Keywords

4.1 Introduction

Synthetic pesticides are excessively applied in current agriculture practices to protect crops from various diseases and damages caused by fungi, insects, mites, and nematodes, to protect crops from abundant growth of weeds, and to control vectors responsible for certain diseases like malaria, dengue in human beings (Freedman 1995; Palanisami et al. 2009). These pesticides are known to be persistent in nature, causing toxicity and teratogenicity. They also cause deleterious side effects, not only in the cultivated soils where they are applied, but also can be accumulated into food crops; but also can be accumulated into food crops; and finally enter in food chain (El-Bestawy et al. 2007). In agro-ecosystems, they affect the growth of nontarget organisms such as beneficial microorganisms which play very crucial role in soil fertility and enhance plant growth (Araujo et al. 2003). Apart from this, they can enter into aquatic ecosystems by spraying, drifting, leaching, surface runoff, discharges from the pesticide manufacturing plants, and by accidental spills; this leads to the killings of fishes and aquatic invertebrates (Akhtar et al. 2009).

Bioremediation is an effective and eco-friendly approach for the decontamination of synthetic pesticides from agro- and aquatic ecosystems; it is a microorganism-mediated transformation or degradation of pollutants into nontoxic or less-toxic substances (Singh et al. 2011a, b, c). The application of various organisms like bacteria, actinomycetes, algae, methanotrophs (Singh and Gupta 2016), and cyanobacteria for efficient bioremediation of pesticide has been reported. Cyanobacteria are successively applied in wastewater treatment to remove nitrogen and phosphorus, textile dyes, and heavy metals (Palanisami et al. 2009; Singh et al. 2016). Cyanobacteria have been shown to be highly effective degraders of pesticides (Megharaj et al. 1994; Singh et al. 2011a, b, c).

Cyanobacteria , generally known as blue–green algae , are considered among the oldest photosynthetic organisms on planet Earth that existed since about 2.6–3.5 billion years ago (Hedges et al. 2001). They show diverse morphology including unicellular, filamentous, and colonial forms; benthic as well as planktonic (Whitton and Potts 2000; Burja et al. 2001). Cyanobacteria can flourish in a variety of habitats: from marine to freshwater and to terrestrial ecosystems; from arctic to Antarctica and to tropical deserts (Kulasooriya 2011). Some filamentous cyanobacteria have endowed with specialized cells known as heterocysts , known for the sites of nitrogen fixation (Capone et al. 2005).

This chapter gives us little information on synthetic pesticides and their fate and impact on agro- and aquatic ecosystems, but prime focus is on cyanobacteria-mediated bioremediation or cyanoremediation of synthetic pesticides and also focuses; how immobilization and genetically engineering enhance the capability to tolerate and degrade the synthetic pesticides.

4.2 Synthetic Pesticides

According to FAO (1989), “Pesticides are natural or synthetic substances or mixture of substances intended for preventing, destroying, or controlling any pest including vectors of human or animal diseases, unwanted species of plants or animals causing harm during, or otherwise interfering with, the production, processing, storage, or marketing of food, agricultural commodities, wood and wood products, or animal feedstuffs, or which may be administered to animals for the control of insects, arachnids or other pests in or on their bodies.” Nowadays, the term “pesticide” is generally applied for synthetic chemicals used to prevent crop loss from various insects, fungi, bacteria, and nematodes; to suppress excess growth of weeds and other substances used for storage and transportation of agricultural commodities.

4.2.1 Classification of Synthetic Pesticides (Based on Zacharia 2011; EPA 2012; Ortiz-Hernández et al. 2013)

Synthetic pesticides could be classified according to their toxicity, chemical group, environmental persistence, target organism, or other features (Tables 4.1 and 4.2). According to their chemical nature, pesticides are divided into following groups:

Table 4.1 Synthetic pesticides: types, mode of action and their examples
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Table 4.2 Synthetic pesticides: classified according to their toxicity
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4.2.1.1 Organochlorines

Organochlorine pesticides are organic compounds with five or more chlorine atoms, and they are widely used as insecticides for the control of a wide range of insects. Organochlorine pesticides also show long persistence in the environment. These pesticides (mostly insecticides) disrupt nervous system, leading to convulsions and paralysis of the insect and its eventual death. DDT, lindane, endosulfan, aldrin, dieldrin, and chlordane are the commonly used organochlorine pesticides.

4.2.1.2 Organophosphorous

Organophosphorous pesticides possess a phosphate group as their basic structure; this is defined by Schrader’s formula:

figure a

In this formula, R1 and R2 are usually methyl or ethyl groups; the O in the OX group can be replaced with S in some compounds, whereas the X group can take a wide diversity of forms. Organophosphorous insecticides are not persistent in the environment (Martin 1968) like organochlorine pesticides, but it is observed that they are more harmful for vertebrates and invertebrates due to cholinesterase inhibitors, leading to paralysis and death. Some of the widely used organophosphorous insecticides include parathion, malathion, diaznon, and glyphosate.

4.2.1.3 Carbamates

Carbamates are organic compounds which are derivatives of carbamic acid and defined through this formula:

figure b

Where R1 is an alcohol group, R2 is a methyl group, and R3 is usually hydrogen. Carbamates (both aryl and oxime) are heavily toxic to insects and mammalians due to cholinesterase inhibitors. Although both carbamates and organophosphorous are cholinesterase inhibitors, the difference is in species specificity and reversibility (Drum 1980). Carbaryl, carbofuran, and aminocarb are the common example of carbamate pesticides.

4.2.1.4 Pyrethoids

Pyrethroids are synthetic equivalents of the naturally occurring pyrethrins extracted from flowers of Chrysanthemum cinerariaefolium . Pyrethroids are known to be very effective against insect pests, with minimal toxicity to mammals and easily biodegradable. The most widely used synthetic pyrethroids include permethrin, cypermethrin, and deltamethrin. Although less toxic and persistent than other groups of insecticides, they can still represent a problem. Pyrethroids display high affinity to Na+-channels and its binding to these channels causes a prolonged channel opening that may result in a complete depolarization of the cell membrane thus blocking neuronal activity.

Other groups of synthetic pesticides that are widely used in control of weeds include among others phenoxyacetic acid under which the herbicide 2,4-D belongs and bipyridyls under which the herbicides paraquat and diquat belong.

There is another group that includes the pesticides which can be applied for the control of fungal infections in crops. There are inorganic and organic fungicides. Inorganic fungicides include Bordeaux mixture, Cu(OH)2.CaSO4 and malachite, Cu(HO)2·CuCO3. Organic fungicides, on the other hand, include among others, benomyl and xine copper (Manahan 2001).

4.3 Fate of Synthetic Pesticides in Agro- and Aquatic Ecosystems

Synthetic pesticides are applied in agriculture through various ways like spraying, dusting or spreading. These pesticides are taken up by pests or crop plants that are converted into degradable products and bio accumulated into plant parts or animal tissues (Babu et al. 2003; Waliszewski et al. 2008). Some parts of the pesticides applied in agricultural fields are also removed upon crop harvesting. The remaining parts of the pesticides can be degraded through chemical reactions and microbial actions in the soil, can be mineralized through sorption onto soil organic matter and clay minerals, and can also be lost to atmosphere through volatilization. Some synthetic pesticides that are not degraded, immobilized, detoxified, or removed with the harvested crop are escaped from the applied sites. The major loss pathways of pesticides to the environment are volatilization into the atmosphere and aerial drift, runoff to surface water bodies in dissolved and particulate forms, and leaching into groundwater basins (Fig. 4.1). The fate and transfer pathways of pesticide applied to crop plants are complex, requiring some knowledge of their chemical properties, their transformations (breakdown), and the physical transport process. Transforms and transport are strongly influenced by site-specific conditions and management practices.

Fig. 4.1
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Fate of synthetic pesticides in agro- and aquatic ecosystems

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4.4 Impact of Pesticides

Synthetic pesticides help enhancing economic potential through increased production of agricultural commodities and prevention of vector-borne diseases (Igbedioh 1991; Forget 1993). This negative impact of pesticides is mainly due to the high toxicity, stable nature, less soluble active ingredients of pesticide .

4.4.1 Soil Contamination

The major portion of the synthetic pesticides remains unused after application and is responsible for the contamination of the soil. In the soil, it can remain persistent, degraded, or transformed. Several researchers reported a variety of transformation products (TPs) from a wide range of pesticides (Barcelo and Hennion 1997; Roberts 1998; Roberts and Hutson 1999). The pesticides and their TPs are sorbed by soils to different degrees, depending on the interactions between soil and pesticide properties. The most influential soil characteristic is the organic matter content; larger the organic matter content, the greater the adsorption of pesticides and TPs (Akhtar et al. 2009).

4.4.2 Surface and Groundwater Contamination

From applied sites, pesticides can escape to surface water through runoff from treated plants and soil. Contamination of surface water by pesticides is a widespread problem. During a survey in India, 58 % of drinking water samples drawn from various hand pumps and wells around Bhopal were contaminated with organochlorine pesticides above the EPA standards (Kole and Bagchi 1995). Once ground water is polluted with toxic chemicals, it may take many years for the contamination to dissipate or be cleaned up. Cleanup may also be very costly and complex, if not impossible (Waskom 1994; O’Neil et al. 1998; US EPA 2001).

4.4.3 Effect on Soil Fertility

Due to indiscriminate use, pesticides have a negative impact on beneficial soil microorganisms. Elaine Ingham stated that if both bacteria and fungi populations are affected, then the soil starts to degrade. Overuse of chemical fertilizers and pesticides have the same side effects on the soil organisms that are similar to human overuse of antibiotics. Although after application of chemicals, it takes days, months, or years to be sort out or escape, but after a while, there aren’t enough "beneficial soil organisms to hold onto the nutrients” (Savonen 1997). Soil microorganisms play a vital role in plants in terms of transformation of atmospheric nitrogen into nitrates, which plants can use, enhancing bioavailability on nutrients .

4.4.4 Nontarget Organisms

Nowadays, synthetic pesticides are found as common contaminants in soil, air, water, and our urban landscapes. They can also harm plants and animals ranging from nontarget insects, plants, fish, birds, and other wildlife. Synthetic pesticides are continuously applied and can be responsible for the extinction of useful organisms present in the agro- and aquatic ecosystems. Pesticide residues not only affect the soil features but also affect useful organisms like earthworms, bees, spiders, and plants (Singh et al. 2014).

4.4.5 Contamination of Vegetation

Pesticide application can directly affect nontarget vegetation or can drift or volatilize from the applied area and contaminate air, soil, and nontarget plants. Some pesticide drift occurs during every application, even from ground equipment (Glotfelty and Schomburg 1989). Pesticide drift can be responsible for a loss of 2–25 % of the pesticide being applied, which can spread over a distance of a few yards to several hundred miles and after few days of application, up to 80–90 % of an applied pesticide can be volatilized (Majewski and Capel 1995).

4.4.6 Human Health

Increase in the use of pesticides can result in various health and environmental problems like poisoning of farmers and farm workers, leading to cardiopulmonary, neurological, and skin disorders, fetal irregularities, miscarriages, lowering the sperm count of applicators, etc. These are categorized into acute and chronic poisoning: (a) Acute pesticide poisoning causes fatigue, headaches and body aches, skin discomfort, skin rashes, poor concentration, feelings of weakness, circulatory problems, dizziness, nausea, vomiting, excessive sweating, impaired vision, tremors, panic attacks, cramps, etc., and in severe cases, coma and death (Bödeker and Dümmler 1993; Alavanja et al. 2004); (b) Chronic poisoning due to pesticide use or due to long-term ingestion of small amounts of these substances include weakening of the immune system and effects on the reproductive system, which can lead to miscarriage, still birth , and premature birth or to low birth weight(WWF 2002; UNEP 2004; Terre Des Hommes 2011).

4.5 Bioremediation of Synthetic Pesticides

Conventionally, bioremediation of synthetic pesticides is attained through the use of microorganisms; but nowadays, several other gents such as plants, fungi, algae, or enzymes (obtained from organisms) are also used in bioremediation which extends the application of bioremediation in various aspects. Bioremediation of synthetic pesticides includes two terms, biodegradation and biotransformation, recognized similar to each other but they are quite different.

Biodegradation involves the biological reactions that modify the chemical structure of the compound so this implies a decrease in toxicity, while biotransformation reduces the pollutant concentration by either modification or translocation. Thus, biotransformation could end decreasing or increasing the undesirable effects. Their difference is clear in the case of pollutants translocation when biodegradation does not occur but biotransformation does. Biotransformation concept has been developed for biological detoxification systems (Alexander 1999; Parkinson 2001). When microorganisms are imported to a contaminated site to enhance degradation, the process is known as bioaugmentation (Murali et al. 2014).

4.6 Factors Affecting the Bioremediation of Synthetic Pesticides

The biodegradation or biotransformation of synthetic pesticides is a complex process, and it is influenced by several physical and chemical attributes such as structure and concentration of pesticide, environmental conditions (temperature, pH, moisture), salinity, and sustainable population of microorganisms.

4.6.1 Structure and Concentration of Pesticide

The structure of synthetic pesticides is an important attribute; pesticides have some of their own physical and chemical properties which varyfrom pesticide to pesticide. Cork and Krueger 1991 stated that in a pesticide polar group such as OH, COOH and NH2 +3 are available to the microbial system; it could be an easier site for attack but if the pesticide molecule is available as a substituent of halogen or alkyl, it makes it more resistant to biodegradation. The rate of degradation of pesticides can be influenced by minor difference in the arrangement or nature of substituent in pesticides of the same class (Topp et al. 1997). Beside the structure, the concentration of pesticide considerably affects the bioremediation of pesticides. The rate of degradation decreases generally quantitatively with the residual pesticide concentration (Topp et al. 1997).

4.6.2 Effect of Temperature, pH, and Moisture

Various environmental factors such as temperature, pH, and moisture also affect the process of biodegradation of synthetic pesticides. According to Alexander 1977, the entire process of biodegradation is carried out at mesophillic (30–37 °C with optimum temperature 35 °C) and thermophillic (50–60 °C with optimum temperature 55 °C) temperature ranges. The optimal temperature required for both the ranges is not invariably critical for the biodegradation.

Soil pH is a crucial factor for adsorption of pesticides for the abiotic and biotic degradation processes, and it also effects the adsorption behavior of pesticide molecules on clay and organic surfaces. This also affects the chemical speciation, mobility, and bioavailability (Burns 1975; Hicks et al. 1990). Racke et al. (1997) reported that degradation of a given pesticide depends mostly on the soil alkaline or acidic pH. In fact, the biodegradation of pesticides depends upon the susceptibility of the microorganism in the optimum pH of the medium. Moisture is another environmental factor which affects the rate of biodegradation; water facilitates as medium for the movement and diffusion of pesticides; it is necessary for microbial availability of pesticides.

Moisture maintains the osmotic pressure and pH of agro- and aquatic ecosystems; it also affects the exchange of respiratory gases in pore spaces of soil. Under saturated conditions, oxygen can be consumed faster than it is replenished in the soil space and the soil becomes anaerobic; this leads to slowing the rate of biodegradation and also changes metabolic activity of microorganisms to occur. Soil moisture content should be between 25 and 85 % of the water holding capacity (with optimum range of 50–80 %) for effective biodegradation of synthetic pesticides.

4.6.3 Effect of Salinity

There is not much information about the effects of salinity on the degradation of synthetic pesticides . Salinity is a big problem in many arid, semiarid, and coastal regions; it could affect the biodegradation of synthetic pesticides. Reddy and Sethunathan (1985) reported that parathion degradation is faster in nonsaline soils. It is also reported that the stability of pesticides in estuarine and sea water, varying degrees of salinity; high salt content in seawater may be barrier for biodegradation (Walker 1976) or inhibit biodegradation of pesticides (Weber 1976; Kodama and Kuwatsuka 1980).

4.6.4 Sustainable Population of Microorganisms

Although microorganisms are able to survive in subzero temperatures, extreme heat, desert conditions, in aerobic or anaerobic conditions, with the presence of hazardous compounds but for the effective biodegradation of synthetic pesticides, it is necessary to meet these variables such as availability of pesticide or metabolite to the microorganisms, physiological status of the microorganisms, survival and/or proliferation of pesticide degrading microorganisms at contaminated site and most important is sustainable population of these microorganisms (Singh 2008).

4.7 Cyanoremediation

Cyanoremediation is the use of cyanobacteria for the removal or degradation or transformation of pollutants including heavy metal, dyes, or pesticides from wastewater or contaminated soil. Figure 4.2 illustrates the advantages using cyanobacteria over other microbes for bioremediation of pesticide contamination. There are numerous examples of cyanobacterial genera which are successfully implemented for the bioremediation of synthetic pesticides (Table 4.3).

Fig. 4.2
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Advantages of using Cyanobacteria over other microbes for bioremediation

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Table 4.3 Some synthetic pesticides and cyanobacteria species responsible for their degradation
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According to Hatzios (1991), pesticide degradation is a process involving three phases: (a) Phase I involves oxidation, reduction, or hydrolysis, which makes the pesticide more water soluble and less toxic pesticide metabolites. In this phase, oxygenation is the crucial step in the degradation of pesticides and many of oxygenation reactions are carried out by oxidative enzymes, e.g., cytochrome P450s, peroxidases, and polyphenol oxidases. (b) Phase II involves conjugation of a pesticide or pesticide metabolites to a sugar, amino acid or glutathione, which enhances the water solubility and reduces the toxicity compared to parent pesticide compound. Generally, metabolites obtained from Phase II have little or no toxicity and may be stored in cellular organelles. In this phase, enzyme Glutathione S-transferase plays a great role which catalyzes the nucleophilic attack of the sulfur atom of GSH by the electrophilic center of the substrate (Armstrong 1994; Marrs 1996); (c) Phase III involves conversion of Phase II metabolites into secondary conjugates , which are also nontoxic.

In the degradation process, pesticides produce singlet oxygen and other active oxygen species at various sites of photosynthetic electron transport chain. These active oxygen species are scavenged by cellular systems through raising antioxidative machinery such as superoxide dismutase, catalase, and peroxidase (Palanisami et al. 2009).

4.7.1 Organochlorine Insecticides

Organochlorines are chlorinated hydrocarbon chemicals used to control various agricultural, horticultural, and public health pests (Lal et al. 2010). Their residues cause serious problems, not only in the cultivated soils where they are applied, but also in the crops that systematically retain part of these residues in nontarget organisms (El-Bestawy et al. 2007; González et al. 2012).

Among organochlorines, lindane (a common A-hexachlorocyclohexane (HCH) formulation) is a wisely applicable pesticide, mainly used for rice crop protection in rice-producing countries (Abdullah et al. 1997). Lindane persists in the environment (Alexander 1994) and can be noticed in the air, rain, and surface water at 90 % of sites long after its application (Majewski and Capel 1995). Singh (1973) reported that some cyanobacterial strains isolated from paddy fields, i.e., Cylindrospermurn sp ., Aulosira fertilissirna , and Plectonema boyanurn , are able to tolerate commercial preparations of lindane in concentrations up to 80 pg/mL. Kuritz and Wolk (1995) also showed that two laboratory strains, Anabaena sp. PCC7120 and Nostoc ellipsosporurn , degraded A-HCH to a mixture of 1,2,3-and 1,2,4-trichlorobenzenes (and, possibly, beyond) via pentachlorocyclohexene as an intermediate. It is also observed that lindane did not affect the growth rates of these cyanobacteria at concentrations up to 20 pg/mL (Singh 1973).

It is reported that Anabaena sp. Strain PCC 7120 and Anabaena flos-aquae biotransformed endosulfan into endodiol, primary product and trace the amount of endosulfan sulfate (Lee et al. 2003). Endodiol is a nontoxic metabolite to fish and other organisms. But endosulfan sulfate has a similar toxicity compared to parent compound endosulfan, and it has a much longer tolerance into soil environment in comparison to endosulfan (Kennedy et al. 2001).

4.7.2 Organophosphorous Insecticides

Organophosphorous insecticides are esters of phosphoric acids and commonly known as organophosphates, which include aliphatic, phenyl, and heterocyclic derivatives and have one of the basic building blocks as a part of their much more complex chemical structure. They are applied for a variety of sucking, chewing, and boring insects, spiders and mites, aphids and pests attacking crops like cotton, sugarcane, peanuts, tobacco, vegetables, fruits, and ornamentals. Some of the main agricultural products are parathion, methyl parathion, chloropyrifos, malathion, monocrotofos, and dimethoate (Kanekar et al. 2004).

Organophosphorus pesticides are less environmentally persistent than organochlorine compounds; however, they still can be detected in air and water due to heavy use (Majewski and Capel 1995). In aquatic environments, nonenzymatic hydrolysis of organophosphates is responsible for their slow decomposition to more toxic and persistent para-nitrophenol (Megharaj et al. 1994). To overcome this problem, microalgae (including cyanobacteria)-mediated degradation could be an effective approach for their cleanup in the environment (Megharaj et al. 1994). Cyanobacteria are not so much affected by organophosphorus pesticides at working concentrations and concentrations present in wastewaters (Singh 1973; Doggett and Rhodes 1991; Megharaj et al. 1994; Subramanian et al. 1994). Pure cultures of Nostoc, Oscillatoria, and Phomidium isolated from methyl parathion-enriched soil, grew in media supplemented with methyl parathion or other organophosphorus pesticides as a sole source of organic phosphorus and nitrate (Megharaj et al. 1987; Orus and Marco 1991; Megharaj et al. 1994; Subramanian et al. 1994) and utilized phosphorus from the pesticide for growth and development (Megharaj et al. 1994; Subramanian et al. 1994). Megharaj et al. (1994) stated that cyanobacteria are also able to oxidize the nitro group of para-nitrophenol accompanied by the release of nitrite into growth media, but enzymatic system which is involved in this process is not known. The metabolism/assimilation of the released nitrite is likely to depend on the activity of nitrite reductase encoded by the nir operon. Subramanian et al. (1994) also noted that the link between nitrogen metabolism and the effectiveness of phosphorus utilization from organophosphorus pesticides; however, the authors did not analyze possible effects of various sources of fixed nitrogen on biodegradation (Kuritz 1999).

Palanisami et al. (2009) reported that cyanobacterium Phormidium valderianum BDU 20041 tolerant to chloropyrifos exposure showed increased activity of oxido-reductase enzymes to degradation of chloropyrifos. Sprirulina platensis are able to grow in media containing up to 80 ppm choloropyrifos and converted to its primary metabolite TCP(3, 5, 6-trichloro-2-pyridinol) through the enzyme alkaline phosphatase (ALP) (Thengodkar and Sivakami 2010). Singh et al. (2011) concluded that cyanobacterium Synechocystis sp. Strain PUPCCC 64 is able to degrade the pesticide chloropyrifos. Three strains of cyanobacteria Anabaena oryzae, Nostoc muscorum, and Spirulina platensis are able to degrade and utilize malathion as a source of phosphorous. These strains grow under high concentration of malathion with enhancement of biomass carbohydrate and protein content (Ibrahim et al. 2014). It is also reported that cyanobacterium Anabaena sp. Strain PCC 7120 reduced the nitro group of methyl parathion to an amino group via a nitroso group intermediate under aerobic conditions (Barton et al. 2004).

4.7.3 Herbicides

Gimsing et al. (2004) reported that the degradation rate of pesticide is strongly correlated with the population size of soil microbes in case of Pseudomonas spp. Lipok et al. (2007) concluded that mixed culture of Spirulina sp. exhibited a remarkable ability to degrade glyphosate and the rate of glyphosate disappearance from the aqueous medium was independent of its initial concentration. They also suggested that the degradative pathway for glyphosate in Spirulina sp. might differ from those exhibited in other bacteria. In fact, Lipok et al. (2009) reconfirmed the ability of the cyanobacterium S. platensis and bacterium Streptomyces lusitanus to catalyze glyphosate metabolism. Four cyanobacterial strains (Anabaena sp., L. boryana, M. aeruginosa, and N. punctiforme) are able to use the glyphosate as the only source of phosphorus (Forlani et al. 2008). Dyhrman et al. (2006) also stated that marine cyanobacteria Trichodesmium erythraeum showed existence of phosphorous-dependent glyphosate transformation. However, reports on the utilization of glyphosate as a source of nitrogen by cyanobacteria are not yet available in the literature. Ravi and Balakumar (1998) reported that extracellular phosphatases are able to hydrolyze the C-P bond of glyphosate with working on cyanobacterium A. variabilis; however, this claim has not been reiterated so far by the other authors. Forlani et al. (2008) stated that extracellular phosphatases seem unlikely to contribute any substantial scale to glyphosate degradation. Cyanobacterial strains which possess the ability to use this phosphonate as a source of phosphorus is of practically significance because such strains could effectively be employed for the cleanup of pesticides (Arunakumara et al. 2013).

4.8 Cyanobacterial Immobilization

The concept of immobilization of microorganisms in matrix or material may enhance the current benefits from the mass culture of the microorganism by degrading a specific metabolite or removing pollutants (De-Bashan and Yoav Bashan 2010; Ortiz-Hernández et al. 2011, 2013). And it can be employed for the bioremediation of synthetic pesticides because it confers the possibility of maintaining catalytic activity over long periods of time (Martín et al. 2000; Richins et al. 2000; Chen and Georgiou 2002). There are many advantages of immobilization of microorganisms over free-living microorganisms, such as the maintaining high cell density, the minimum cell washout, even at high dilution rates, easy separation of cells from the reaction system, repeated use of cells, and better protection of cells from the toxic effects of hazardous compounds and harsh environments. Immobilization can increase the cells’ survival and metabolic activity in bioremediation systems (Moslemy et al. 2002; Tao et al. 2009; Ha et al. 2008, 2009; Sun et al. 2010). Two types of immobilization are as follows:

4.8.1 Passive Immobilization

Some microorganisms (including some groups of microalgae/cyanobacteria) have a natural tendency to attach to surfaces and grow on them (Robinson et al. 1986). This characteristic can be exploited in order to immobilize cells on carriers of different types (Codd 1987). In passive immobilization, carriers (adsorbent materials) can be natural or synthetic, and this process is reversible (Cohen 2001; Moreno-Garrido 2008). The natural carrier loofa biomass is widely used and accepted for passive immobilization while synthetic materials, polyvinyl and polyurethane, are widely used in experiments involving passive immobilization (Urrutia et al. 1995).

4.8.2 Active Immobilization

For active immobilization, a variety of carriers such as flocculant agents, chemical attachment, and gel entrapment are currently in use. Among flocculants, chitosan has been the most widely employed. Chemical attachment is carried out through the chemical interaction (mainly due to covalent bonding, cross-linking) by common carriers such as glutaraldehyde, or cells. Apart from flocculant and chemical attachment, gel entrapment can be performed by the use of synthetic polymers (acrylamide, photocrosslinkable resins, polyurethanes), proteins (gelatine, collagen, or egg white), or natural polysaccharides (Taha et al. 2013).

Entrapment in natural polymeric gels has become the best suitable technique for the immobilization of cells (Mallick 2002); however, immobilized cells on supports have been used more frequently in xenobiotics biodegradation than for pesticides (Lusta et al. 1990). For cyanoremediation of synthetic pesticides, it is important to search for materials with favorable characteristics for the immobilization of cells, including aspects such as physical structure, ease of sterilization, and the possibility of using it repeatedly. Above all, the support must be affordable enough to allow its future use for pesticide degradation .

4.9 GE Cyanobacteria as Biopesticides

Gene manipulation offers a way of engineering microorganisms to deal with a pollutant, including pesticides that may be present in the contaminated sites. The simplest approach is to extend the degradative capabilities of existing metabolic pathways within an organism either by introducing additional enzymes from other organisms or by modifying the specificity of the catabolic genes already present. Cyanobacteria have long been studied as model organisms for photosynthesis (Vermaas 2001; Dong and Golden 2008); the engineering of cyanobacteria for applied purposes remains an underdeveloped field of interest. The potential of genetically modified cyanobacteria is still in the initial stages of exploration. Only a handful of cyanobacterial species have been investigated as host organisms for industrial and bioremediation purposes (Table 4.4). As new species are discovered and sequenced and new tools become available for genetic manipulation, the rich diversity of cyanobacterial phenotypes and genotypes can be exploited for new applications. Increased knowledge of native cyanobacterial genetics, metabolism, and regulatory systems will provide targets for increased production, enabling the synthesis of new products and improving the ability to predict the effects of targeted genetic manipulation.

Table 4.4 Some GE cyanobacterial strains and their transformation methods
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Genetic engineering in filamentous N2-fixing cyanobacteria usually involves Anabaena sp. PCC 7120 and several other non-aggregating species. Mass culture and harvest of such species are more energy consuming relative to aggregating species. To establish a gene transfer system for aggregating species, Qiong et al. (2010) tested many species of Anabaena and Nostoc and identified Nostoc muscorum FACHB244 as a species that can be genetically manipulated using conjugative gene transfer system. To promote biodegradation of organophosphorous pollutants in environments, they introduced a plasmid containing the organophosphorous degradation gene (opd) into Anabaena sp. PCC 7120 and N. muscorum FACHB244 by conjugation. The opd gene was driven by a story promoter, PpsbA. From both species, they obtained transgenic strains having organophosphorous degradation activities. The genetic manipulation of cyanobacteria could be utilized in the elimination of pollutants and large-scale production of valuable proteins or metabolites.

4.10 Conclusions

Although the use of chemical pesticides in agriculture is helpful in the increment of crop production, soil productivity, and products quality, it is also reflected in economic benefits, vector disease control, and in general, in public health. But approximately only 10 % of applied pesticides reach the target organism and rest of the applied pesticides is deposited into soil, water, and sediments which affects the nontarget organism in agro- and aquatic ecosystems besides affecting public health. For this reason, it is necessary to generate strategies for the removal of pesticide contamination from polluted sites, and the biological treatment is an important technology from an economical and environmental point of view for the cleanup of pesticide contamination.

The choice of the bioremediation strategy should be made on the basis of the type of pesticide, environment, and the target organisms present in the ecosystem. Since, the target organism is the only major concern and the information about features, advantages or disadvantages of target organisms can be helpful for better and successive bioremediation. Some parameters like pH, temperature, cell count, biomass growth rate, substrate bioavailability, and moisture, which are crucial for microbial population, can be addressed for bioremediation (Velázquez-Fernández et al. 2012). Moreover, it is important to understand the molecular mechanisms involved in enzymatic catalysis, which will be possible to design new alternatives and/or efficient tools for the treatment of pesticide residues or for the bioremediation of contaminated sites.

Use of cyanobacteria and microlage for the degradation of synthetic pesticides either by increasing the degradation capability of the cyanobacterial community to remove the pollutant is cost-effective and safe technology (Kumar and Singh 2016). Among the cyanobacterial genera, the high tolerance of some cyanobacterial genera toward synthetic pesticides resulted in colonized contaminated environments. It should also be kept in mind that cyanobacteria provide high product selectivity, simple catalyst preparation, and a recycling system.

Moreover, in implementing strategies to increase the efficiency of degradation, such as immobilization of cyanobacterial cells, we may have tools to decline the existence of obsolete pesticides and waste generated; it will reduce the danger of pesticides on the environment and health (Ortiz-Hernández et al. 2013). However, there is a suggestion that immobilization affects the cell’s behavior, but many of the observations, particularly with respect to productivity are contradictory. It is therefore, there is a need to increase understanding on the effects of immobilization on cyanobacterial cell physiology and biochemistry. The leakage problem is one of the key concerns in cell immobilization since it obviates the primary purpose of delimiting viable cells in a confined matrix.

Despite the uncertainty regarding the development of GE algae as production strains, development of genetic tools is still imperative from a research standpoint. Understanding the basic biology that will inform such aspects as lateral gene transfer, potential for toxin production, potential for large-scale blooms and subsequent anoxic zone formation, and choice of cultivation methods in terms of organism containment, are very important.