. + Variovorax sp. and R. leguminosarum (M5) + Pseudomonasfluorescens (K23) + Luteibacter sp. + Variovorax sp. to test phytoremediation efficiency of Lathyrus sativus plants as well as soil quality and fertility. An increase of 47% and 22% in shoot length is observed, roots dry weights increased by 22% and 29%, nodules number increased by 48% and 31% respective to inoculated PGPRs consortium. The later inoculated consortium found to be effective in phytoremediation capacity of Lathyrus sativus for heavy metal accumulation. The above ground tissue for lead accumulation was recorded 1180.85 mgkg−1 of dry weight. Below the ground at the rhizosphere, it was observed a reduction in total Pb by 46% and Cd by 61%. Also, the later consortium efficiently improves the soil quality by enhancing the total nitrogen of the soil by 35%, phosphorus by 100%, as β-glucosidase by 16%, urease by 32%, and alkaline phosphatase activities by 12% (Abdelkrim et al., 2020).

1.5.6.2 Arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) can remove heavy metal contamination by direct involvement on the fungal surface for metal adsorption and immobilization in the soil (Ferreira Vilela & Barbosa, 2019). The hyphae of AMF contributes with the dispersal of heavy metal by chelation and sequestration of heavy metals by their fungal structure that is the direct involvement of fungi to provide a physical barrier to the heavy metals to take entry into the plants (Riaz et al., 2021) AMF assisted accumulation and sequestration of heavy metal by glomalin AMF depict extremely diverse resistance to heavy metals. For example, mycorrhizal species Rhizophagus irregularis with host plant Phragmites australis studied on Cu using mechanism phyto–rhizoremediation (Wu, Wang, Zhao, Huang, & Ma, 2020). Mycorrhizal species Glomus aggregatum, Glomus etunicatum, Glomus intraradices, Glomus tortuosum, Glomus versiforme by amendment of organic manure with host plant Trifolium repens researched on heavy metal cadmium contaminated sterilized soil. Additive effects of AMF inoculation and organic manure on the growth and nutrient contents of Trifolium repens occurred in the Cd-contaminated sterilized soil but not in the natural soil, implying the limited function of nonnative AMF introduced into natural soils (Xiao, Zhao, Chen, & Li, 2020).

Lu, Lu, Peng, Wan, and Liao (2014) researched on phytoremediation of a PCB-contaminated soil by ryegrass grown for 180 days with inoculation of arbuscular mycorrhizal fungus (AMF) Glomus caledoniun L. and epigeic earthworms (Eisenia foetida). The effects of inoculation ryegrass with AMF showed decreased PCB by 74.3%, ryegrass coinoculated with AMF and earthworms 79.5% (Lu et al., 2014).

1.6 Economic importance of microbe assisted phytoremediation of environmental contaminants

Microbe-plant association do almost all of the needed work on their own unlike other conventional process especially regarding the removal of contaminants. They are significant cost savings as well because of the low operation and maintenance requirements; the technology is predominately self-repairing and generally resilient. There is a more positive public reaction and acceptance by using green and low-tech remedial technology (Yang et al., 2020). This technique process remediates contaminants from soil, water also from air in sustainable manner. Through this technique the soil, water air qualities are improved without further damage. This allows retaining the natural flora and fauna of a contaminated site. The process may be used in combination with other restoration or migration goals and creates new habitat or supplements existing habitat creating more ecological diversity (Bharagava et al., 2018; Kishor et al., 2018).

1.7 Conclusion

Different types of contaminants like PAHs, PCBs, pesticides, POPs and heavy metals, cause severe environmental problems. These substances disturb soil profile leading destruction in microbial community leading abrupt soil quality. These contaminants reach down groundwater and surface water bodies which disturb the aquatic ecosystem. These environmental deteriorations impact indirect or direct harmful consequences to human health leading to respiratory illness, nervous system damage, hormonal imbalance, dermal allergic issues, on a serious note which can also be lethal. Mitigation of these contaminants is important for environment safety and human health. Conventional methods used for managing these contaminants are found effective, but not up to the optimum and also leave residues behind. The microbe assisted phytoremediation technique is eco-friendly with minimum or no residues of contaminants and can be used effectively in treatment/remediation of contaminated environments.

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