However, dif- ferences were presented during previous days, where the amendment with straw decreased the formation of 14 CO2 compared to the unamended soil after 2 days of incubation, assuming that the strawly amendment had a last- ing effect on the amount of slowly mineralizable radiocarbon, maybe due to incorporation of radiocarbon into the biomass of the soil.
By contrast, straw addition had a major effect on biotic parameters such as the microbial activity measurement; after 64 days the straw amendment led to a threefold value of the microbial activity compared to the control samples, due to the stimulating effect of the readily bioavailable carbon source Wanner et al. The table 5 shows the biodegradation of pesticides in soil amendment and the amendment applied.
Both responses are associated with the decrease in extreme result in significant reduction in the biological efficacy of several soil-applied pesticides, resulting in emergence, for exam- ple, of weeds in crops Golovlena et al.
An accelerated degradation of pesticides can result from extensive and repeated use of pesticides, due to the pesticides be- ingdegraded so quickly that they are unable to carried out its control Singh et al.
This can be due to enrichment in the populations of microbial degraders in the soil, from increased enzymatic activity, or from combination of these factors Di Primo et al.
Nowadays, studies carried have been focused on the microorganisms able to degrade pesticudes quickly. Recently, Karpouzas et al. Dinitroaniline Trifluralin OC 2. Diazinon OC 0. Response associated to increase in microorganism number. Phosphoglysine, Glyphosate pH 6. Methyl isotiocianate OM 0. Contrary effects in weed and disease control have been reported for or- ganic amendments such as compost, organic fertilizer manures , and sludge Craft and Nelson, ; Boulter et al.
Compost is known to suppress plant disease through a com- bination of physiochemical and biological characteristics. Physicochemical characteristics include any physical or chemical aspects of compost that re- duce disease severity by directly or indirectly affecting the pathogens or host capacity for growth. Examples of these aspects include nutrient level, OM, moisture, pH, and other factors.
Biological characteristics include compost- inhabiting microbial populations in competition for nutrients with pathogens, antibiotic productions, lytic and other extracellular enzyme productions, par- asitism and predation, induction of host-mediated resistance in plants, and other interactions that increase microbial activity or decrease disease devel- opment.
However, high levels of microbial activity in compost have been postulated as the primary mechanism of disease control Miyasaka et al. Boulter et al. High compost application 9. Brown and Tworkoski assessed the effects of compost mulch on abundance of weeds, ap- ple scab, growth of the brown rot fungus, and two arthropod pests in apple orchards. With compost application no difference in apple scab was present, while a sig- nificant reduction in brown rot growth was found active compost compared with sterilized compost.
In insect control, the addition of compost presents a major abundance, with spiders predominant. In this study it was possi- ble to observe that compost was preferable to herbicide application, since beneficial predators were more abundant, herbivorous ones were reduced, and weed suppression was greater in mulched plots compared to herbicide treatment for at least the first year after compost application. However, in amended soil a weed and pest control has been reported such that the effects of competition by nutrients and the highest activity in soil amendment are the main factors for those controls.
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Khna, Z. Influence of soil components on adsorption-desorption of hazardous organic-development of low cost technology for reclamation of hazardous waste dumpisites, J. Laboratory investigation into the effects of the pesticides mancozeb, chlothalonil, and prosulfuron on nitrous oxide and nitric oxide production in fertilized soil, Soil Biol.
Kumar, S. Molecular aspects of pesticides degradation by microorganisms, Crit. Li, K. Elizabeth Howell. Related Audiobooks Free with a 30 day trial from Scribd. On Animals Susan Orlean. Pastoral Song James Rebanks. A Wild Idea Jonathan Franklin. Biodegradation of pesticides 1. Part II Roll No. Organisms must have necessary catabolic activity required for degradation of contaminant at fast rate to bring down the concentration of contaminant.
The target contaminant must be bioavailability. Cost of bioremediation must be less than other technologies of removal of contaminants. Biostimulation: Practice of addition of nitrogen and phosphorus to stimulate indigenous microorganisms in soil. Bioventing: Process of Biostimulation by which gases stimulants like oxygen and methane are added or forced into soil to stimulate microbial activity.
Composting: Piles of contaminated soils are constructed and treated with aerobic thermophilic microorganisms to degrade contaminants.
Periodic physical mixing and moistening of piles are done to promote microbial activity. Phytoremediation: Can be achieved directly by planting plants which hyperaccumulate heavy metals or indirectly by plants stimulating microorganisms in the rhizosphere. Mineralization: Complete conversion of an organic contaminant to its inorganic constituent by a species or group of microorganisms.
Bacterial degradation: Most bacterial species degrade pesticides. Fungal degradation:. Fungi degrade pesticides by introducing minor structural changes to the pesticides rendering it non toxic and are released to soil, where it is susceptible to further biodegradation by bacteria.
Enzymatic degradation: Enzymes have a great potentiality to effectively transform and detoxify polluting substances because they have been recognized to be able to transform pollutants at a detectable rate and are potentially suitable to restore polluted environments.
Detoxification: Conversion of the pesticide molecule to a non- toxic compound. A single chance in the side chain of a complex molecule may render the chemical non-toxic. Thirum fungicide is degraded by a strain of Pseudomonas and the degradation products are dimethlamine, proteins, sulpholipaids, etc.
Congugation: In which an organism make the substrate more complex or combines the pesticide with cell metabolites. Microorganisms have the ability to interact, both chemically and physically, with substances leading to structural changes or complete degradation of the target molecule. Among the microbial communities, bacteria, fungi, and actinomycetes are the main transformers and pesticide degraders [ 49 ].
Fungi generally biotransform pesticides and other xenobiotics by introducing minor structural changes to the molecule, rendering it nontoxic. The biotransformed pesticide is released into the environment, where it is susceptible to further degradation by bacteria [ 50 ].
Fungi and bacteria are considered as the extracellular enzyme-producing microorganisms for excellence. White rot fungi have been proposed as promising bioremediation agents, especially for compounds not readily degraded by bacteria. This ability arises from the production of extracellular enzymes that act on a broad array of organic compounds.
Some of these extracellular enzymes are involved in lignin degradation, such as lignin peroxidase, manganese peroxidase, laccase and oxidases. Several bacterial that degrade pesticide have been isolated and the list is expanding rapidly. The three main enzyme families implicated in degradation are esterases, glutathione S-transferases GSTs and cytochrome P [ 51 ]. Enzymes are central to the biology of many pesticides [ 52 ].
Applying enzymes to transform or degrade pesticides is an innovative treatment technique for removal of these chemicals from polluted environments. Enzyme-catalyzed degradation of a pesticide may be more effective than existing chemical methods. Enzymes are central to the mode of action of many pesticides: some pesticides are activated in situ by enzymatic action, and many pesticides function by targeting particular enzymes with essential physiological roles.
Enzymes are also involved in the degradation of pesticide compounds, both in the target organism, through intrinsic detoxification mechanisms and evolved metabolic resistance, and in the wider environment, via biodegradation by soil and water microorganisms [ 53 ].
For pesticides degradation, three are mainly enzyme systems involved: hydrolases, esterases also hydrolases , the mixed function oxidases MFO , these systems in the first metabolism stage, and the glutathione S-transferases GST system, in the second phase [ 55 ]. Several enzymes catalyze metabolic reactions including hydrolysis, oxidation, addition of an oxygen to a double bound, oxidation of an amino group NH 2 to a nitro group, addition of a hydroxyl group to a benzene ring, dehalogenation, reduction of a nitro group NO 2 to an amino group, replacement of a sulfur with an oxygen, metabolism of side chains, ring cleavage.
The process of biodegradation depends on the metabolic potential of microorganisms to detoxify or transform the pollutant molecule, which is dependent on both accessibility and bioavailability [ 47 ]. Metabolism of pesticides may involve a three-phase process. In Phase I metabolism, the initial properties of a parent compound are transformed through oxidation, reduction, or hydrolysis to generally produce a more water-soluble and usually a less toxic product than the parent.
The second phase involves conjugation of a pesticide or pesticide metabolite to a sugar or amino acid, which increases the water solubility and reduces toxicity compared with the parent pesticide.
The third phase involves conversion of Phase II metabolites into secondary conjugates, which are also non-toxic. In these processes fungi and bacteria are involved producing intracellular or extra cellular enzymes including hydrolytic enzymes, peroxidases, oxygenases, etc [ 16 , 56 ].
Due to the diversity of chemistries used in pesticides, the biochemistry of pesticide bioremediation requires a wide range of catalytic mechanisms, and therefore a wide range of enzyme classes. Information for some pesticide degrading enzymes could be founded in Table 3.
Relevant enzymes in the bioremediation of pesticides [ 52 - 53 ]. Hydrolases are a broad group of enzymes involved in pesticide biodegradation. Hydrolases catalyze the hydrolysis of several major biochemical classes of pesticide esters, peptide bonds, carbon-halide bonds, ureas, thioesters, etc.
As an example of the catalytic activity of enzymes hydrolases, the degradation pathway of carbofuran, a pesticide the group of carbamates is presented Figure 3. This pesticide can be transformed in the environment and different metabolites are generated and accumulated in potentially contaminated sites soil, water and sediments, mainly.
Different organisms isolated from contaminated sites that have been identified and characterized as transformers of carbofuran, resulting in different metabolites [ 57 ]. Among the hydrolases involved in the degradation of pesticides are including different types such as:. Among the most studied pesticide degrading enzymes, the PTEs are one of the most important groups [ 58 ]. These enzymes have been isolated from different microorganisms that hydrolyze and detoxify organophosphate pesticides OPs.
The first isolated phosphotriesterase belongs to the Pseudomonas diminuta MG species; this enzyme shows a highly catalytic activity towards organophosphate pesticides. The phosphotriesterases are encoded by a gene called opd organophosphate-degrading. The gene was cloned and sequenced by [ 64 ]. These enzymes specifically hydrolyze phosphoester bonds, such as P—O, P—F, P—NC, and P—S, and the hydrolysis mechanism involves a water molecule at the phosphorus center [ 65 ].
Different microbial enzymes with the capacity to hydrolyze MP have been identified, such as organophosphorus hydrolase OPH; encoded by the opd gene , methyl-parathion hydrolase MPH; encoded by the mpd gene , and hydrolysis of coroxon HOCA; encoded by the hocA gene , which were isolated from Flavobacterium sp.
Degradation pathway of carbofuran. In a several bacteria are involved in the hydrolysis of metabolites and b fungal degradation of carbofuran may occur via hydroxylation at the three position and oxidation to 3-ketocarbofuran University of Minnesota. The phosphotriesterase enzyme is a homo-dimeric protein with a monomeric molecular weight of 36 Kda. As a first step in the PTE organophosphorous pesticide hydrolysis mechanism, the enzymatic active site removes a proton from water, activating this molecule, them, the activated water directly attacks the central phosphorus of the pesticide molecule producing an inversion in its configuration The oxygen is polarized by the active site, with the participation of a zinc atom [ 6 , 69 ], Figure 4.
This enzyme has potential use for the cleaning of organophosphorus pesticides contaminated environments [ 65 ]. Proposed mechanism for PTE activity. Esterases are enzymes that catalyze hydrolysis reactions over carboxylic esters carboxiesterases , amides amidases , phosphate esters phosphatases , etc.
In the reaction catalyzed by esterases, hydrolysis of a wide range of ester substrates occurs in their alcohol and acid components as following:. Many insecticides organophosphates, carbamates and pyrethroids have associated a carboxylic ester, and the enzymes capable of hydrolyze such ester bond are known with the name of carboxylesterases. At present, multiple classification nomenclature systems are used for these enzymes.
Another common classification is the nomenclature divides the esterases into three groups according to the nature of their interactions with organophosphorus insecticides. Carboxylesterases belong, according to this classification, the group of ali-esterases and B-esterases. Esterases are a large family of enzymes in arthropods [ 71 ]. The esterases are a group of enzymes highly variable, which has been recognized as one of the most important in the metabolism of xenobiotics and its mechanism is associated with the mass production of multifunctional hydrolytic enzymes Organophosphate pesticides can be hydrolyzed by such enzymes [ 72 - 74 ].
There are different types of esterases and with very different distribution in tissues and organisms. Esterases A, contain a Cys residue in the active center and esterases B contain a Ser residue. Organophosphates that bind to the esterase B stoichiometrically inhibit its enzymatic activity.
Esterases are a diverse group that protects the target site acetylcholinesterase by catalyzing the hydrolysis of insecticides, or acting as an alternative blank [ 75 ]. Esterases in general have a wide range of substrate specificities; they are capable of binding to phosphate triesters, esters, thioesters, amides and peptides [ 76 ].
Oxidoreductases are a broad group of enzymes that catalyze the transfer of electrons from one molecule the reductant or electron donor to another the oxidant, or electron acceptor. Many of these enzymes require additional cofactors, to act as either electron donors, electron acceptors or both. Oxidoreductases have been further sub classified into 22 subclasses EC 1.
Several of these have applications in bioremediation, albeit their need for cofactors complicates their use in some applications. In these reactions, oxygen is reduced to water H 2 O or hydrogen peroxide H 2 O 2. The oxidases are a subclass of the oxidoreductases [ 53 ]. As an example of the many functions of these enzymes in the degradation of pesticides, as an example we present the ensodulfan degradation pathway. In this process not only oxidoreductase enzymes are involved, but different microorganisms and catalytic activities, in combination, can lead to complete mineralization of a pesticide Figure 5.
Endosulfan 1,2,5,6,7,7-hexachloronorbornene-2,3- dimethanolcyclic sulfite is an organochlorine insecticide of the cyclodiene family of pesticides. It is highly toxic and endocrine disruptor, and it is banned in European Union and several countries. Because it has been extensively applied directly to fields, it can be detected a considerable distance away from the original site of application.
Contamination of drinking water and food, as well as detrimental effects to wildlife are important concerns [ 77 ]. The end-use product of endosulfan is a mixture of two isomers, typically in a ratio. Microorganisms play a key role in removal of xenobiotics like endosulfan from the contaminated sites because of their dynamic, complex and complicated enzymatic systems which degrade these chemicals by eliminating their functional groups of the parent compound.
This pesticide can undergo either oxidation or hydrolysis reactions. Several intensive studies on the degradation of endosulfan have been conducted showing the primary metabolites to normally be endosulfan sulfate and endosulfan diol endodiol. Endosulfan sulphate will be present in the environment as a result of the use of endosulfan as an insecticide.
If endosulfan sulphate is released to water, it is expected to absorb to the sediment and may bioconcentrate in aquatic organism. This metabolite has a similar toxicity as endosulfan and has a much longer half-life in the soil compared to endosulfan. Therefore, production of endosulfan sulfate by biological systems possesses an ecological hazard in that it contributes to long persistence of endosulfan in soil. Endodiol is much less toxic to fish and other organisms than the parent compound.
Thus, it is important to note that some microbial enzymes are specific to one isomer, or catalyze at different rates for each isomer [ 78 ]. For example, a Mycobacterium tuberculosis ESD enzyme degrades beta-endosulfan to the monoaldehyde and hydroxyether depending on the reducing equivalent stoichiometry , but transforms alpha-endosulfan to the more toxic endosulfan sulfate.
However, oxidation of endosulfan or endosulfan sulfate by the monooxygenase encoded by ese in Arthrobacter sp. KW yields endosulfan monoalcohol [ 79 ]. Both ese and esd proteins are part of the unique Two Component Flavin Dependent Monooxygenase Family, which require reduced flavin. They are conditionally expressed when no or very little sulfate or sulfite is available, and endosulfan is available to provide sulfur in these starved conditions. Alternatively, hydrolysis of endosulfan in some bacteria Pseudomonas aeruginosa , Burkholderia cepaeia yields the less toxic metabolite endosulfan diol [ 80 ].
Endosulfan can spontanteously hydrolyze to the diol in alkaline conditions, so it is difficult to separate bacterial from abiotic hydrolysis. The diol can be converted to endosulfan ether or endosulfan hydroxyether and then endosulfan lactone [ 81 ].
Hydrolysis of endosulfan lactone yields endosulfan hydroxycarboxylate. These various branches of endosulfan degradation all result in desulfurization while leaving the chlorines intact, exhibiting the recalcitrance to bioremediation found in many organohalogen aromatics. They are also known as dependent cytochrome P monooxygenases or P system. The genes encoding the different isozymes comprise a superfamily of over genes grouped into 36 families based on their sequence similarity.
Cytochrome P enzymes are active in the metabolism of wide variety of xenobiotics [ 82 ]. The cytochrome P family is a large, well characterized group of monooxygenase enzymes that have long been recognized for their potential in many industrial processes, particularly due to their ability to oxidize or hydroxylate substrates in an enantiospecific manner using molecular oxygen [ 83 ].
Many cytochrome P enzymes have a broad substrate range and have been shown to catalyse biochemically recalcitrant reactions such as the oxidation or hydroxylation of non-activated carbon atoms. These properties are ideal for the remediation of environmentally persistent pesticide residues. Over subfamilies of P enzymes have been found across various prokaryotes and eukaryotes.
All contain a catalytic iron-containing porphyrin group that absorbs at nm upon binding of carbon monoxide. In common with many of the other oxidoreductases described before, P enzymes require a non-covalently bound cofactor to recycle their redox center most frequently NAD P H is used , which limits their potential for pesticide bioremediation to strategies that employ live organisms.
In insects, MFOs are found in the endoplasmic reticulum and mitochondria, are involved in a large number of processes such as growth, development, reproduction, detoxification, etc. MFO are involved in the metabolism of both endogenous and exogenous substances, for this reason these compounds promote their induction Due to its high inspecificity, the MFOs metabolize a wide range of compounds such as organophosphates, carbamates, pyrethroids, DDT, inhibitors of the chitin synthesis, juvenile hormone mimics, etc.
Degradation pathway of endosulfan University of Minnesota. In this reaction, the thiol group of glutathione reacts with an electrophilic place in the target compound to form a conjugate which can be metabolized or excreted, and they are involved in many cellular physiological activities, such as detoxification of endogenous and xenobiotic compounds, intracellular transport, biosynthesis of hormones and protection against oxidative stress [ 85 ].
Representation of the conjugation reaction catalyzed by glutathione S-transferase GST. In order to investigate genetic basis of pesticides biodegradation, several works with special emphasis on the role of catabolic genes and the application of recombinant DNA technology, had been reported. Pesticide-degrading genes of only a few microorganisms have been characterized. Most of genes responsible for catabolic degradation are located on the chromosomes, but in a few cases these genes are found in plasmids or transposons.
The recent advances in metagenomics and whole genome sequencing have opened up new avenues for searching the novel pollutant degradative genes and their regulatory elements from both culturable and nonculturable microorganisms from the environment. Mobile genetic elements such as plasmids and transposons have been shown to encode enzymes responsible for the degradation of several pesticides. The isolation of pesticide degrading microorganisms and the characterization of genes encoding pesticide degradation enzymes, combined with new techniques for isolating and examining nucleic acids from soil microorganisms, will yield unique insights into the molecular events that lead to the development of enhanced pesticide degradation phenomenon.
An understanding of the genetic basis of the mechanisms of how microorganisms biodegrade pollutants and how they interact with the environment is important for successful implementation of the technology for in situ remediation [ 86 ]. Different microbial enzymes with the capacity to hydrolyze pesticides have been identified [ 57 ], such as organophosphorus hydrolase OPH; encoded by the opd gene. This gene has been found in bacterial strains that can use organophosphate pesticides as carbon source; these have been isolated in different geographic regions.
These plasmids show considerable genetic diversity, but the region containing the opd gene is highly conserved. Methyl-parathion hydrolase MPH; encoded by the mpd gene , Are Pseudaminobacter , Achrobacter , Brucella and Ochrobactrum genes, they were identified by comparison with the gene mpd from Pleisomonas sp. In the various isolates of microorganisms capable of degrading pesticide, several genes have been described, in the table 4 shown the most studied. Microorganisms respond differently to various kinds of stresses and gain fitness in the polluted environment.
This process can be accelerated by applying genetic engineering techniques. Various genetic approaches have been developed and used to optimize the enzymes, metabolic pathways and organisms relevant for biodegradation [ 90 ]. New information on the metabolic routes and bottlenecks of degradation is still accumulating, requiring the need to reinforce the available molecular toolbox.
Nevertheless, the introduced genes or enzymes, even in a single modified organism, need to be integrated within the regulatory and metabolic network for proper expression [ 89 - 91 ]. Detoxification of organophosphate pesticides was the first demonstrated by genetically engineered microorganisms and the genes encoding these hydrolases have been cloned and expressed in P.
The potential of plants to degrade organic pollutants can be further enhanced by engineering plants by introduction of efficient heterologous genes that are involved in the degradation of organic pollutants [ 98 ]. Unfortunately, the rates of hydrolysis several enzymes differ dramatically for members of the family of OP compounds, ranging from hydrolysis at the diffusion-controlled limit for paraoxon to several orders of magnitude slower for malathion, chlorpyrifos, and others pesticides [ 99 ].
Although site-directed mutagenesis has been used to improve the substrate specificity and stereoselectivity of OPH [ 99 - ], the ability to deduce substitutions that are important for substrate specificity is still limited to the active-site residues. Two interesting papers have shown that an biological solution for efficient decontamination might be to direct evolution.
Directed evolution has recently been used to generate OPH variants with up to fold improvements in hydrolysis of methyl parathion [ ], a substrate that is hydrolyzed fold less efficiently than paraoxon, and other report the directed evolution of OPH to improve the hydrolysis of a poorly hydrolyzable substrate, chlorpyrifos 1,fold less efficient than paraoxon.
Up to fold improvement was obtained, and the best variant hydrolyzes chlorpyrifos at a rate similar to that of the hydrolysis of paraoxon by wild-type OPH [ ]. The complexity of microbial diversity results from multiple interacting parameters, which include pH, water content, soil structure, climatic variations and biotic activity.
During the last two decades, development of methods to isolate nucleic acids from environmental sources has opened a window to a previously unknown diversity of microorganisms. Analysis of nucleic acids directly extracted from environmental samples allows researchers to study natural microbial communities without the need for cultivation [ - ].
Metagenome technology metagenomics has led to the accumulation of DNA sequences and these sequences are exploited for novel biotechnological applications [ , ].
Due to the overwhelming majority of non-culturable microbes in any environment, metagenome searches will always result in identification of hitherto unknown genes and proteins [ - ]. In its broadest definition, functional genomics encompasses many traditional molecular genetics and biological approaches, such as the analysis of phenotypic changes resulting from mutagenesis and gene disruption [ ].
Functional genomics has emerged recently as a new discipline employing major innovative technologies for genome-wide analysis supported by bioinformatics. These new techniques include proteomics for protein identification, characterization, expression, interactions and transcriptomic profiling by microarrays and metabolic engineering [ ].
The application of proteomics in environmental bioremediation research provides a global view of the protein compositions of the microbial cells and offers a promising approach to address the molecular mechanisms of bioremediation.
With the combination of proteomics, functional genomics provide an insight into global metabolic and regulatory networks that can enhance the understanding of gene functions. The fundamental strategy in a functional genomics approach is to expand the scope of biological investigations from studying a single gene or protein to studying all the genes or proteins simultaneously in a systematic fashion.
The classic approach to assess gene function is to identify which gene is required for a certain biological function at a given condition through gene disruption or complementation. With the combination of technologies, such as transcriptomics and proteomics complementing traditional genetic approaches, the detailed understanding of gene functions becomes feasible [ - ].
Metabolic engineering combines systematic analysis of metabolic and other pathways with molecular biological techniques to improve cellular properties by designing and implementing rational genetic modifications [ ].
Understanding microbial physiology, will adapt to the host cells to support changes and become more efficient bioremediation processes, events that would be difficult to acquire during evolution [ ].
With these new genomics tools scientists are in a better position to answer questions such as how oxygen stress, nutrient availability, or high contaminant concentrations along differing geochemical gradients or at transitional interfaces impact the organohalide respiring community structure and function. Ultimately, by tracking the overall microbial community structure and function in addition to key functional players, informed decisions can then be made regarding how to best manipulate the field conditions to achieve effective bioremediation of, e.
Cell immobilization has been employed for biological removal of pesticides because it confers the possibility of maintaining catalytic activity over long periods of time [ - ]. Whole-cell immobilization has been shown to have remarkable advantages over conventional biological systems using free cells, such as the possibility of employing a high cell density, the avoidance of 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 harsh environments.
Previous reports have suggested that this higher productivity results from cellular or genetic modifications induced by immobilization. There is evidence indicating that immobilized cells are much more tolerant to perturbations in the reaction environment and less susceptible to toxic substances, which makes immobilized cell systems particularly attractive for the treatment of toxic substances like pesticides [ ].
In addition, the enhanced degradation capacity of immobilized cells is due primarily to the protection of the cells from inhibitory substances present in the environment.
The degradation rates for repeated operations were observed to increase for successive batches, indicating that cells became better adapted to the reaction conditions over time [ ]. There are two types of processes for cell immobilization: those based on physical retention entrapment and inclusion membrane and those based on chemical bonds, such as biofilm formation [ ].
In cell immobilization methods may be used various materials or substrates inorganic clays, silicates, glass and ceramics and organic cellulose, starch, dextran, agarose, alginate, chitin, collagen, keratin, etc. Entrapment in polymeric gels natural has become the preferred technique for the immobilization of cells, however, immobilized cell on supports have been used more frequently in xenobiotics biodegradation as pesticides [ ].
In order to degrade pesticides, is important to search for materials with favorable characteristics for the immobilization of cells, including aspects such physical structure, ease of sterilization, the possibility of using it repeatedly, but above all, the support must be cheap than allow in the future apply it for pesticide degradation.
Table 5 describes the main methods of immobilization [ , - ]. Thus, the methods can be grouped in two ways: the active that induce the capture of microorganisms in a matrix, and the passive that uses the tendency of microorganisms to attack surfaces either natural or synthetic, which form biofilms.
By the other hand, a biofilm can be defined as a coherent complex structure of microorganism organized in colony and cell products such as extracellular polymers exopolymer , which either spontaneously or in forming dense granules, grow attached to a solid surface static static biofilm or in a suspension bracket [ , ].
The biofilm formation process is performed in several steps starting with the attack or recognition to the surface, followed by growth and utilization of various carbon and nitrogen sources for the formation of products with adhesive properties. In parallel a stratified organization dependent on oxygen gradients and other abiotic conditions takes place.
This process is known as colonization. Then an intermediate period of maturation of the biofilm is carried out which varies depending on the presence of nutrients from the medium or friction with the surrounding water flow. Finally, a period of aging biofilm where a detachment of cells may occur and colonize other surfaces [ ]. The hydrodynamic plays an important role in the development of biofilm as these organizations develop in a solid-liquid interface, where the flow rate passing through it influences the physical detachment of microorganisms.
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