Potential Application of Plant-microbe Interaction for Restoration of Degraded Ecosystems

Content

INTRODUCTION

HEAVY METAL POLLUTION

Remedial Measures

Analytical Techniques

Recent Advances in Heavy Metal Bioremediation

PESTICIDE POLLUTION

Bioremediation of pesticides

Advantages and Limitations of Bioremediation

Analytical Techniques

Recent Advances in Pesticide Bioremediation

ECOLOGICAL DEGRADATION AND ITS CAUSES

Eco-restoration of Degraded Ecosystems

FUTURE RESEARCH DIRECTIONS

CONCLUSION

REFERENCES

ADDITIONAL READINGS

KEY TERMS AND DEFINITIONS

ABSTRACTRapidly increasing human population, urbanization, industrialization, and mining

activities has become one of the serious environmental issue of today’s world. These

activities have added substantial quantities of organic and inorganic pollutants such

as xenobiotics (pesticides, pharmaceuticals, petroleum & compounds), toxic and

radioactive heavy metals in the environment, which causes hazardous effects on

living organisms and impair environmental quality. Coal and mineral mining has

resulted forest degradation, biodiversity loss, acid mine drainage, air, water and soil

quality deterioration in various parts of the world. Conventional physico-chemical

remediation methods are highly expensive and often generate secondary waste.

However, bioremediation/phytoremediation of contaminated ecosystems using

indigenous microbes and plants or amalgamation of both has been recognized as a

cost effective and eco-friendly method of remediation as well as restoration of mine

degraded ecosystems. Further, variety of pollutant attenuation mechanisms

possessed by microbes and plants makes them more feasible for remediation of

contaminated land and water over physico-chemical methods. With respect to their

direct roles in remediation processes, microbes and plants use several strategies for

dealing with environmental pollutants. Plants and microbes act cooperatively to

improve the rates of biodegradation and biostabilization of environmental

contaminants. Ecological restoration embraces a broad suite of goals, ranging from

amelioration of highly degraded abiotic conditions i.e. toxic pollutant levels and the

absence of topsoil on old mine sites, enhancement of key ecosystem functions e.g.

production, erosion control, water flow and quality, to the reestablishment of a target

biotic community such as rare species, native species, increased biodiversity and

eradication of invasive species. In terrestrial ecosystems, plant–microbe interactions

are the foundation for effective and sustainable achievement of any of these goals.

This chapter aims to emphasize on potential application of microbes and plants to

attenuate the organic and inorganic contaminants from the contaminated sites as

well as eco-restoration of mine degraded/jhom lands by way of biodegradation and

phytoremediation technologies.

INTRODUCTION

Fast growing population, industrialization, mineral mining, oil exploration, modern

agricultural practices and related anthropogenic activities in the world has resulted

elevated levels of toxic metal and xenobiotic pollutants in the environment (Bernhoft,

2012). Mineral mining, oil exploration and various metal processing industries has

led to the dramatic increase in concentration of toxic heavy metals and metalloids

such as iron, chromium, Nickel, cadmium mercury, lead, zinc, arsenic etc (Giri et al.

2014); petroleum hydrocarbons (PHC), and polycyclic aromatic hydrocarbons

(PAHs). However, intensive agriculture, and crop protection strategies led to the

build up of variety of persistent organic pollutants such as insecticides, fungicides,

herbicides, rodenticides, nematicides and other toxic organic compounds in the air,

water and soil. In order to cater the demands of fast growing population, the rapid

expansion of industries, food, health care, vehicles, etc. is necessary, but it is very

difficult to maintain the quality of environment with all these new developments,

which are unfavourable to the environment. The adverse effects of metals and

pesticide toxicity have been well documented. These pollutants impose hazardous

impacts on living organisms and ecosystem health (Bernhoft, 2012; Godt et al.

2006; Jomova et al. 2011; Patrick, 2006; Auger et al. 2013).

Therefore, remediation of these contaminants is becoming one of the serious

environmental issues in the world (Chaudhry et al. 2005; Euliss et al. 2008). The

common remedial measures for restoration of contaminated environment include

various Conventional physico-chemical methods. These remediation technologies

required high energy or large input of chemicals causing pollution (Yang et al. 2009);

and all these methods are not cost-effective because of secondary waste generation

(Rawat et al. 2014). Phytoremediation has now emerged as a promising strategy for

in-situ removal of many organic and inorganic contaminants (Susarla, et al. 2002;

Macek et al. 2000; Pulford, & Watson, 2003; Pilon-Smits, 2005; Greenberg, 2010).

Microbe-assisted phytoremediation, including rhizoremediation, appears to be

effective for removal and/or degradation of contaminants from contaminated

environment, particularly when used in conjunction with appropriate agronomic

techniques (Kuiper et al. 2004; Singer et al. 2004; Chaudhary et al. 2005; Hauang et

al. 2005 & Zhuang et al. 2007). However, restoration of mine degraded and jhom

land represents an indefinitely long-term commitment of ecosystem restoration

process. Natural recovery in mine spoils/jhom land is a very slow process which

may take many years of natural succession on a mine degraded land for the total

nutrient pool recovery to the level of native forest soil. The first step in any

restoration program is to protect the disturbed habitat and communities from being

further wasted followed by to accelerate re-vegetation process for increasing

biodiversity and stabilizing nutrient cycling. As a result of natural succession by

planting desirable plant species on mine degraded ecosystems/jhom lands a self-

sustaining ecosystem may be developed in a short period of time (Bhattacharya,

20005; Giri et al. 2014). This chapter provides an overview of plant microbe

interaction for restoration of degraded environment (Anderson et al. 1993; Siciliano

and Germida, 1998).

HEAVY METAL POLLUTION

Metals are found naturally in soil, water and sediments in background concentration

and have been used by humans for thousands of years. Human activity releases

them into the environment in much higher concentration that may have adverse

impact on ecosystem functioning. Metals with atomic mass over 20 and specific

gravity above 5 g cm-3 are known as heavy metal. They can be metalloids that have

toxic effect on biological components of an ecosystem even at low concentration.

Metals in soil may range in different concentrations from less than one to as high as

100000 mg kg−1 (Pal and Rai, 2010). Although some metals viz., Co, Cu, Fe, Mo,

Mn, Zn and Ni are essential for cell as they are required for normal growth and

metabolism for all life forms, while other (e.g. As, Cd, Hg, Pb, and Se) are toxic

and/or non essential due to complex compound formation within the cell. Once

introduced into the environment heavy metals cannot be degraded easily and persist

indefinitely for longer period and pollute the ecosphere.

Rapid industrialization and consumerist life style has led to an unprecedented

increase of such toxic substances in natural environment. Although several long term

health effects of heavy metals are well known for a long time, exposure to these toxic

substances is continue and even increasing in some parts of the world, particularly in

developing and less developed countries. Heavy metal pollution occurs both at the

industrial production level as well as the end use of products and run-off. They enter

the human body through food, water and inhalation of polluted air, use of cosmetics,

drugs, poor quality herbal formulations particularly ‘Ayurvedic/Sidha bhasamas’,

(herbo-mineral preparations) and `Unani’ formulations, and even items like toys

which have paints containing lead (INSA, 2011). Some industrial sources of heavy

metal pollution are presented in table 1.

Injudicious applications of synthetic fertilizers such as phosphate have

deposited heavy metals in much higher concentrations on earth surface than natural

background sources. Phosphate fertilizers show big source of cadmium. For

example in Scandinavia, cadmium concentration in agriculture soil increases by 0.2

% per year (Mohammed et al. 2011). In recent years the use of energy-saving CFL

bulbs has increased enormously. According to a report CFL bulbs production has

increased 500 million in 2010 from 19 million in 2002. These bulbs can prove to be a

major health hazard as each contains 3-12 mg of mercury, with no system to recover

these bulbs and safe disposal (INSA, 2011).

Table 1: Sources of heavy metals

Metal IndustryChromium (Cr) Mining, industrial coolants, chromium salts manufacturing,

leather tanning

Lead (Pb) lead acid batteries, paints, E-waste, Smelting operations, coal-

based thermal power plants, ceramics, bangle industry

Mercury (Hg) Chlor-alkali plants, thermal power plants, fluorescent lamps,

hospital waste (damaged thermometers, barometers,

sphygmomanometers), electrical appliances etc.

Arsenic (As) Geogenic/natural processes, smelting operations, thermal

power plants, fuel burning

Copper (Cu) Mining, electroplating, smelting operations

Vanadium (Va) Spent catalyst, sulphuric acid plant

Nickel (Ni) Smelting operations, thermal power plants, battery industry

Cadmium (Cd) Zinc smelting, waste batteries, e-waste, paint sludge,

incinerations & fuel combustion

Molybdenum (Mo) Spent catalyst

Zinc (Zn) Smelting, electroplating

(Source: INSA, 2011)Remedial Measures

Strategies for remedy of heavy metal pollution involve reducing the bioavailability,

mobility and toxicity of heavy metals. This can be achieved by three complementary

functions viz., technological, management and regulatory. Technological methods

involve development of the treatment system whereby pollution load in the waste or

effluent is brought within the safe limits before discharge in the environment.

Technology for remediation should be cost effective and environmentally

sustainable. Management function is important to ensure that the right technologies

are being adopted and also monitor the end results. Regulations ensure the safety

and health of workers as well as the public in general by regulating the toxic metal

levels in effluent release in the environment. Most conventional techniques such as

thermal processes, physical separation, electrochemical methods, washing,

stabilization/solidification and burial are too expensive, require high energy and may

generate secondary pollutants that affect biological functioning of an ecosystem.

Therefore, alternate techniques such as bioremediation, particularly plants

microbe interaction is gaining much attention for heavy metal pollution and eco-

restoration of contaminated environment. Bioremediation technologies are more

acceptable and offer many advantages over conventional treatment methods, for

example, cost effectiveness, high efficiency, minimizing the disposable sludge

volume and it also offers the flexibility for desorption techniques for biomass

regeneration and/or recovery of metals (Eapen & D’Souza, 2005). Plants based

technology capable of extracting and accumulating significant level of heavy metal.

Phytoremediation approaches with its subset (e.g. phytoextraction,

phytovolatilisation and phytostabilisation) for heavy metal pollution abatement has

been well documented in recent years. At present, more than 400 plant species of 45

families are known accumulate heavy metals (Pal & Rai, 2010; Reeves & Baker,

2000; Guerinot & Salt, 2001). Several plant species e.g. Alyssum bertolonii, Brassica

juncea, Eichhornia crassipes, and Iberis intermedia have been found to sequester

various metals in their tissues (Pinto et al. 1987; Robinson et al. 1997; Brooks et al.

1998; Anderson et al. 1999; Boominathan et al. 2004). Success of the process of

absorption and transformation of heavy metals into plant system strictly depends on

their solubility and complexity (Rungwa, et al. 2013). However, Plants have

constitutive and adaptive mechanisms for extracting, accumulating and tolerating

high concentrations of their rhizospheric contaminants. Plants have developed range

of potential mechanism to tolerate and avoid toxic effect of high metal concentration,

which are as follows:

1. Immobilization of heavy metals in cell walls, preventing their contact with

protoplasm. Plant cell wall acts as a cation exchanger and can hold variable

quantities of metal (Rauser, 1999).

2. Compartmentalization and formation of complexes with inorganic and organic

acid, phenol derivatives and glycosides in the vacuole (Singh et al. 2010).

3. Chelation in the cytoplasm by peptide ligands such as metallothioneins (MTs)

and phytochelatins (PCs). MTs are cysteine-rich polypeptides. PCs are trace

metal binding peptides play key role in metal tolerance. PCs protect plant

enzymes from trace metal poisoning (Pal & Rai, 2010; Singh et al. 2010).

Microorganisms associated with plants root system also play significant role in plants

mediated heavy metal remediation technologies. Such microbial community can be

classified into two major group viz., michorrhizal fungi and plant growth promoting

rhizobacteria (PGPR). These microbes in rhizosphere provide a critical link between

plant and soil, which is described in Figure 1.

Figure 1. Rhizosphere microorganisms as a critical link between plants and soil

(Source: Richardson et al. 2009; Hrynkiewicz & Christel, 2012)

Michorrhizal fungi form major component of rhizosphere and show mutualistic

association with most plants (Azcón – Aguillar & Barea, 1992; Marques et al. 2009).

Michorrhizal fungi such as, arbuscular mycorrhizal fungi (AMF) can benefits plant

(Marques, et al. 2009) in following ways:

1. Improve nutrient absorption through extensive extra radical hyphal networks,

which explore the soil, absorb nutrients, and translocate them to the roots.

2. Modify root system resulting in a more extensive length and increased

branching and therefore enhanced nutrient absorption capacity of roots.

3. Changes the chemical composition of root exudates and influences soil pH

thus quantitatively affecting the microbial populations in the rhizosphere.

4. Improve soil structure.

5. Regulate hormones.

6. Tolerance and protection against biotic and abiotic stress such as soil-borne

plant pathogens, insect herbivores, drought and high levels of heavy metals.

On the basis of relationship with plants, plant growth-promoting rhizobacteria

(PGPR) communities can be divided into two groups (a) symbiotic bacteria and (b)

free-living rhizobacteria (Khan, 2005). These organisms are able to enhance plant

growth through various mechanisms (Marque et al. 2009), such as:

1. Allowing plants to develop longer roots during early stages of growth by

reducing ethylene production.

2. Nitrogen fixation.

3. Specific enzymatic activity.

4. Supply bioavailable phosphorous and other trace elements for plant uptake.

5. Production of phytohormones such as auxins, cytokinins, and gibberellins.

6. Produce antibiotic that protect plants from diseases.

7. Increase plant tolerance against flooding, salt stress, and water deprivation.

8. Produce siderophores (low molecular mass compounds, 400-1000 K dalton).

Play key role in solubilizing unavailable forms of heavy metal bearing minerals

by complexation reaction (Rajkumar, et al. 2012).

Different microorganisms apply different mechanisms for growth and metal tolerance

in plants, so it can be beneficial to design the process of phytoremediation in

combination with appropriate microbial consortium, which may include AMF and

PGPR.

Analytical Techniques

Analysis of pollution load is an integral part of environmental management. In

environmental samples heavy metals can exist in a range of physicochemical forms

such as, hydrated metal ions and inorganic and organic complexes. There are many

good analytical methods for analyzing the heavy metals in environment such as,

atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission

spectrometry (ICP/AES), inductively coupled plasma mass spectrometry (ICP/MS),

X-ray fluorescence (XRF) and ion chromatography (IC). Most of these techniques

required sample digestion before quantification of metal. The aim of digestion is to

achieve a selective or complete extraction of metals from the samples. Mostly, the

digestion procedures are based on the addition of inorganic acids such as, aqua

regia, HNO3-HF, HFHNO3-H2SO4-HClO4, HNO3-HClO4 in a closed vessel, which may

be heated on different sources (Jeneper & Hayao, 2005; Nieuwenhuize et al. 1991;

Scancar et al. 2000; Hseu et al. 2002).

Atomic Absorption spectroscopy is based on absorption of radiation by atoms.

Absorption results in the excitation of electrons of atoms which jump to the higher

energy levels. The amount of energy absorbed in the form of photons by sample is

measured by AAS. The energy required for an electron to leave an atom is known as

ionization energy and is specific to each chemical element. Absorbance is directly

proportional to the concentration of the analyte present in the sample (Garcia &

Baez, 2012).

Inductively coupled plasma atomic emission spectrometry (ICP/AES) is based

on principle that atoms emit light when excited by plasma. Plasma is ionized gas with

very high temperature range from 7000 to 10000 ˚K. Excited atom emit characteristic

spectra (Wang, et al. 2003). Inductively coupled plasma mass spectrometry (ICP-

MS) is a very powerful, highly sensitive and specific technique for the analysis of

trace (ppb-ppm) and ultra-trace (ppq-ppb) element and isotope. ICP-MS is

composed of plasma (a high temperature i.e. 8000 ˚K ionization source), quadrupole

mass spectrometer (MS) analyzer (sensitive rapid scan detector) and a distinctive

interface. The detection of elements is done by their mass-to-charge ratio (m/z) and

intensity of a specific peak in the mass spectrum is proportional to the amount of that

isotope (element) in the original sample. ICP/AES and ICP/MS are the future

techniques for heavy metal detection in environmental samples because of

accuracy, rapid and multi element analysis (Tu et al. 2010).

X-ray fluorescence is a non-destructive method for analyzing samples.

Fluorescence involves emission of an X-ray photon after ionization of atom by a

primary X-ray beam. When primary X-ray beam strikes a sample, it interacts with

electron and knocks it out of its inner shell forming voids. These voids present an

unstable condition of atom, which stabilized when the void promptly filled by outer

shell electron and give off X-ray with specific wavelength. This characteristic X-ray is

the measure of elemental composition of a sample (Meirer et al. 2010).

Recent Advances in Heavy Metal Bioremediation

The process of phytoremediation has gained much attention in last few years to

explore molecular and biochemical pathways involve in heavy metal uptake,

transport and storage in plants (Clemens et al. 2002; Pilon-Smits and Pilon, 2002;

Pollard et al. 2002; Eapen & D’Souza, 2005). However, the process of

phytoremediation is rather slow; an improved technique via biotechnological

approach can overcome the problem. Genetic modifications in plants to enhance the

efficiency of remediation technique require a deep insight into the complete

mechanism of heavy metal extraction by plant.

Development of transgenic plants with increased metal selective organic acid,

ligands and phytochilatins could have promising applications in heavy metal

decontamination. It is well known that organic acids and peptide ligands form

complexes with metals. For example, free histidine is found as metal chelator in

xylem exudates of Ni hyperaccumulators, therefore, by modifying histidine

concentration in xylem exudates Ni accumulating capacity of plants can be improve.

Cellular targeting manipulation specifically in metal transporters and vacuoles is

important since the compartmentalization of heavy metals is safe mechanism

adopted by most plants without disturbing the cellular functions. Great successes

have been achieved in the development of transgenic plants with enhanced heavy

metal accumulating capacity but majority of genes have been transferred from other

plants or organisms (Eapen & D’Souza, 2005).

To develop plant species better suited for phytoremediation of metal

contaminated sites Thlaspi caerulescens has been used as source of genes by

various workers (Brewer et al. 1999; Gleba et al. 1999; Lombi et al. 2001). Brewer et

al. (1999), developed somatic hybrids between T. caerulescens and Brassica napus.

The selected high biomass hybrids for Zn tolerance were found to capable of

accumulating Zn level that would otherwise toxic to B. napus. In other study somatic

hybrids from T. caerulescens and B. juncea were also able to remove significant

amounts of Pb (Gleba et al. 1999). Transgenic B. juncea showed efficient affinity for

Se uptake with enhanced Se tolerance than the wild species (Pilon-Smits et al. 1999,

Huysen et al. 2004). Transgenic B. juncea with Se tolerance was developed by

transferring the selenocysteine methyltransferase (SMT) from the A. bisulcatus (Se

hyperaccumulator). SMT transgenic plants of B. juncea accumulate 60% more Se

than the wild-type when grown in a contaminated soil (Zhao & McGrath, 2009;

Rascio & Navari-Izz, 2011). Transgenic plants have proved to be a promising

biotechnological approach, but only few field studies have been performed till now

(Zhao & McGrath, 2009; Rascio & Navari-Izzo, 2011).

Application of mixed microorganisms with plant species can provide effective future

measures for heavy metal decontamination. However, several obstacles need to

overcome for commercial application of such treatment system (Hrynkiewicz &

Baum, 2012) such as,

1. Commercially cost-effective mass-production and formulation of microbial

inoculums.

2. Microbial inoculum should be relatively universal for various plants and soils

and its effectiveness should be relatively easy to evaluate.

3. Effectiveness of microbial consortium to function in natural conditions.

4. Knowledge of possible interactions between plants and associated soil

microorganisms in natural environment.

However, additional research is expected to overcome these problems (Rajkumar et

al. 2012), for example

1. Complete physiological and molecular characterization of several

environmentally relevant microorganisms.

2. Exploration of mechanism followed by microbial chelators-metal complex

uptake in plants.

3. Effects of factors influencing the solubility and plant availability of

nutrients/heavy metals.

4. Identification of signaling processes that occur between plant roots and

microbes.

5. Effect of manipulation in rhizosphere zone processes such as coinoculating

ecologically diverse microorganisms on phytoremediation process.

Such knowledge may enable us to exploring the mechanism of metal-microbes-plant

interactions and to improve the performance and use of beneficial microbes as

inoculants for microbial assisted phytoremediation (Rajkumar et al. 2012).

PESTICIDE POLLUTION

Pesticides have long history since the emergence of agriculture. Human beings are

facing the development of pests (including weeds, insects and pathogenic agents)

causing considerable agricultural losses. If these pests are not controlled, they

diminish the quality and quantity of crop production (Richardson, 1998). In the

beginning, either some inorganic chemicals or compounds extracted from plants

were used as pesticides. The pyrethrine was extracted from Chrysanthemum flowers

and used to control the pest development during winter storage of crop. This was

reported by the Greek civilization and authorization of this compound is still going on.

However, agricultural revolution in the 19 th century has lead to the intensive and

diversified use of the pesticides corresponding to compounds derived from minerals

and plants. As an example, the development of Bouillie Bordelaise (Bordeaux

mixture) in 1880, consisting of copper sulphate and lime allowed better control of

cryptogamic diseases in Bordeaux and French vineyard. It is still in use for vineyard

and fruit tree protection. Development and application of pesticides for the control of

various insectivorous and herbivorous pests is considered as fundamental

contributor to this “Green Revolution”. The use of synthetic organic pesticides began

during the early decades of 20th century and increased tremendously after the World

War II, with the introduction of synthetic organic molecules such as DDT, aldrin (two

insecticides) and the herbicide 2,4-D in the agricultural market. Due to their

advantages of being effective and cheap, use of synthetic pesticides is continously

increasing in the whole world.

Although pesticide application ensures better yield in agricultural production,

however, when they contaminate the soil and water resources, became harmful for

the environment and living beings through the food chain (Briceno et al. 2007). Due

to their intensive and repeated application, and their relative recalcitrance to

biodegradation, pesticide residues are persistent in the environment where they

have often been detected beyond the permissible limits in different compartments of

the environment as well as in food chain. In many parts of the world, particularly in

developing countries, clean drinking water is a limited resource and, in this context,

intensive agricultural production is a major environmental and health problem

because pesticide residues accumulate in surface and under ground water

(Rasmussen et al. 2005). Contamination with pesticides is restricted not only to

developing countries but also in Europeon countries, where pesticide residues have

often been detected in surface and ground water resources (Gooddy et al. 2002). As

a result, the use of pesticides in conventional agriculture has attracted much

attention in recent years due to rising public and governmental concerns about their

impact not only on environmental contamination but also on human and animal

health.

Pesticide exposure to environment is dependent on various factors like

production, formulation processing and application doses. A pesticide enter in to the

environment via (1) direct intentional application to soil to control pre emergent

weeds, plant pathogens, soil insects /pests, and/or (2) indirect unintentional entry

followed by foliar application for post emergent weeds and insects/pests (Mathews,

2006; Brieceno et al. 2007). In addition to this a certain portion of pesticides may

undergo spillage from formulation plants during processing and waste disposal

process as well. Adverse impact of pesticides on soil biology and ecosystem have

been described by many researchers (Sacki and Toyota, 2004).

These recalcitrant compounds build up regularly in the environment, as they

are not at all biodegradable, and even if degraded, very slowly. Owing to low water

solubility, pesticides have strong affinity for particulate matter and consequently

enter in to water sediments (Giri et al. 2014). For instance Lindane, the most

commonly used isomer of HCH is known to accumulate in food chains, causing

toxicity in wild/domestic animals and human beings. Apart from food contamination,

human beings are exposed to lindane by inhalation, polluted water and dermal

contact (Giri et al. 2014).

Repeated applications of haloginated insecticide endosulfan causes its

accumulation in the soil and water environment. Consequent upon accumulation, it is

extremely toxic to aquatic fauna, while provoking chronic symptoms like testicular

and prostate cance, breast cancer, sexual abnormality, genotoxicity and

neurotoxicity in various mammalian species (Giri and Rai, 2012).

Bioremediation of Pesticides

Bioremediation is the use of microorganisms for degradation of hazardous chemicals

in soil, sediments, water, or other contaminated materials. Often the microorganisms

metabolize the chemicals to produce carbon dioxide or methane, water and biomass.

Alternatively, the contaminants may be enzymatically transformed to metabolites that

are less toxic or environmentally innocuous. It should be noted that in some

instances, the metabolites formed are more toxic than the parent compound. For

example, perchloroethylene and trichloroethylene may degrade to vinyl chloride

which is highly toxic in nature than parent compound (Sacki and Toyota, 2004).

There are a number of possible pesticide degradation pathways in the soil

and water environment including chemical treatment, volatilization,

photodecomposition and incineration. However, most of them are not applicable for

the diffused contamination with low concentration because of being expensive, less

efficient and environmental friendly. Thus, keeping in view the environmental

concerns associated with pesticides/recalcitrant compounds, there is a need to

develop safe, convenient and economically viable methods for its remediation. In this

context, several researchers have focused their attention to study the microbial

biodegradation which has been reported as a primary mechanism of pesticide

dissipation from the environment (Cox et al. 1996, Pieuchot et al. 1996). Although

bioremediation strategies are more acceptable to the society because of their

reduced impact on the natural ecosystem (Zhang and Quiao, 2002). However,

complexity of the mechanisms responsible for pesticide degradation has made it

slow to emerge as an economically viable remediation method (Nerud et al. 2003). It

is noteworthy that bioremediation strategies have been developed extensively for

taking care of sites heavily contaminated with organic pollutants, however, up to

now, the sites diffusely contaminated are only monitored and natural attenuation is

the process of interest leading to contaminant abatement.

Microbial biodegradation occurs mostly in the soil solution. Pesticide microbial

biodegradation is carried out by soil microorganisms like bacteria fungi and

actinomycetes possessing a large set of enzymes susceptible to transform these

pesticides. It is the principal mechanism for diminishing the persistence of pesticides

in soil environment (Arbeli and Fuentes, 2007). Soil serves as a potential habitat for

variety of microorganisms which have the ability to interact not only with other living

components but also the physical elements including pesticides for the fulfilment of

their energy requirement. When pesticides are applied in the soils, enzyme-driven

biochemical reactions carried out by the indigenous soil microorganisms result in

modification of the structure and toxicological properties of pesticides leading to their

complete conversion into harmless inorganic end products (Hussain et al. 2009a).

Pesticides degradation by soil microbial communities has been reported by several

researchers (Fenlon et al. 2007; Hussain et al. 2007a; Shi and Bending, 2007;

Hussain et al. 2009b; Sun et al. 2009) and it has been described as a primary

mechanism of pesticide dissipation from the environment (Fournier et al. 1975; Cox

et al. 1996; Pieuchot et al. 1996). The efficiency of pesticide biodegradation varies

considerably between different groups of the microorganisms and even between the

different members belonging to the same group of microorganisms. Although a

strong diversity of microbial species is found in the soil, however, the adaptability of

these different degrading microbial species in the contaminated soils assures the

continuity of biodegradation process. Microbial biodegradation of pesticides in the

soil can be categorized into two principal types based on the mode and pathway of

degradation i.e. metabolic and co-metabolic.

Metabolic degradation

Metabolic pesticides degradation is carried out by soil microbial population

harbouring specific catabolic enzymes allowing complete mineralization of target

compound. A large number of pesticide degrading fungal and bacterial strains have

been isolated and characterized from the soil environment (Hussain et al. 2009b).

Although often metabolic biodegradation can leads to incomplete degradation

resulting in the formation of metabolites (Turnbull et al. 2001; Hangler et al. 2007;

Badawi et al. 2009). However, up to now, the full mineralization of the pesticides has

been found to be taking place only by the soil bacteria. The enzymes required for

metabolic degradation of pesticides are either harboured by a single microorganism

or scattered in various microbial populations working as a cooperative consortium,

jointly involved in the degradation of the pesticides (Fournier et al. 1996).

Co-metabolic degradation

The co-metabolic degradation corresponds to the non specific degradation of

xenobiotic molecule by microorganisms. In most of the cases, this is a non-inducible

phenomenon occurring because of the presence of detoxifying enzymes able to

degrade xenobiotics depicting homologies with their substrate. In this case, the

target pesticides do not contribute to the growth of the degrading organisms (Dalton

and Stirling, 1982; Novick and Alexander, 1985). For this reason, the degradation

rate of pesticide in a given environment depends primarily on the size of microbial

biomass and on the competitiveness of the degrading microbial population towards

sources of energy and nutrients in the soil. In other words, pesticide degradation rate

is dependent on size of the biomass (Fournier et al. 1996). In general, co-

metabolism does not yield in extensive degradation of the molecule but rather

causes incomplete transformation such as oxidation, hydroxylation, reduction, N-

dealkylation or hydrolysis (Fournier et al. 1996) which may lead to the formation of

metabolites that may prove even more toxic and recalcitrant than the parent

compound (De Schrijver and De Mot, 1999).

Some compounds can only be partially metabolized by microbial populations

and transformed into metabolites that may either accumulate in the environment or

be metabolized further by other microbial species. These metabolic reactions do not

provide benefit to the responsible organism because they do not gain either carbon

or energy. These processes are typically fortuitous and occur because the

responsible population produces one or more enzymes that are comparatively

nonspecific and can react with structural analogues compounds of the “normal”

substrate for enzyme(s). Co-metabolism is important for the degradation of many

environmental contaminants particularly chlorinated pesticides solvents (e.g.

trichloroethylene,), polychlorinated biphenyls, and many polyaromatic hydrocarbons

(Fournier et al. 1996). Giri and Rai (2012), studied Biodegradation of endosulfan

isomers in broth culture and soil microcosm by Pseudomonas fluorescens. After 15

days incubation, maximum 92.80% α and 79.35% β endosulfan isomers were

degraded in shake flask culture at 20 mg/L concentration, followed by 50 and 100

mg/L, while the corresponding values in static condition were 69.15 and 51.39%,

respectively.

Advantages and Limitations of Bioremediation

The use of intrinsic or engineered bioremediation processes offers several

potential advantages that are attractive to site owners, regulatory agencies, and

the public. These include:

1. Lower cost than conventional technologies.

2. Contaminants usually converted to innocuous products.

3. Contaminants are destroyed, not simply transferred to different environmental

media.

4. Nonintrusive, potentially allowing for continued site use.

5. Relative ease of implementation.

However, there are some limitations to bioremediation as well, these include:

1. Difficult to control the laboratory optimized conditions in the field.

2. Amendments introduced into the environment to enhance bioremediation may

cause other contamination problems.

3. May not reduce concentration of contaminants to the required levels.

4. Requires more time.

5. May require more extensive monitoring.

.

Analytical Techniques

The development of new technologies and their implementation in the analysis of

pesticides in environmental samples has greatly affected the way we perceive and

use pesticides. In the 1940’s, pesticides were perceived as miracle chemicals that

gave tremendous gain in crop yields and they were used without adequate regard to

health and the environment. At the time, thin layer chromatography (TLC) with semi-

quantitative detection was the primary means of analysis. Gas liquid chromatography

(GLC or GC) with packed columns became the method of choice as commercial

instruments improved and selective quantitative detectors were developed in the late

1950’s, to mid 1960’s. By the time of publication of Rachel Carson’s Silent Spring in

1962, GC was predominant method of pesticide analysis (Hawthorne et al. 1994).

When the environmental and ecological impacts of pesticides were come in to lime

light, the perception of pesticides began to change. Laws that established regulatory

controls on the use of pesticides and their presence in the environment required

residue analysis using state-of-the-art instrumentation (Hopper, 1996). With the

development of improved capillary columns for GC in the 1970’s, tremendous gains

in separation power were achieved and the capabilities of multi-residue methods

improved accordingly. During the same time frame, high performance liquid

chromatography (HPLC) was commercialized and its implementation in pesticide

residue analysis permitted detection of many compounds that were not analysed

easily. Through the complementary nature of GC and HPLC, a wide range of

pesticides could be analysed and many environmentally safer pesticides were

developed and registered using these sophisticated technologies (Richter et al.

1996). Presently potentially advanced and sophisticated pesticide analytical methods

such Gas chromatography mass spectrometry (GC-MS) Liquid chromatography

mass spectrometry (LC-MS), etc, have been developed and commercialized. Figure

2 represent a schematic representation of pesticide analysis method in

environmental samples.

Preparation Sample

Extraction Organic Solvents

Clean-up

Liquid-Liquid Partitioning SPE GPC Concentration

Separation/ Analysis GC HPLCUse of selective detectors and

multiple analyses are common

(SPE= solid phase extraction, GPC= gel permeation chromatography, GC= gas

chromatography, HPLC= high performance liquid chromatography)

Figure 2. Pesticide residue analysis method in environmental samples(Source: Hawthorne et al. 1994)

Recent Advances in Pesticide BioremediationPesticide degrading catabolic gene and their respective enzymes of microorganisms

have been isolated and identified by several researchers. For example lindane

(Kumari et al. 2002), endosulfan (Sutherland et al. 2002; Hussain et al. 2007), DDT

(Barraga et al. 2007) and monocrotophos degrading microbial genes and enzymes

have been (Subhas and Singh 2003; Das and Singh, 2006), isolated and identified.

Genetic studies revealed that plasmids are the main place to harbour pesticide

catabolising genes in microbial community. Sutherland et al. (2002) had reported

Esd gene having sequence homology to monooygenase family which uses reduced

flavin, provided by a separate flavin reductase enzyme, as co-substrates in

Mycobacterium smegmatis. Esd catalyzes the oxygenation of β-endosulfan to

endosulfan monoaldehyde to endosulfan hydroxyether. Esd did not degrade either α-

endosulfan or the metabolites of endosulfan and endosulfan sulphate. Wier et al.

(2006) have reported that Ese gene of Arthrobacter sp. encoding enzyme from

monooxygenase family is capable of degrading both the isomers of endosulfan.

After, understanding the gene of interest and enzyme involved, the Superbugs can

be created to achieve the desired result at fast rate in short time frame. Lal et al. [25]

has reviewed the degradation of HCH and distribution of lin gene in Sphingomonads.

S. indicum B90A was found to contain two non-identical linA genes (designated as

linA1 and linA2). The linA-encoded HCH dehydrochlorinase (LinA) mediates the first

two steps of dehydrochlorination of γ-HCH (Singh, 2008). Besides, genetically

modified microbes are used to enhance the capability of pesticide degradation.

However, genetically engineered technology for environment use is still controversial

because an adverse genotype can be readily mobilized in the environment. In a

development of technology following points should be taken care i.e. (i)

heterogeneity of contaminant. (ii) concentration of contaminant and its effect on

biodegradative microbe, (iii) persistence and toxicity of contaminant, (iv) behaviour of

contaminant in soil environment and (v) conditions favourable for biodegradative

microbe or microbial population (Singh, 2008). The degradation of persistent

chemical compounds by microorganisms in the natural environment has revealed a

larger number of enzymatic reactions with high bioremediation potential (Finley et al.,

2010). These biocatalysts can be obtained in quantities by recombinant DNA

technology, expression of enzymes, or indigenous organisms, which are employed in

the field for removing pesticides from polluted sites. The microorganisms contribute

significantly for the removal of toxic pesticides used in agriculture and in the absence

of enzymatic reactions many cultivable areas would be impracticable for agriculture

(Abramowicz, 1995).

Although, significant advances have been made in understanding the roles of

plant associated microbial pesticide degradation and application of these processes

in field scale bioremediation (Joshi and Juwarkar, 2009; Li et al., 2010; Shi et al.,

2011). An exciting alternative to the use of plant-associated bacteria to degrade toxic

organic compounds in soil is the use of recombinant DNA technology to generate

transgenic plants expressing bacterial enzymes resulting in improved plant tolerance

and metabolism of toxic organic compounds in soil. Transgenic plants have been

produced for phytoremediation of both heavy metals and organic pollutants (Eapen

et al. 2007). Transgenic poplar plantlets expressing bacterial mercuric reductase

were shown to germinate and grow in the presence of toxic levels mercury.

Arabidopsis thaliana was engineered to express a modified organomercurial lyase

(Rugh et al. 1992) and those transgenic plants grew vigorously on a wide range of

concentrations of highly toxic organomercurials, probably by forming ionic mercury

which should accumulate in the disposable plant tissues. The first report of

genetically modified plant for the transformation of xenobiotic contaminants to

nontoxic material was reported (French et al. 1999). They previously reported that

Enterobacter cloacae PB2 is capable of growth with trinitrotoluene (TNT) as a

nitrogen source (Bhatia and Malik, 2011).

ECOLOGICAL DEGRADATION AND ITS CAUSES

Ecosystem degradation resulting from resource extraction, land-use change, shifting

cultivation, invasion by exotic species, forest fire and subsequent biodiversity loss

alters the functions and services provided by forest ecosystems. Mineral mining

exerts a long lasting impact on landscape, ecosystem and socio-cultural economic

considerations. Mining and its subsequent activities have been found to degrade the

land to a significant extent. Overburden removal from the coal field results in

significant forest and top soil loss (Figure 3). Most of the mining wastes are inert

solid materials and toxic in nature (Guha Roy, 1991). These toxic substances are

inherently present in the ore, e.g. heavy metals such as iron, mercury, arsenic, lead,

zinc, cadmium, etc (Giri et al. 2014). These heavy metals leach out of the stored

waste piles and contaminate immediate environment. However, some toxic

chemicals are also found in waste, as they are added intentionally during extraction

and processing. The major environmental impacts due to coal mining are changes in

soil stratification, reduced biotic diversity, and alteration of structure and functioning

of ecosystems; these changes ultimately influence water and nutrient dynamics as

well as trophic interactions (Giri et al. 2014). Land degradation due to forest

clearance, shifting cultivation and mining activities is the cumulative effect of air and

water pollution, soil quality degradation and biodiversity loss (Sankar et al. 1993).

This process works through a cycle known as land degradation cycle. The

magnitude and impact of mining on environment varies from mineral to mineral and

also depends on the potential of the surrounding environment to attenuate the

negative effects of mining, geographical disposition of mineral deposits and size of

mining operations. A list of minerals has been prepared by Department of

Environment, which is supposed to have serious impact on environment. These

minerals include coal, iron ore, zinc, lead, copper, gold, pyrite, manganese, bauxite,

chromite, dolomite, limestone, apatite and rock phosphate, fireclay, silica sand,

kaolin, barytes. Mineral production generates enormous quantities of waste/

overburden and tailings / slimes (Rai, 1996, Giri et al. 2014).

Figure 3 (a) Open cast coal mining (b) acid mine drainage (c &d) coal dumping in

Margherita Assam, India

Acid mine drainage is a serious environmental issue of coal/mineral mining

activities. This occurs when sulphide ores are exposed to the atmosphere, which can

be enhanced through mining and milling processes where oxidation reactions are

initiated. Mining increases the exposed surface area of sulfur-bearing rocks allowing

for excess acid generation beyond natural buffering capabilities found in host rock

and water resources (Caruccio, 1975). Once acid drainage is created, metals are

released into the surrounding environment, and become readily available to

biological organisms. When fishes are exposed directly to metals and H+ ions

through their gills, impaired respiration may result chronic and acute toxicity. Fishes

are also exposed indirectly to metals through ingestion of contaminated sediments

and food materials. A common weathering product of sulfide oxidation is the

formation of iron hydroxide (Fe (OH)3), a red/orange coloured precipitate found in

thousands of miles of streams affected by acid mine drainage (Figure 3). Iron

a b

c d

hydroxides and oxyhydroxides may physically coat the surface of stream sediments

and streambeds destroying habitat, diminishing availability of clean gravels used for

spawning, and reducing fish food items such as benthic macro invertebrates. Acid

mine drainage, characterized by acidic metalliferous conditions in water, is

responsible for physical, chemical, and biological degradation of aquatic ecosystems

(Ashraf et al. 2010). Acidic water adversely affects the soil environment by way of

making the soil acidic and rich in inorganic component and poor in organic content.

Deterioration of soil quality has severely affects the crop growth and yield in the area

mainly due to high concentrations of hydrogen ions, which inactivate most enzyme

systems, restrict respiration, and root uptake of salts and water by plants. It also

leads to deficiency of nitrogen, phosphorous, calcium, magnesium, molybdenum and

boron as well as iron and manganese toxicity. Solubilisation and transport of

phosphorus from soil to the water environment due to acidity is an important issue

associated with decreased agriculture productivity (Giri et al. 2014). Open cast coal

mining and other mineral mining activities resulted forest degradation, biodiversity

loss and severe environmental pollution in mining areas. These mineral mining

activities are being carried out in various parts of the country such as Madhya

Pradesh, Jharkhand, Chhattisgarh, Orissa, Assam, Meghalaya, Arunachal Pradesh

and Nagaland.

Shifting cultivation also called slash and burn agriculture is the clearing of

forested land for raising or growing the crops until the soil nutrients are exhausted

and/or the site is overtaken by weeds and then moving on to clear more forest. It has

been often reported as the main cause of deforestation and land degradation (Dick,

1991; Barbier et al., 1994 and Ross, 1996). Mostly all reports indicate shifting

agriculture is responsible for about one half of tropical deforestation in the world

(Figure 4 a & b). In India shifting cultivation/jhom cultivation is predominant in

Northeast part of the country, particularly in Assam, Nagaland, Meghalaya and

Mizoram. Shifting cultivation has been considered one of the major causes for

ecological degradation and deforestation in the country, which has become a serious

environmental issue.

Figure 4 Shifting Cultivation and Ecosystem Degradation in Mizoram

ECO-RESTORATION OF DEGRADED ECOSYSTEMS

Ecological restoration embraces a broad suite of goals, ranging from amelioration of

highly degraded abiotic conditions (e.g., toxic pollutant levels and the absence of

topsoil on old mine sites), to the reinstatement or enhancement of key ecosystem

functions (e.g., production, erosion control, water flow and quality), to the

reestablishment of a target biotic community (e.g., rare species, native species, high

diversity, eradication of invasive species). In terrestrial ecosystems, plant–soil

interactions are the foundation for effective and sustained achievement of any of

these goals. Soil conditions constrain plant performance and community composition

(Grime 2001; Pywell et al. 2003), and attempts to restore plant communities are

likely to fail if they do not consider the limitations imposed by soil conditions. In

contrast, plant composition can impact almost every aspect of soil structure and

function Ecological restoration is the practice of restoring ecosystems as performed

by practitioners at specific project sites, whereas restoration ecology is the science

upon which the practice is based (Eviner and Haukes, 2008).

Restoration ecology ideally provides clear concepts, models, methodologies and

tools for practitioners in support of their practice. Sometimes the practitioner and the

restoration ecologist are the same person the nexus of practice and theory. The field

of restoration ecology is not limited to the direct service of restoration practice.

Restoration ecologists can advance ecological theory by using restoration project

sites as experimental areas. For example, information derived from project sites

a b

could be useful in resolving questions pertaining to assembly rules of biotic

communities. Further, restored ecosystems can serve as references for set-aside

areas designated for nature conservation (SER, 2004). Ecological restoration is one

of several activities that strive to alter the biota and physical conditions at a site.

These activities include reclamation, rehabilitation, mitigation, ecological engineering

and various kinds of resource management, including wildlife, fisheries and range

management, agro forestry, and forestry. At the heart of plant–soil interactions lies

the microbial community. Microbial communities (Eviner et al. 2008):

1. are ultimately responsible for most biogeochemical transformations in soil,

2. can play a significant role in impacting soil structure, and

3. Can have strong effects on plant growth and competitive dynamics.

Success in eco-restoration studies requires the presence of key microbial groups,

particularly those microbes that are obligate or facultative symbionts with plant roots.

Plant seedlings grow substantially better when planted into a community with

established mycorrhizal connections than in disturbed sites or in isolation Eviner et

al. 2008. In some cases, such as with pine trees, establishment requires

simultaneous introduction of plants and ectomycorrhizal fungi if these root symbionts

are not already present. Addition of symbiont inoculum can also facilitate restoration

efforts when microbial communities have been disturbed or altered (Eviner et al.

2008). For example mycorrhizal inoculations, have been shown to increase plant

establishment and growth (Cuenca & Lovera 1992); soil organic matter, nitrogen,

aggregation (Requena et al. 2001), and alter succession by shifting competitive

interactions between plants (Allen & Allen 1990). In addition, inhibiting microbial

symbiont establishment can be used as a tool to reduce establishment and growth of

undesirable species. For example, in a study, absence of arbuscular mycorrhizal

fungi (AMF) and actinorhizal Frankia, native oleaster shrub growth decreased by 4-

fold (Visser et al. 1991), whereas growth of an invasive leguminous shrub decreased

by 5-fold in the absence of specific Bradyrhizobium strains (Parker et al. 2006;

Eviner et al. 2008).

FUTURE RESEARCH DIRECTIONS

Isolation of various plants associated microbes and characterization of its beneficial

metabolites/processes are time consuming, since it requires the analysis of more

than thousands of isolates. Thus strong molecular research effort is required in order

to find specific biomarker associated with the beneficial microbes for efficient

microbe assisted bioremediation. Although promising results have been reported

under laboratory conditions, showing that inoculation of beneficial microbes

particularly plant growth promoting bacteria and/or mycorrhizae may stimulate heavy

phytoextraction or phytostabilization. Only a few studies have demonstrated the

effectiveness of the microbial assisted plant bioremediation of pesticides and toxic

metals in field conditions (Brunetti et al., 2011; Juwarkar and Jambhulkar, 2008; Wu

et al., 2011a; Yang et al., 2012). Genetically engineered organisms with novel

pathways will to generate new or improved activities hold a great potential for

enhanced bioremediation. Using genes encoding the biosynthetic pathway of bio-

surfactants can enhance biodegradation rates by improving the bioavailability of the

substrates and genes encoding resistance to critical stress factors may enhance

both the survival and the performance of designed catalysts. Thus, genetic

engineering of indigenous microflora, well adapted to local environmental conditions,

may offer more efficient bioremediation of contaminated sites and making the

bioremediation more viable and eco-friendly technology. Complete genome

sequences for several environmentally relevant microorganisms, mechanism of

pesticide solubility, uptake and availability of nutrients/ pesticides, signalling

processes that occur between plant roots and microbes, these types of analysis will

surely prove useful for exploring the mechanism of pollutants-microbes-plant

interactions. Moreover, such knowledge may enable us to improve the performance

and use of beneficial microbes as inoculants for microbial bioremediation.

Emphasis should be placed when developing bioremediation systems using

plant-associated bacteria, to choose wild type bacteria, or bacteria enhanced using

natural gene transfer, to avoid the complications of national and international

legislation restricting and monitoring the use of genetically modified microbes

(GMMs). However, with a global political shift towards sustainable and green

bioremediation technologies, the use of plant-associated bacteria to degrade toxic

synthetic organic compounds in environmental soil may provide an efficient,

economic, and sustainable green remediation technology for future environment

(Bhatia and Malik, 2011).

Much is still unknown about tolerances, degradative capacities and ecological

interactions of organisms that have potential use in eco-restoration. However, it is

clear, that plants and microbes act cooperatively to improve the rates of

biodegradation and biostabilization of environmental contaminants as well as

improve nutrient contents in degraded lands.. Knowledge of the microbial community

structure resident to the rhizosphere of plants that are resistant to a given

contaminant will improve the chances of successfully increasing biodegradation

rates when co-inoculating plants and microbes into contaminated environments. In

designing a eco-restoration program the oxidative capacity of a plant should be

considered in terms of its action on the contaminant itself and for its potential to

support rhizospheric microbes with the capacity to enhance biodegradation.

Additional basic biological and ecological information in these areas will allow us to

make better informed decisions on how to widen bottlenecks in bioremediation/eco-

restoration processes (Cohen et al. 2004).

CONCLUSIONS

Since the plant-associated microbes possess the capability of plant growth

promotion and/or metal mobilization/immobilization. There has been increasing

interest in the possibility of manipulating plant microbe interactions in contaminated

soils (Aafi et al., 2012; Azcón et al., 2010; Braud et al., 2009b; Dimkpa et al., 2008,

2009a,b; Hrynkiewicz et al., 2012; Kuffner et al., 2010; Luo et al., 2011; Luo et al.,

2012; Maria et al., 2011; Mastretta et al., 2009; Orłowska et al., 2011; Sheng et al.,

2008a,b). Bioremediation is cost effective, faster than natural attenuation, high public

acceptance and generates less secondary wastes and emerged as an integrated

tool for environmental cleanup as well as ecosystem service provider. (Dickinson et

al. 2009). The potential role of plants and associated rhizomicrobial population in

facilitating microbial degradation for in situ bioremediation of surface soils

contaminated with hazardous organic compounds is substantial. Support for this

concept comes from the fundamental microbial ecology of the rhizosphere,

documented acceleration of microbial degradation of agricultural chemicals and

mobilization/immobilization of metals in the root zone.

Further understanding of the critical factors influencing the plant-microbe-

toxicant interaction in soils will permit more rapid realization of this new approach to

in situ bioremediation (Dubey and Fulekar, 2013). To effectively restore an

ecosystem or ecological community, it is often critical to consider multiple species,

multiple functions, and their interactions. Furthermore, the restoration of self-

maintaining systems is increasingly requiring the consideration of human-induced

local- to global-scale environmental changes. Studies on plant–soil interactions vis-

à-vis plant microbe interaction provide an important foundation for eco-restoration. In

order to help managers with the challenge of designing successful restoration

techniques at a specific site, we need to embrace the variability of ecological studies

and develop frameworks to understand this variability (Bever, 2002; Eviner and

Hawkes, 2008). Bioremediation is not a Panacea to restore all the contaminated

environmental sites, however, in comparison to other remediation processes i.e.

incineration, thermal disposition, land farming etc. it has a better future in

development of technology for removal of contaminants from actual site and

restoration of degraded lands (Singh, 2008). With a global political shift towards

sustainable and green technologies, the use of plant-associated microorganisms to

degrade toxic synthetic organic and inorganic pollutants in environmental soil may

provide an efficient, economic, and sustainable green remediation technology for

future environment (Bhatiya and Malik, 2011).

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KEY TERMS AND DEFINATIONS

Bioremediation: Bioremediation is the use of living organisms such as microbes

and plants for mitigation and wherever possible, complete elimination of the noxious

effects caused by environmental pollutants.

Biodegradation: Biodegradation is a natural process, where the degradation of a

xenobiotic chemical or pesticide by an organism is primarily a strategy for their own

survival.

Biosorption: Biosorption is a physiochemical process that occurs naturally in

certain biomass which allows it to passively concentrate and bind contaminants onto

its cellular structure.

Bioavailability: The fraction of contaminant actually available to microorganisms is

said to be bioavailable.

Environmental Pollution: Introduction of contaminants into the natural

environment that cause adverse effects on living organisms and ecosystems.

Ecological Restoration: Ecological restoration is the process of assisting the

recovery of an ecosystem that has been degraded, damaged, or destroyed.

Heavy metals: A heavy metal is a metallic element which has high density, specific

gravity or atomic weight and usually toxic in nature.

Metabolic degradation: Metabolic biodegradation of the organic pollutants is

carried out by the soil microbial populations harbouring specific catabolic enzymes

leading to the complete mineralization of target compound.

Co-metabolism: The co-metabolic degradation corresponds to the non specific

degradation of xenobiotic molecule by microorganisms.

Phytoremediation: Phytoremediation is the process of removing/eliminating

inorganic toxic metals and organic compounds using plants and trees from

contaminated environment.

Phytovolatilization: contaminants taken up by the roots pass through the plants to

the leaves and are volatized through stomata, where gas exchange occurs.

Phytostabilization: plants are used to reduce the mobility and bioavailability of

environmental pollutants.

Phytoextraction: plant roots take up contaminants and store them in stems and

leaves

Xenobiotics: A synthetic organic compound such as drug, pesticide, or carcinogen

that is foreign to a living organism is called xenobiotic compounds.

Lower Metabolic Pathway: The organic pollutant degradation pathway involving

cleavage of the aromatic ring structure is called lower metabolic pathway.

Upper Metabolic Pathway: The organic pollutant degradation pathway leading to

formation of some key intermediates/secondary product is called upper metabolic

pathway.

ADDITIONAL READINGSCritical Reviews in Environmental Science and Technology

Giri, K. and Rai, J.P.N. 2012. Biodegradation of endosulfan isomers in broth culture

and soil microcosm by Pseudomonas fluorescens isolated from soil. International

Journal of Environmental studies, 69 (5): 729-742.

Giri, K. and Rai, J.P.N. 2014. Bacterial Metabolism of Petroleum Hydrocarbons In

J.N., Govil, (ed.). Biodegradation and Bioremediation, Biotechnology 11: 73-93.

Studium Press LLC, New Delhi

Giri, K. Rawat, A.P., Rawat, M. and Rai, J.P.N. 2014. Biodegradation of

Hexachlorocyclohexane by Two Species of Bacillus Isolated from Contaminated Soil.

Chemistry and Ecology, 30 (2): 97-109.

INSA, (2011). Hazardous metals and minerals pollution in india. Indian National

Science Academy, Bahadurshah Zafar Marg, New Delhi, Angkor Publishers (P) Ltd.,

Noida. pp. 1- 24.

Marques, A. P. G. C., Rangel, A. O. S. S., & Castro, P. M. L. (2009). Remediation of

heavy metal contaminated soils: phytoremediation as a potentially promising clean-

up technology. Critical Reviews in Environmental Science and Technology, 39(8),

622–654.

Mohammed, A. S., Kapri, A., & Goel, R. (2011). Heavy metal pollution: source,

impact, and remedies. In M. S. Khan, A. Zaidi, R. Goel & J. Musarrat (Eds.),

Biomanagement of Metal-Contaminated Soils (pp 1-28). Springer Netherlands.

Prasad, M.N.V. 2011. A State-of-the-Art report on Bioremediation, its Applications to

Contaminated Sites in India, Ministry of Environment and Forests Government of

India, pp.90