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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
Rev. Int. Contam. Ambie. 29 (Número especial sobre plaguicidas) 85-104
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION:
OPPORTUNITY FOR WASTE, SOILS AND WATER CLEANING
Ma. Laura ORTIZ-HERNÁNDEZ
1*
, Enrique SÁNCHEZ-SALINAS
1
,
María Luisa CASTREJÓN GODÍNEZ
2
, Edgar DANTAN GONZÁLEZ
3
y Elida Carolina POPOCA URSINO
3
1
Centro de Investigación en Biotecnología, Universidad Autónoma del Estado de Morelos
2
Programa de Gestión Ambiental Universitario, Universidad Autónoma del Estado de Morelos
3
Centro de Investigación en Biotecnología, Universidad Autónoma del Estado de Morelos
* Autora responsable: ortizhl@uaem.mx
(Recibido julio 2013, aceptado agosto 2013)
Key words: pesticide, biodegradation, biobeds, cells immobilization
ABSTRACT
Pesticides are substances or mixtures of substances intended to prevent, destroy or con-
trol any pest, and they are widely used mainly in agriculture, industry and the domestic
sector. These compounds have been extensively used for decades and have signiFcantly
increased food production. However, a large amount of applied pesticides often never
reach their intended target due to their degradation, volatilization and leaching, resulting
in serious environmental problems. This article reviews the main problems that the use
of these compounds causes to the environment and health and discusses the basis for
biodegradation that can be used for remediation of contaminated sites. It also provides
information about the cell immobilization of speciFc microorganisms on different types
of supports, as a strategy to increase the efFciency of pesticide degradation. We also
review and discuss the use of biobeds as an economic, clean and efFcient strategy to
provide a tool for the
in situ
degradation of pesticide residues.
Palabras clave: plaguicidas, biodegradación, biocamas, inmovilización celular
RESUMEN
Los plaguicidas son sustancias o mezclas de sustancias que se destinan a prevenir,
destruir o controlar cualquier plaga y son ampliamente utilizados en el sector agrícola,
industrial y doméstico, principalmente. Estos compuestos se han usado por décadas
y por ello se ha incrementado signiFcativamente la producción de alimentos. Sin
embargo, de la cantidad total de plaguicidas aplicados, un gran porcentaje no alcanza
el sitio blanco, ya que pueden degradarse, volatilizarse y/o lixiviarse, dando como
resultado serios problemas ambientales. Este artículo revisa los principales problemas
que se causan al ambiente y a la salud por la utilización de estos compuestos y discute
las bases para la biodegradación para que sus principios puedan ser utilizados para la
remediación de sitios contaminados. También se proporciona información acerca de la
inmovilización de células de microorganismos especíFcos sobre diferentes soportes,
como una estrategia para incrementar la eFciencia de degradación de los plaguicidas.
Septiembre 2013
M.L. Ortiz-Hernández
et al.
86
Por otro lado, se revisa y discute acerca del empleo de las biobeds, como una estrategia
económica, limpia y efciente para proveer una herramienta in situ para la degradación
de residuos de plaguicidas.
INTRODUCTION
Because of human activities, a large number of
pollutants and waste are currently dispersed within
the environment. Approximately 6×10
6
chemical
compounds have been produced, 1000 new products
are synthesized annually, and between 60 000 and
95 000 chemicals are commercially used (Shukla
et al.
2010). Among these substances are chemical
pesticides, which are used extensively in most areas
of crop production to minimize pest infestations, to
protect the crop yield losses and to avoid reducing the
product quality. A pesticide is any substance or mix-
ture of substances intended for preventing, destroy-
ing, repelling or mitigating any pest (insects, mites,
nematodes, weeds, rats, etc.), including insecticides,
herbicides, fungicides and various other substances
used to control pests (EPA 2012).
Pesticides belong to a category of chemicals used
worldwide to prevent or control pests, diseases,
weeds and other plant pathogens in an effort to
reduce or eliminate yield losses and maintain high
product quality (Damalas and Eleftherohorinos
2011). The positive aspect of the application of
pesticides has resulted in enhanced crop/food pro-
ductivity and a drastic reduction of vector-borne dis-
eases (Damalas 2009, Agrawal
et al.
2010). Chemi-
cal pesticides can be classifed in diFFerent ways,
but they are most commonly classifed according
to their chemical composition. This method allows
the uniForm and scientifc grouping oF pesticides to
establish a correlation between structure, activity,
toxicity and degradation mechanisms, among other
characteristics.
Table I
shows the most important
pesticides and their general characteristics, and
fg
-
ure 1
shows examples of some chemical structures
of pesticides.
Global insecticide use in 2007 has been estimated
at 404 000 metric tons of active ingredient (Grube
et al.
2011). The agricultural sector is the primary
user of pesticides, consuming over four million tons
of pesticides annually; however, a large amount of
applied pesticides often never reach their intended
target due to their degradation, volatilization and lea-
ching, leading to serious ecological problems (Chen
et al.
2009, Chevillard
et al.
2012). Under actual
agricultural practices, different groups of pesticides
are often simultaneously or consecutively applied and
consequently interact with each other (Myresiotis
et
al.
2012). A population inhabiting a contaminated
site may be subjected to selective pressure from
the contamination, which may result in an elevated
TABLE I.
GENERAL CHARACTERISTICS OF SOME PESTICIDES (Badii and Landeros 2007)
Pesticides
Characteristics
Main composition
Organochlorines
Soluble in lipids, they accumulate in fatty tissue
of animals, are transferred through the food chain;
toxic to a variety of animals, long-term persistent.
Carbon atoms, chlorine, hydrogen and occasionally
oxygen. They are nonpolar and lipophilic
Organophosphates
Soluble in organic solvents but also in water. They
infltrate reaching groundwater, less persistent than
chlorinated hydrocarbons; some affect the central
nervous system. They are absorbed by plants and
then transferred to leaves and stems, which are the
supply of leaf-eating insects or feed on wise.
Possess central phosphorus atom in the molecule. In
relation whit organochlorines, these compounds are
more stable and less toxic in the environment. The
organophosphate pesticides can be aliphatic, cyclic
and heterocyclic.
Carbamates
Carbamate acid derivatives; kill a limited spectrum
of insects, but are highly toxic to vertebrates.
Relatively low persistence
Chemical structure based on a plant alkaloid
Physostigma venenosum
Pyrethroids
Affect the nervous system; are less persistent than
other pesticides; are the safest in terms of their use,
some are used as household insecticides.
Compounds similar to the synthetic pyrethrins
(alkaloids obtained from petals of
Chysanthemun
cinerariefolium
).
Biological
Only the
Bacillus thuringiensis
(Bt) and its
subspecies are used with some frequency; are
applied against forest pests and crops, particularly
against butter±ies. Also aFFect other caterpillars.
Viruses, microorganisms or their metabolic products
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
87
resistance in this population compared to resistance
in a population of conspeciFcs living at a clean site
(Klerks
et al.
2011).
The unregulated and indiscriminate application
of pesticides can cause adverse effects to human
health, to different life forms and to the ecosystems.
The extent of these effects depends on the degree of
sensitivity of the organisms and the toxicity of the
pesticides. The continued application of pesticides
has increased its concentration in soils and waters and
their effects can also be magniFed through the food
chain. Dispersion mechanisms have also increased
the level of environmental risk for the occupationally
exposed population and the inhabitants of surround-
ing villages. Pesticides cause serious health hazards
to living systems because of their rapid fat solubil-
ity and bioaccumulation in non-target organisms
(Agrawal
et al.
2010). The main forms of pollution
are the direct application of pesticides to agricultural
crops, accidental spills during transport and manufac-
turing, as well as waste from tanks where cattle are
treated for ectoparasite control (EPA 2012).
In addition, liquid and solid wastes and obsolete
products are stored or disposed in an inappropri-
ate manner, which has favored the appearance of
signiFcant environmental liabilities. An obsolete
pesticide may be recognized as one that is undesir-
able or impossible to use and must be eliminated
(Martinez 2004, Karstensen
et al.
2006, Shah and
Devkota 2009, Dasgupta
et al.
2010). In the absence
of a clear obsolete pesticide management strategy,
over the years, signiFcant amounts of obsolete pes
-
ticides have been stockpiled in developing countries
(Dasgupta
et al.
2010). There are more than half
a million tons of obsolete, unused, forbidden or
outdated pesticides, in several developing and tran-
sitional countries, which endanger the environment
and health of millions of people (Ortiz-Hernández
et al.
2011). Obsolete pesticides have accumulated
in almost every developing country or economy in
transition over the past several decades (Dasgupta
et al.
2010). It is estimated that there are more than
100,000 tons of these products in Africa and the
Middle East, almost 200 000 tons in Asia and a simi-
lar quantity in Eastern Europe and the former Soviet
Union. Currently, the FAO is recording the inven-
tories of Latin America (Farrera 2004, Karstensen
et al.
2006, Ortiz-Hernández and Sánchez-Salinas
2010). However, it is difFcult to estimate the exact
quantities of obsolete pesticides because many of
the products are very old and documentation is often
lacking (Vijgen and Egenhofer 2009).
For the total destruction of these obsolete pes-
ticides, the results of projects undertaken by IHPA
(International HCH & Pesticides Association)
suggest that the cost for cleaning up, repackaging,
A)
B)
C)
D)
O
S
O
O
Cl
CH
3
H
3
C
H
3
C
CH
3
CH
3
CH
3
O
O
O
P
S
S
O
O
O
O
O
CH
3
–NH–C–O
Cl
Cl
Cl
Cl
Cl
Cl
O
O
O
Cl
Endosulfan (
6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-
hexahydro-6,9-methano-2,4,3-benzadioxathiepin 3-
oxide
)
Permethrin
(
3-phenoxybenzyl (1RS)-cis,trans-3-(2,2-
dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate
)
Carbofuran
(
2,2-Dimethyl-2,2-dihydrobenzofuranyl-7
N-methylcarbamate
)
Malathion (
Diethyl 2-dimethoxyphosphinothioyl
sulfanylbutanedioate
)
Fig. 1.
Examples of chemical structure of pesticides A) Organochloride, B) Pyrethroid, C) Carbamate,
and D) Organophosphate
M.L. Ortiz-Hernández
et al.
88
transporting and fnal ultimate destroying oF obsolete
pesticide to be at 4000 USD per ton. The FAO as-
sumes roughly similar fgures. ±or AFrica the costs
are estimated to be in the order of 4000-7000 USD
per ton (Vijgen and Egenhofer 2009).
BIODEGRADATION AS A STRATEGY TO
REDUCE THE NEGATIVE IMPACT OF
PESTICIDES
Due to the problems mentioned above, the de-
velopment of technologies for environmental re-
mediation or waste destruction that guarantees their
elimination in a saFe, eFfcient and economical way
is important. The mechanisms for the cleanup of
pesticides in soil such as chemical treatment, volatil-
ization and incineration have met public opposition
because of problems such as the production of large
volumes of acids and alkalis that must subsequently
be disposed. The potentially toxic emissions and the
elevated economic costs are also signifcant concerns.
Overall, most of these physical-chemical cleaning
technologies are expensive and ineFfcient (Nyakundi
et al.
2011). A methodology for degradation that has
gained acceptance is the bioremediation, which is
conducted through the biodegradation of these chemi-
cal compounds. According to the defnition by the
International Union of Pure and Applied Chemistry,
the term biodegradation is defned as the breakdown
of a substance catalyzed by enzymes
in vitro
or
in
vivo
. Biodegradation may be defned For the purpose
of hazard assessment into the following categories
(Meleiro-Porto
et al.
2011):
1. Primary. Alteration of the chemical structure
oF a substance resulting in loss oF a specifc
property of that substance.
2. Environmentally acceptable. Biodegradation
to such an extent as to remove undesirable
properties of the compound. This change often
corresponds to primary biodegradation but it
depends on the circumstances under which the
products are discharged into the environment.
3. Ultimate. Complete breakdown of a compound
to either fully oxidized or reduced simple
molecules (such as carbon dioxide/methane,
nitrate/ammonium and water). It should be
noted that the biodegradation products can be
more harmful than the substance degraded.
The microbial degradation of pesticides in the
environment is an important route for the removal
of these compounds. The biodegradation of these
compounds is often complex and involves biochemi-
cal reactions. Although many enzymes eFfciently
catalyze the biodegradation of pesticides, the full
understanding of the biodegradation pathway of-
ten requires new investigations. Several pesticide
biodegradation studies have shown only the total of
degraded pesticide, but have not investigated in depth
the new biotransformed products and their fate in the
environment (Meleiro-Porto
et al.
2011).
As an eFfcient, economical and environmentally
friendly technique, biodegradation has emerged as a
potential alternative to the conventional techniques.
However, the biodegradation process of many pesti-
cides has not been fully investigated (Sun
et al.
2010).
With knowledge oF the biodegradation processes, is
possible to apply it to improve the bioremediation of
sites contaminated with pesticides. Bioremediation
enables the destruction of many organic contaminants
at a reduced cost, and in recent years, bioremediation
technology has progressed for the degradation of a
wide range of pollutant compounds. Bioremedia-
tion can oFFer an eFfcient and cheap option For the
decontamination of polluted ecosystems and the de-
struction of pesticides (Blackburn and Hafker 1993,
Vidali 2001, Dua
et al.
2002, Singleton 2004, Singh
and Walker 2006).
MICROORGANISMS INVOLVED IN THE
BIODEGRADATION OF PESTICIDES
Different microorganisms have been used to bio-
transform pesticides. A fraction of the soil biota can
quickly develop the ability to degrade certain pesti-
cides, when they are continuously applied to the soil.
These chemicals provide an adequate carbon source
and electron donors for certain soil microorganisms
(Torres 2003), thereby generating a method for the
treatment of pesticide-contaminated sites (Araya and
Lakhi 2004, Qiu
et al.
2007). However, the transfor-
mation of such compounds depends not only on the
presence of microorganisms with appropriate degrad-
ing enzymes but also on a wide range of environ-
mental parameters (Aislabie and Lloyd-Jones 1995,
Alves
et al.
2010). Additionally, some physiological,
ecological, biochemical and molecular aspects play
an important role in the microbial transformation of
pollutants (Iranzo
et al.
2001, Vischetti
et al.
2002,
Becker and Seagren 2010, Megharaj
et al.
2011).
There are different sources of microorganisms
with the ability to degrade pesticides. Because
pesticides are mainly applied to agricultural crops,
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
89
soil is most affected by these chemicals. Industry’s
efFuent-sediment, sewage sludge, activated sludge,
wastewater, natural waters, sediments, areas sur-
rounding the manufacture of pesticides, and even
some live organisms are also affected. In general,
microorganisms that have been identi±ed as pesticide
degraders have been isolated from a wide variety of
sites contaminated with some type of pesticide. At
present, in different laboratories around the world
there are collections of microorganisms character-
ized by their identi±cation, growth and degradation
of pesticides. The isolation and characterization of
microorganisms that are able to degrade pesticides
makes it possible to utilize new tools to restore pol-
luted environments or to treat wastes before their ±nal
disposition (Ortiz-Hernández
et al.
2011).
PRINCIPLES OF PESTICIDE
BIODEGRADATION
Biodegradation is a process that involves the
complete breakdown of an organic compound in its
inorganic constituents. The microbial transforma-
tion may be driven by energy needs or a need to
detoxify the pollutants, or it may be fortuitous in
nature (co-metabolism) (Becker and Seagren 2010).
The search for pollutant-degrading microorganisms,
understanding their genetics and biochemistry and
developing methods for their application in the
±eld have become an important human endeavor
(Megharaj
et al.
2011). The ubiquitous nature of
microorganisms, their numbers and large biomass
relative to other living organisms on earth, their
more diverse catalytic mechanisms (Paul
et al.
2005), and their ability to function even in the ab-
sence of oxygen and other extreme conditions are
greatly important in the use of microorganisms for
the degradation of pesticides.
The microbial populations of soil or aquatic
environments are composed of diverse, synergistic
or antagonistic communities rather than a single
strain. In natural environments, biodegradation
involves the transfer of substrates and products
within a well-coordinated microbial community,
a process referred to as metabolic cooperation
(Abraham
et al.
2002). Microorganisms have the
ability to interact both chemically and physically
with substances, leading to structural changes or
the complete degradation of the target molecule.
Pesticides interact with soil organisms and their
metabolic activities and may alter the physiological
and biochemical behavior of soil microbes. Many
recent studies have revealed the adverse impacts
of pesticides on soil microbial biomass and soil
respiration; generally, a decrease in soil respiration
reFects the reduction in microbial biomass. Some
microbial groups are capable of using applied
pesticides as a source of energy and nutrients for
their multiplication, whereas the pesticide may
be toxic to other organisms. Likewise, sometimes
the application of pesticides reduces microbial
diversity but increases the functional diversity of
microbial communities. Pesticide application may
also inhibit or kill certain groups of microorganisms
and outnumber other groups by reducing competi-
tion (Hussain
et al.
2009). Among the microbial
communities, bacteria, fungi and actinomycetes
are the main transformers and pesticide degraders
(Briceño
et al.
2007). Fungi generally biotransform
pesticides and other xenobiotics by introducing mi-
nor 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 (Diez 2010).
Fungi and bacteria are considered excellent
extracellular enzyme-producing microorganisms.
Moreover, the ability of fungi to form extended
mycelial networks, the low speci±city of their
catabolic enzymes and their independence from
organic chemicals as a growth substrate make fungi
well suited for bioremediation processes (Harms
et
al.
2012). Fungi are critical to the biogeochemi-
cal cycles and are responsible for the bulk of the
degradation of environmental xenobiotics in the
biosphere (Liang
et al.
2005). White rot fungi
have been proposed as promising bioremediation
agents, especially for compounds that are not read-
ily 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
species that degrade pesticides 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 P450 (Bass and Field 2011).
Enzymes are central to the biology of many
pesticides (Riya and Jagatpati 2012). Applying
enzymes to transform or degrade pesticides is an
innovative treatment technique for the removal
of these chemicals from polluted environments.
Enzyme-catalyzed degradation of a pesticide may
be more effective than existing chemical methods.
M.L. Ortiz-Hernández
et al.
90
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 physi-
ological roles. Enzymes are also involved in the
degradation of pesticide compounds, both in the
target organism, through intrinsic detoxifcation
mechanisms and evolved metabolic resistance, and
in the wider environment, via biodegradation by
soil and water microorganisms (Scott
et al.
2008).
Trigo
et al.
(2009) suggested that (i) the central
metabolism of the global biodegradation networks
involves transferases, isomerases, hydrolases and
ligases; (ii) linear pathways converging on par-
ticular intermediates form a funnel topology; (iii)
the novel reactions exist in the exterior part of the
network; and (iv) the possible pathway between
compounds and the central metabolism can be ar-
rived at by considering all the required enzymes in
a given organism and the intermediate compounds
(Ramakrishnan
et al.
2011).
The metabolism of pesticides may involve a
three-phase process. In Phase I metabolism, the
initial properties of a parent compound are trans-
formed 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 the conjugation of a pes-
ticide or pesticide metabolite to a sugar or amino
acid, which increases the water solubility and re-
duces toxicity compared with the parent pesticide.
The third phase involves conversion of Phase II
metabolites into secondary conjugates, which are
also non-toxic. Fungi and bacteria are involved in
these processes and produce intracellular or extra-
cellular enzymes including hydrolytic enzymes,
peroxidases, oxygenases, etc. (Van Eerd
et al.
2003,
Ortiz-Hernández
et al.
2011).
Due to the diversity of chemicals used in pesti-
cides, the biochemistry of pesticide bioremediation
requires a wide range of catalytic mechanisms, and
therefore a wide range of enzyme classes. Informa-
tion for some pesticide-degrading enzymes can be
found in
table II
.
Among the enzymes that degrade pesticides, the
hydrolases catalyze the hydrolysis of several major
biochemical classes of pesticide (esters, peptide
bonds, carbon-halide bonds, ureas, thioesters, etc.)
and generally operate in the absence of redox cofac-
tors, making them ideal candidates for all of the cur-
rent bioremediation strategies (Scott
et al.
2008).
In this group, we can fnd the phosphotriesterases
(PTEs), which are one of the most important classes
(Chino-Flores
et al.
2012). These enzymes have
been isolated from different microorganisms that
hydrolyze and detoxify organophosphate pesticides
(OP). This reaction reduces OP toxicity by decreas-
ing the ability of OP to inactivate AchE (Ghanem
and Raushel 2005, Singh and Walker 2006, Porzio
et al.
2007, Shen
et al.
2010, Theriot and Grunden
2010). The frst isolated phosphotriesterase belongs
to
Pseudomonas diminuta
MG; this enzyme shows
a highly catalytic activity towards organophosphate
pesticides. The PTEs are encoded by a gene called
opd
(organophosphate-degrading).
Flavobacterium
ATCC 27551 contains the
opd
gene that encode a
PTE (Latif
et al.
2012). These enzymes specif
-
cally 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.
This enzyme has a potential use for the cleaning of
organophosphorus pesticide-contaminated environ-
ments (Ortiz-Hernández
et al.
2003). There are other
enzymes involved in the overall degradation of a
pesticide. The parathion degradation pathway is an
example of this process (Abo-Amer 2012) (
Fig. 2
).
Esterases are enzymes that catalyze the hydrolysis
of carboxylic esters (carboxyesterases), amides (ami-
dases), phosphate esters (phosphatases), etc. (Bansal
2012). In the reaction catalyzed by esterases, a wide
range of ester substrates can be hydrolyzed into their
alcohol and acid components as follows:
R = O-OCH
3
+ H
2
O
R = O-OH + CH
3
OH
Many insecticides (organophosphates, carbamates
and pyrethroids) have a carboxylic ester component,
and the enzymes capable of hydrolyzing this type of
ester bond are known as carboxylesterases.
Oxidoreductases are a broad group of enzymes
that catalyzes the transfer of electrons from one
molecule (the reductant or electron donor) to an-
other (the oxidant or electron acceptor). Many of
these enzymes require additional cofactors to act as
electron donors, electron acceptors or both. These
enzymes have applications in bioremediation, dur-
ing which they catalyze an oxidation/reduction
reaction by including molecular oxygen (O
2
) as the
electron acceptor. In these reactions, oxygen is re-
duced to water (H
2
O) or hydrogen peroxide (H
2
O
2
).
The oxidases are a subclass of the oxidoreductases
(Scott
et al.
2008).
As an example of the many functions of these en-
zymes in the degradation of pesticides, we present the
malathion degradation pathway. This process involves
esterases and oxidoreductase enzymes, and different
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
91
microorganisms and catalytic activities can lead to the
complete mineralization of a pesticide (
Fig. 3
).
A fungus capable of using chlorpyrifos as the
sole carbon source was isolated from organophos-
phate-contaminated soil and was characterized as
Cladosporium cladosporioides
(collection number
CCTCC M 20711) (Gao
et al.
2012). Based on the
metabolic products formed, the degradation path-
way for chlorpyrifos by the strain was proposed
(
Fig. 4
). Specifcally, the parent chlorpyriFos was
frst metabolized by hydrolysis to produce 3,5,6-tri
-
chloro-2-pyridinol (TCP) and diethylthiophosphoric
acid (DETP). Subsequently, the hydrolysis product
TCP was further transformed by ring breakage,
resulting in its complete detoxifcation (Chen
et
al.
2012). A novel chlorpyrifos hydrolase from cell
extract was purifed 35.6-Fold to apparent homoge
-
neity with 38.5 % overall recovery by ammonium
sulFate precipitation, gel fltration chromatography
and anion-exchange chromatography. The enzyme
is a monomeric structure with a molecular mass of
38.3 kDa. The pI value was estimated to be 5.2. The
optimal pH and temperature oF the purifed enzyme
were 6.5 and 40 ºC, respectively. No cofactors were
required for the hydrolysis of chlorpyrifos (Gao
et
al.
2012).
Lu
et al.
(2013), reported a bacterial strain,
Cu-
priavidus
sp. DT-1, capable of degrading chlorpyrifos
and 3,5,6-trichloro-2-pyridinol (TCP) and using these
compounds as sole carbon source was isolated and
characterized. Investigation of the degradation path-
way showed that chlorpyriFos was frst hydrolyzed to
TCP, successively dechlorinated to 2-pyridinol, and
then subjected to the cleavage of the pyridine ring
TABLE II.
RELEVANT MICROBIAL ENZYMES IN PESTICIDE BIODEGRADATION (Singh and Walker 2006, Scott
et al.
2008, Riya and Jagatpati 2012)
Enzyme
Organism
Pesticide
Bioremediation strategy
Oxidoreductases
Gox
Pseudomonas
sp. LBr
Agrobacterium
strain T10
Glyphosate
Plant
Monooxygenases
ESd
Mycobacterium
sp.
Endosulphan and endosulphato
-
Ese
Arthrobacter
sp.
Endosulphan, aldrin, malation,
DDDT and endosulphate
-
Cyp1A1/1A2
Rats
Atrazine, nor±urazon and
isoproturon
Plant
Cyp76B1
Helianthus tuberosus
Linuron, chlortoluron and
isoproturon
Plant
P450
Pseudomonas putida
Hexachlorobenzene and
pentachlorobenzene
-
Dioxygenases
TOD
Pseudomonas putida
Herbicides tri±uralin
-
E3
Lucilia cuprina
Synthetic pyrethroids and
insecticides phosphotriester
-
Phosphodiesterases
PdeA
Delftia acidovorans
Organophosphorus compounds
-
Phosphotriesterases
OPH
OpdA
Agrobacterium radiobacter
Pseudomonas diminuta
Flavobacterium
sp.
Insecticides phosphotriester:
Parathion, methyl parathion,
malathion, coumaphos, others.
Bioremediation and free
enzymes
Phosphonatase
Phn
Escherichia coli
Sinorhizobium meliloti
Organophosphorus compounds
-
Haloalkane
dehalogenases
LinB
Sphingobium
sp.
Shingomonas
sp.
Hexachlorocyclohexane
(
β
and
δ
isomers)
Bioaugmentation
AtzA
Pseudomonas
sp. ADP
Herbicides chloro-s-trazine
Plants and bacteria
TrzN
Nocardioides
sp.
Herbicides chloro-s-trazine
-
LinA
Sphingobium
sp.
Shingomonas sp.
Hexachlorocyclohexane
(γ isomers)
Bioaugmentation
TfdA
Ralstonia eutropha
2,4-dichlorophenoxyacetic acid
and pyridyl-oxyacetic
Plant
DMO
Pseudomonas maltophilia
Dicamba
Plant
C-P-lyase
Glp A&B
Pseudomonas pseudomallei
Organophosphorus compounds
-
ND
hocA
Pseudomonas monteilli
Organophosphorus compounds
-
mpd
Pleisomonas
sp.
Organophosphorus compounds
-
ND= not determined
M.L. Ortiz-Hernández
et al.
92
and further degradation. The
mpd
gene, encoding
the enzyme responsible for chlorpyrifos hydrolysis
to TCP, was cloned and expressed in
Escherichia
coli
BL21. Inoculation of chlorpyrifos-contaminated
soil with strain DT-1 resulted in a degradation rate of
chlorpyrifos and TCP of 100 % and 94.3 %, respec-
tively as compared to a rate of 28.2 % and 19.9 % in
uninoculated soil.
A)
B)
C)
O
-
+
O
O
HO
OH
O
O
O
O
O
CH
2
CH
3
CH
2
CH
3
Parathion
Phosphotriesterase
CH
2
CH
3
CH
2
CH
3
P
P
S
S
O
N
N
Phosphotriesterase
p-Aminophenol
Benzoquinone
4-aminophenol
dehidrogenase
Phosphotriesterase
Diethyl thiophosphoric acid
Diethyl thiophosphoric acid
Diethyl thiophosphoric acid
p-Nitrophenol
HO
O
O
CH
2
CH
3
CH
2
CH
3
P
S
-
O
-
+
O
O
O
Parathion
CH
2
CH
3
CH
2
CH
3
P
S
O
N
Parathion
NAD(P)H nitroreductase
Aminoparathion
Paraoxon
Parathion oxidoreductase
O
-
O
O
O
CH
2
CH
3
CH
2
CH
3
P
O
O
N
O
-
O
O
O
CH
2
CH
3
CH
2
CH
3
P
S
O
N
H
2
N
HO
OH
O
CH
2
CH
3
CH
2
CH
3
P
S
O
O
O
O
O
CH
2
CH
3
CH
2
CH
3
P
S
O
H
2
N
Fig. 2
.
Parathion degradation pathway. A) Aerobic pathway involves initial hydrolysis of parathion to p-nitro-
phenol and diethylthiophosphoric acid. B) Other aerobic reaction involves the oxidation of parathion to paraoxon
and then it follows the same way as A). C) Under anerobic conditions, parathion is reduced to aminoparathion,
which is hydrolyzed to p-aminophenol and diethylthiophosphoric acid (modifed From University oF Minnesota.
Biocatalysis/Biodegradation Database, http://umbbd.ethz.ch/pthn/pthn_map.html)
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
93
Fig. 3
. Malathion degradation pathway (University of Minnesota. Biocatalysis/Biodegradation Database, http://www.umbbd.
ethz.ch/end/end_map.html).
O
O
O
O
S
S
S
S
S
S
S
S
S
P
P
P
P
P
P
P
O
O
O
O
HO
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
SH
H
3
C
H
3
C
H
3
C
H
3
C––OH
H
3
C
HO
CH
3
CH
3
CH
3
CH
3
O
O
O
O
COO
COO
COO
COO
COO
O
O
O
O
O
O
O
O
O
O
O
O
S
P
O
O
O
O
P
O
O
O
O
O
O
O
O
P
CH
3
O
O
O
O
A
Malathion
Desmethyl
malathion
Diethyl malate
malathion
phosphodiesterase
O
O
S
S
P
O
O
O
O
CH
3
CH
3
CH
3
Malathion
esterase
Malathion
monocarboxylate
Ethanol
Malathion
monocarboxylate
esterase
Malathion
dicarboxylate
oxidoreductase
Dimethyl-
dithiophosphate
Dimethyldithiophosphate
oxidoreductase
Dimethylthiophosphate
phosphodiesterase
Malathion
dicarboxylate
Oxaloacetate
Dimethyldithiophosphate
Dimethylphosphate
Methylphosphate
Methylphosphate
phosphomonoesterase
Methylthiophosphate
Thiophosphate
Thiophosphate
oxidoreductase
Methylthiophosphate
phosphomonoesterase
Methanol
Phosphate
Dimethylphosphate
phosphodiesterase
Dimethyl-
thiophosphate
reductase
M.L. Ortiz-Hernández
et al.
94
Lu
et al.
(2013) reported a bacterial strain,
Cu-
priavidus
sp. DT-1, that is capable of degrading
chlorpyrifos and 3,5,6-trichloro-2-pyridinol (TCP).
The strain was isolated and characterized by using
these compounds as a sole carbon source. Investiga-
tion of the degradation pathway showed that chlor-
pyrifos was Frst hydrolyzed to TCP, successively
dechlorinated to 2-pyridinol, and then subjected to the
cleavage of the pyridine ring and further degradation.
The
mpd
gene that encodes the enzyme responsible
for chlorpyrifos hydrolysis to TCP was cloned and
expressed in
Escherichia coli
BL21. Inoculation
of chlorpyrifos-contaminated soil with strain DT-1
resulted in chlorpyrifos and TCP degradation at rates
of 100 % and 94.3 %, respectively, compared to rates
of 28.2 % and 19.9 % in uninoculated soil.
CELLS IMMOBILIZATION TO IMPROVE
THE EFFICIENCY OF PESTICIDE
DEGRADATION
An immobilized cell is deFned as a living cell
that, by natural or artiFcial means, is prevented from
moving independently from its original location to all
parts of an aqueous phase of a system. The underlying
concept is that immobilized microorganisms in matri-
ces, either biological or inert, may enhance the required
biotechnological beneFts from the mass culture of the
microorganism by degrading a speciFc metabolite or
removing pollutants (de-Bashan and Bashan 2010).
Microorganisms do not live as pure cultures of
dispersed single cells but instead accumulate at
interfaces to form polymicrobial aggregates such
as Flms, mats, ±ocs (±oating bioFlms), sludge or
bioFlms. Multispecies aggregates can form stable mi
-
croconsortia, develop physiochemical gradients, and
undergo horizontal gene transfer and intense cell–cell
communication. These consortia therefore represent
highly competitive environments (Flemming and
Wingender 2010). Immobilization of microorgan
-
isms on inert supports has generated an increasing
inter
est because of the beneFts that can be obtained
from the process (Jo
et al.
2010). An immobilized cell
is deFned as a living cell that, by natural or artiFcial
means, is prevented from moving independently from
its original location to all parts of an aqueous phase
of a system (Tampion and Tampion, 1987).
Cell immobilization has been employed for the
biological removal of pesticides because it confers
the possibility of maintaining catalytic activity over
long periods of time (Martin
et al.
2000, Richins
et al.
2000, Chen and Georgiou 2002). Whole-cell
immobilization has been shown to have remarkable
advantages over conventional biological systems us-
ing 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 the toxic effects of hazardous
compounds and harsh environments. Immobilization
can increase the cells’ survival and metabolic activity
in bioremediation systems (Tao
et al.
2009, Moslemy
et al.
2002). Previous reports have suggested that this
higher productivity results from cellular or genetic
modiFcations induced by immobilization. There is
evidence indicating that immobilized cells are much
more tolerant to perturbations in the reaction envi-
ronment and less susceptible to toxic substances,
which makes immobilized cell systems particularly
attractive for the treatment of toxic substances such
as pesticides (Ha
et al.
2008). In addition, the en-
hanced degradation capacity of immobilized cells
is due primarily to the protection of the cells from
inhibitory substances present in the environment
(Sun
et al.
2010). 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 (Ha
et al.
2009).
There are two types of processes for cell immobili-
zation: those based on physical retention (entrapment
and inclusion membranes) and those based on chemi-
cal bonds, such as bioFlm formation (Kennedy and
Cabral 1983). Cell immobilization methods may use
various materials or substrates both inorganic (clays,
silicates, glass and ceramics) and organic (cellulose,
starch, dextran, agarose, alginate, chitin, collagen,
keratin, polyacylamide hydrazide, activated pumice
and activated carbon) (Arroyo 1998, Jo
et al.
2010).
The applicability of several natural or synthetic poly-
mers as matrices for immobilization of viable cells
motivated the study of the use of different gels such
as alginate, agar-agar and agarose (Taha
et al.
2013).
Cl
+
Cl
Cl
Cl
O
OC
2
H
5
OC
2
H
5
OC
2
H
5
CO
2
+ H
2
O
OC
2
H
5
N
HO
S
S
P
P
DETP
Chlorpyrifos
ring breakage
Cl
Cl
OH
N
TCP
Fig. 4.
Biodegradation of chlorpyrifos and its hydrolysis pro-
duct 3,5,6-trichloro-2-pyridinol by a new fungal strain
Cladosporium cladosporioides
(Chen
et al.
2012).
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
95
Entrapment in natural polymeric gels has become the
preferred technique for the immobilization of cells;
however, immobilized cells on supports have been
used more frequently in xenobiotics biodegradation
than for pesticides (Lusta
et al.
1990).
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 and the pos-
sibility of using it repeatedly. Above all, the support
must be affordable enough to allow its future use
for pesticide degradation.
Figure 5
describes the
main methods of immobilization (Kennedy and Ca-
bral 1983, Heitkamp
et al.
1990, Wang
et al.
1997,
a) Cells contained behind a barrier (microencapsulation)
b) Selft aggregation of cells (natural flocculation)
c) Entrapment within a porous matrix
e) Covalent binding on a surface
f) Cross-linking on a surface (artificial flocculation)
g) Cross-linking (artificial flocculation)
d) Attachment or adsorption to a preformed carrier
Fig. 5
. Cell immobilization methods
M.L. Ortiz-Hernández
et al.
96
Karamanev
et al.
1998, Pedersen and Christensen
2000). The methods can be grouped into two types:
the active that induce the capture of microorganisms
in a matrix, and the passive use the tendency of
microorganisms to attack either natural or synthetic
surfaces, which enables them to form bioFlms.
The supports used for immobilization may be of
synthetic or natural origin (
Table III
).
Bacterial bioFlms are deFned as sessile com
-
munities characterized by cells that are attached to
a substratum, to an interface or to each other. Large
amounts of extracellular matrix material are often
produced during biofilm formation. This matrix
holds the cells in association with each other and
with the surface, and it commonly contains exo-
polysaccharides (EPS), proteins, DNA, surfactants,
lipids, glycolipids, and ions such as Ca
2+
, which form
dense granules, grow attached to a static solid surface
(static bioFlm) or in a suspension bracket (Davey
and O’Toole 2000, Nicolella
et al.
2000, Flemming
and Wingender 2010, Prigent-Combaret
et al.
2012).
BioFlms form in several steps starting with the attack
or recognition of the surface, followed by growth and
the utilization of various carbon and nitrogen sources
for the formation of products with adhesive proper-
ties. In parallel, a stratiFed 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 bioFlm
takes place which varies depending on the presence
of nutrients from the medium or friction with the
surrounding water ±ow. ²inally, a period of bioFlm
aging may occur during which cells detach and
colonize other surfaces (Yañez-Ocampo
et al.
2009).
TABLE III.
SUPPORTS FOR IMMOBILIZATION OF MICROORGANISMS IN XENOBIOTICS REMOTION
Support
Microorganism
Xenobiotic
Reference
Glass beads
Escherichia coli
(transformed)
Coumaphos
Mansee
et al.
2005
Ceramic
Pseudomonas GCH1
Propachlor
Martín
et al.
2000
Polyurethane, alginate,
alginate poly vinyl alcohol
Pseudomonas spp.
Phenol
Chivita and Dussán 2003
Coffee beans
Pseudomonas aeruginosa
Flavimonas oryzihabitans
Dichlorodiphenyltrichloroethane
Endosulfan
Barragán
et al.
2007
Ca Alginate beads
Escherichia coli (OPH)
Coumaphos, diethylphosphate and
chlorferon
Ha
et al.
2009
Tezontle
Pseudomonas fuorescens
2,4-dichlorophenoxiacetic acid
Dichlorodiphenyltrichloroethane
Santacruz
et al.
2005
Ca Alginate beads, Tezontle Bacterial consortia
Methyl parathion, tetrachlorvinphos Yáñez-Ocampo
et al.
2009, 2011
Tezontle
Flavobacterium
sp. ATCC
27551
Methyl parathion
Abdel-Razek
et al.
2013
Corncob
Rhodococcus sp.
Pseudomonas sp.
n-Hexadecane
n-heptadecane
Rivelli
et al.
2013
Montmorillonite
Arthrobacter chlorophenolicus
A6
4-chlorophenol (4-CP)
Lee
et al.
2013a
Alginate
Bacillus sphaericus
strain CT7
Pseudomonas
sp. strain W4
Nonylphenol (NP),
Hsu
et al.
2013
Coir, banana stem, bulrush,
water hyacinth stem
Burkholderia cepacia
PCL3
Carbofuran
Laocharoen
et al.
2013
Luffa aegyptiaca
Mill
.
Metarhizium anisopliae
and bac-
terial consortium
Parathion methyl and coumaphos
Moreno-Medina 2011
Alginate
Dermocarpella
sp.
Ammonium
Lee
et al.
2013b
Alginate, silica gel,
agarose
Arthrospira platensis
(SAG257.80)
Plumb
Duda-Chodak
et al.
2013
Ca Alginate beads
Candida tropicalis YMEC14
Poliphenols
Ettayebi
et al.
2003
Alginate
Candida xylopsoci
Mercury
Amin and Latif 2013
Agave tequiliana Webber
(blue)
Trametes versicolor
Pleurotus ostreatus
Klebsiella sp.
Acid blue 113
Disperse blue 3
Basic green 4
Garzón-Jiménez 2009
Polyurethane foam
Phanerochaete chrysosporium
strain 1198
Bagasse
Shararia
et al.
2013
Alginate beads
Streptomyces
spp.
(A2, A5, A11, and M7)
Chlorpyrifos and pentachlorophenol Fuentes
et al.
2013
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
97
Tezontle is a native volcanic rock of Morelos state
(central Mexico) and has yielded good results in the
degradation of mixtures of pesticides (in Nahuatl,
“tezt” means “rock” and “zontli” means “hair”). This
rock is highly porous, provides a large contact surface
and can also be sterilized and reused. The presence
of micropores allows the establishment of bacterial
microcolonies (
Fig. 6
).
The immobilization method with this material is
based on the colonization of the tezontle micropores
through the formation of a bioFlm. Subsequently, a
current with the pesticides wastes is passed through
to allow contact with the immobilized microorgan-
isms so that biodegradation can be executed. This
strategy has been very efFcient and can be used for
the degradation of pesticide wastes. Yáñez-Ocampo
et al.
(2011) and el Razek
et al.
(2013) immobilized
a bacterial consortium in a bioFlm on tezontle. This
system exhibited a considerable capacity for the re-
moval of a mixture of organophosphate pesticides,
which are the pesticides widely used in agriculture
and stockbreeding in Mexico. In addition, this mate-
rial and immobilized cells were packaged in an up-
±ow reactor, which resulted in a greater viability of
the bacteria and more efFcient removal of pesticides.
Furthermore, there are reports of a variety of
materials that provide the features necessary to
immobilize microorganisms. For example, the use
of various plant Fbers as supports for immobilized
bacterial consortia to degrade xenobiotics has im-
portant advantages. The use of natural structural
materials, such as petiolar felt-sheath of palm, to
entrap the cells has added another dimension to a
variety of immobilization matrices. The advantages
of such biostructures are reusability, freedom from
toxicity problems, mechanical strength and open
spaces within the matrix for growing cells thereby
avoiding rupture and diffusion problems. It is neces-
sary to search diverse plant sources for other types
of biomaterials that may be used for cell entrapment.
The loofa sponge (
Luffa cylindrica
) has been used
as a carrier material for immobilizing various micro-
organisms for the purpose of either the adsorption
or degradation of various xenobiotics. This sponge
has been used as a natural support to immobilize
various organisms such as
Chlorella sorokiniana,
Porphyridium cruentum, Penicillium cyclopium
and
Funalia trogii
for nickel II, cadmium II and dyes
Fig. 6.
Scanning electron micrographs showing tezontle and immobilized
cells on tezontle.
A) Tezontle (2000 X); B) Tezontle with immobilized cells (2000 X); C) Tezontle
(4000 X) and D) Tezontle with immobilized cells (4000 X)
A)
B)
C)
D)
30μm
10μm
30μm
10μm
M.L. Ortiz-Hernández
et al.
98
and chlorinated substances treatment. Loofa grows
well in both tropical and subtropical climates and the
sponges are produced in large quantities in México
where they are currently used for bathing and dish
washing. They are light, cylindrical in shape and
made up of an interconnecting void within an open
network of matrix support materials. Because of their
random lattice of small cross sections coupled with
very high porosity, their potentiality as carriers for
cell immobilization is very high (Akhtar
et al.
2004,
Iqbal and Edyvean 2004, Mazmanci and Unyayara
2005). Moreno-Medina (2011) used this sponge and
reported methyl parathion removal at efFciencies
of 75%.
BIOBEDS: A STRATEGY FOR PESTICIDE
BIODEGRADATION
IN SITU
In response to the environmental and health
problems related to pesticides, the BioBed (BB) was
developed in the early 1990s. Biobeds are a simple
and inexpensive construction designed to collect and
degrade pesticide spills (Torstensson 2000, Juwarkar
et al.
2010).
The original design consists of a hole in the
ground in which a layer of waterproof clay is placed
on the bottom (10 cm). A mixture of straw, peat and
soil in proportions of 50-25-25 respectively and 50
cm in thickness is then added, followed by a layer of
grass on the surface. Straw is the main component
for ligninolytic fungi growth, the soil is used for
adsorption and promotes microbial activity and the
peat contributes to moisture control (Torstensson and
Castillo 1997, Castillo
et al.
2008).
Due to the low maintenance of the work, the short
time required and low costs, the BB has generated
great interest in many countries such as France, Italy,
the United Kingdom and Chile. Its introduction has
led to adaptations of the design according to the
climatic conditions of the location and the avail-
able organic materials such as olive branches, citrus
peels, cotton waste, garden compost and bagasse.
Similarly, the name has been adapted in different
ways to include terms such as bioFlter, biomassbed,
Phytobac®, Biobac and Biotable (Fogg
et al.
2004,
Vischetti
et al.
2004, Coppola
et al.
2007, De Rof-
Fgnac
et al.
2008, Karanasios
et al.
2012a, Tortella
et al.
2012).
An efFcient mix of materials for a BB must in
-
clude wide surfaces for the retention of pesticides,
which will reduce leaching, while providing a
robust and active microbial community (Vischetti
et al.
2008). However, strong adsorption reduces
the bioavailability of the pesticide and limits its
biodegradation. When measuring the adsorption of
a mixture of pesticides in the soil and in a variety
of biomixes, we observed that these biobeds had
greater adsorption compared soil pesticides (Ka-
ranasios
et al.
2010), whereby care must be taken
in choosing the biomix components. Castillo and
Torstensson (2007) have evaluated the effects of the
mixture composition, as well as various other factors
and found that the original conFguration at acidic
pH (5.9), 60% humidity and 20ºC, is the optimal
condition for degradation in Sweden. Another key
parameter is the ±ow of water. Different studies have
shown that under high volumes of water applied at
low frequencies (600 mL per week) results in high
levels of leaching compared to systems with a low
volume applied to high frequencies (100-200 mL
per day) (Karanasios
et al.
2012b).
Under laboratory conditions, biostimulation of
the mixture with inorganic fertilizers (N, P, K) at
low concentrations (0.1% and 0.5%) resulted in a
signiFcant increase in the degradation of chlorpyrifos
in the early days of incubation. However, increasing
N, P and K concentrations (0.5% and 1.0%) resulted
in the accumulation of TCP (the main metabolite of
the pesticide), which caused signiFcant changes in
the bacterial communities and an increased the risk
of leaching (Tortella
et al.
2010).
Bioaugmentation is a process that increases the
soil microbiota by inoculating external microor-
ganisms for the remediation of soil contaminated
by a xenobiotic. To improve the efFciency of bio
-
degradation in the BB, Diez
et al.
(2012) used
bioaugmentation with pellets of
Anthracophyllum
discolor,
a fungus with highly efFcient ligninolytic
activity on atrazine degradation and obtained an
increase of 18 % in the degradation of the pesticide.
Recent studies demonstrated that the addition of
terpenes at relatively low concentrations (50 mg/kg)
signiFcantly enhances the degradation of atrazine
(Tortella
et al.
2013).
Despite its beneFts, there are certain limitations.
Due to exhaustion, in general, the maturity of the
BM affects the performance of the BB, but this pos-
sibility requires further study. A study of the three
stages of biomix maturity (0, 15 and 30 days) with
regard to the degradation of different concentrations
of chlorpyrifos (200, 320 and 480 mg/L) showed
that the maturity did not interfere with the degrada-
tion (Tortella
et al.
2012). In the Feld, the mixture
should be replaced after 6-10 years and composted
to remove pesticide residues (Castillo
et al.
2008).
Although the efFciency of biodegradation is depen
-
MECHANISMS AND STRATEGIES FOR PESTICIDE BIODEGRADATION
99
dent on the dissipation of pesticides in the BB, little
is known about the microbiota and its interaction
with pesticides (Marinozzi
et al.
2012). Specifc
studies are needed on this subject to discern these
metabolic processes and enhance the eFfciency oF
the degradation.
CONCLUSIONS
Chemical pesticides are widely used around
the world and have historically increased the crop
yields for food production. However, they have also
been introduced into the food chain, with effects
on human health and ecosystems. Therefore, it is
important that efforts are made for the disposal of
waste and for the remediation of contaminated sites.
Biodegradation oF pesticides with specifc microor
-
ganisms is economic and environmental and socially
acceptable. By understanding the mechanisms for
degradation, it is possible to develop technologies
to increase the eFfciency oF degradation, such as the
immobilization of cells in different support systems
and the construction and use of biobeds for waste
degradation
in situ
.
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