Bioremediation of pesticides

 Pesticides are the chemical substances use to kill or manage pests at tolerable levels. The extensive use of pesticides has resulted in serious environmental as well as health problems and has effected biodiversity. The use of pesticides not only degrade the soil quality but also aquatic environment. Among the pesticides 98% were classified as acutely toxic for fishes and crustaceans. 

    The pesticide contamination of surface and ground water pose a serious threat to surrounding ecosystems. The organochlorines (DDT, methoxychlor, dieldrin, chlordane, toxaphene, mirex, kepone, lindane, and benzene hexachloride) and organophosphates (malathion, parathion, diazinon, fenthion, dichlorvos, chlorpyrifos, ethion) cause tumors, irritability and convulsions. They also cause serious environmental issues due to biomagnifications

The fate of pesticides is often uncertain, thus decontamination of pesticide polluted areas is very complex process. Low degree of biodegradability has made them as persistent toxic substances. To reduce or eliminate them from the environment, earlier techniques or technologies which were used were landfills, recycling, pyrolysis etc., but these can lead to formation of toxic intermediates and are expensive and difficult to execute especially in case of pesticides.

Bioremediation is a promising technology which utilizes the ability of microorganisms to remove pollution from the environment. It is an eco-friendly, economical and versatile approach.

 

Pesticide	Persistence (Half-life)	Health Effects
Aldrin	20 days to 1 year	Nervous system effects. Probable carcinogen. Large doses: convulsions, death.
Dichlorodiphenyltrichloroethane (DDT)	2 to15 years	Nervous system effects (tremors, seizures); probable carcinogen
Chlordane	4 years	Nervous system, digestive system, liver effects. Higher doses: convulsions and death.
Dieldrin	Up to 7 years	Nervous system effects. Probable carcinogen. Large doses: Convulsions, death.
Heptachlor	0.4 to 2 years	Nervous system damage, liver and adrenal gland damage, tremors

 Factors affecting biodegradation process

Pesticide pollution is a serious environmental problem and their remediation is necessary. Ideally treatment should result in destruction of the compounds without generation of intermediates. In some cases, intrinsic bioremediation occurs because of microbes that are already present in polluted ecosystems, but intrinsic bioremediation is not adequate.

Any factor which can alter growth or metabolism, would also affect biodegradation. Hence, physicochemical characteristics of the environmental matrix, such as temperature, pH, water potential, oxygen and substrate availability, would influence the biodegradation efficiency

The requirements for the process of bioremediation of pesticides are summarised as

Factor	Conditions required
Micro organisms	Aerobic or Anaerobic
Natural biological processes of micro organisms	Catabolism and Anabolism
Environmental factors	Oxygen content
Nutrients	Carbon, Nitrogen, oxygen etc.,
Soil moisture	25-28 % of water holding capacity
Type of soil	Low clay or slit content

 The other factors include co-metabolism and action of microbial consortia. Some biodegraders need other substrates to degrade pollutants. This phenomenon is called co-metabolism and is especially required for organochlorine compounds. In contrast, it has been shown that the presence of other carbon sources decreases organophosphate biodegradation. When pesticide degradation occurs, it usually involves more than one microorganism (microbial consortia), i.e. each microorganism contributes to biodegradation reactions on pesticides, but no single strain can carry out ineralization. Thus, the presence of different microorganisms is essential for an adequate biodegradation.

Fungi belonging to Basidiomycetes or bacteria like Pseudomonas, Aerobacter, Acinetobacter, Moraxella, Plesiomonas, Burkholderia, Neisseria, Sphingomonas, Micrococcus and Flavobacterium perform biodegradation effectively by reactions like de-chlorination, cleavage, oxidation, reduction involving different enzymes such as oxidoreductases, hydrolases, transferases and translocases.

Bioremediation could be by bio stimulating the indigenous biodegraders (bio stimulation) or adding exogenous to the site (bio augmentation).

         Bacteria are preferred for bioremediation due to their fast growth, easy handling and low cost. The complete biodegradation of the pesticide involves the oxidation of the parent compound resulting in to carbon dioxide and water, this provides energy to microbes. The soil where innate microbial population cannot be able to manage pesticides, the external addition of pesticide degrading micro flora is recommended. Degradation of pesticides by microbes not only depends on the enzyme system but also the conditions like temperature, pH and nutrients. Some of the pesticides are easily degraded however some are recalcitrant because of presence of anionic species in the compound. 

Although different enzymes participate in each condition, it seems that both, aerobic and anaerobic degradation are needed for  mineralization. Anaerobic metabolism is more adequate for dechlorination and aerobic metabolism produces a cleavage in aromatic or aliphatic cyclic metabolites. Organochlorine pesticides show higher persistance in aerobic conditions  compared to anaerobic conditions. The removal of heteroatoms (like halogens) or heteroatom-containing groups are among the first steps in biodegradation. These steps are catalyzed by dehalogenases under anaerobic conditions. Thus anaerbic conditions are more adequate for biodegradation of organochlorine pesticides, while aerobic are better for biodegrading hydrocarbon metabolites from pesticides.

Anaerobic biodegradation of dichlorodiphenyltrichloroethane (DDT) occurs by reductive dehalogenation- removal of only on chlorine atom from DDT. Pseudomonas, Aerobacter, Trichoderma, Neisseria, Moraxella and Acinetobacter can degrade  DDT . DDT is converted to TDE (2,2-bis(4-chlor-phenyl)-1,1 -dichloro ethane) and TDE is further degraded to DDE (1,1-dichlor-2,2, bis-(4,chloro phenyl) ethylene). Mineralization of DDT does not usually occur in the environement. 

Anabaena,  Pseudomonas aeruginosa and Burkholderia were shown to be good biodegraders of endosulfan. No mineralization of endosulfan has been observed. Microorganisms from the Pseudomonas, Bacillus, Trichoderma, Aerobacter, Mucor, Micrococcus and Burkholderia genera have been shown to biodegrade dieldrin and endrin. 

    Fungal enzymes especially, oxidoreductases, laccase and peroxidases play a key role in the biodegradation of  xenobiotics compounds. 

Species of fungi	Potential for degrading pesticide
Dichomitus spp,  Hypholoma spp,  Auricularia spp,  Pleurotus spp,	triazine, phenylurea, chlorinated organophosphorus compounds
White-rot fungi	Heptachlor,   atrazine, lindane, metalaxyl, chlordane, mirex,  dieldrin, diuron, aldrin, DDT

        Bioremediations has a tremendous potential for remediation of the soils that are affected by pesticides. Microorganisms that are present in the soils can remove pesticides from the environment. Biopesticide enzymatic degradation of polluted environment represents most important strategy for pollutant removal and degradation of persistent chemical substances that can surely solve the problem of pesticide pollution of soils.