Introduction heavy metal pollution

Bioremediation of Pesticides
Bioremediation is the use of microorganisms for degradation of hazardous chemicals in soil, sediments, water, or other contaminated materials. Often the microorganisms metabolize the chemicals to produce carbon dioxide or methane, water and biomass. Alternatively, the contaminants may be enzymatically transformed to metabolites that are less toxic or environmentally innocuous. It should be noted that in some instances, the metabolites formed are more toxic than the parent compound. For example, perchloroethylene and trichloroethylene may degrade to vinyl chloride which is highly toxic in nature than parent compound (Sacki and Toyota, 2004).
There are a number of possible pesticide degradation pathways in the soil and water environment including chemical treatment, volatilization, photodecomposition and incineration. However, most of them are not applicable for the diffused contamination with low concentration because of being expensive, less efficient and environmental friendly. Thus, keeping in view the environmental concerns associated with pesticides/recalcitrant compounds, there is a need to develop safe, convenient and economically viable methods for its remediation. In this context, several researchers have focused their attention to study the microbial biodegradation which has been reported as a primary mechanism of pesticide dissipation from the environment (Cox et al. 1996, Pieuchot et al. 1996). Although bioremediation strategies are more acceptable to the society because of their reduced impact on the natural ecosystem (Zhang and Quiao, 2002). However, complexity of the mechanisms responsible for pesticide degradation has made it slow to emerge as an economically viable remediation method (Nerud et al. 2003). It is noteworthy that bioremediation strategies have been developed extensively for taking care of sites heavily contaminated with organic pollutants, however, up to now, the sites diffusely contaminated are only monitored and natural attenuation is the process of interest leading to contaminant abatement.

Microbial biodegradation occurs mostly in the soil solution. Pesticide microbial biodegradation is carried out by soil microorganisms like bacteria fungi and actinomycetes possessing a large set of enzymes susceptible to transform these pesticides. It is the principal mechanism for diminishing the persistence of pesticides in soil environment (Arbeli and Fuentes, 2007). Soil serves as a potential habitat for variety of microorganisms which have the ability to interact not only with other living components but also the physical elements including pesticides for the fulfilment of their energy requirement. When pesticides are applied in the soils, enzyme-driven biochemical reactions carried out by the indigenous soil microorganisms result in modification of the structure and toxicological properties of pesticides leading to their complete conversion into harmless inorganic end products (Hussain et al. 2009a). Pesticides degradation by soil microbial communities has been reported by several researchers (Fenlon et al. 2007; Hussain et al. 2007a; Shi and Bending, 2007; Hussain et al. 2009b; Sun et al. 2009) and it has been described as a primary mechanism of pesticide dissipation from the environment (Fournier et al. 1975; Cox et al. 1996; Pieuchot et al. 1996). The efficiency of pesticide biodegradation varies considerably between different groups of the microorganisms and even between the different members belonging to the same group of microorganisms. Although a strong diversity of microbial species is found in the soil, however, the adaptability of these different degrading microbial species in the contaminated soils assures the continuity of biodegradation process. Microbial biodegradation of pesticides in the soil can be categorized into two principal types based on the mode and pathway of degradation i.e. metabolic and co-metabolic.