Introduction heavy metal pollution

Co-metabolic degradation
The co-metabolic degradation corresponds to the non specific degradation of xenobiotic molecule by microorganisms. In most of the cases, this is a non-inducible phenomenon occurring because of the presence of detoxifying enzymes able to degrade xenobiotics depicting homologies with their substrate. In this case, the target pesticides do not contribute to the growth of the degrading organisms (Dalton and Stirling, 1982; Novick and Alexander, 1985). For this reason, the degradation rate of pesticide in a given environment depends primarily on the size of microbial biomass and on the competitiveness of the degrading microbial population towards sources of energy and nutrients in the soil. In other words, pesticide degradation rate is dependent on size of the biomass (Fournier et al. 1996). In general, co-metabolism does not yield in extensive degradation of the molecule but rather causes incomplete transformation such as oxidation, hydroxylation, reduction, N-dealkylation or hydrolysis (Fournier et al. 1996) which may lead to the formation of metabolites that may prove even more toxic and recalcitrant than the parent compound (De Schrijver and De Mot, 1999).

Some compounds can only be partially metabolized by microbial populations and transformed into metabolites that may either accumulate in the environment or be metabolized further by other microbial species. These metabolic reactions do not provide benefit to the responsible organism because they do not gain either carbon or energy. These processes are typically fortuitous and occur because the responsible population produces one or more enzymes that are comparatively nonspecific and can react with structural analogues compounds of the “normal” substrate for enzyme(s). Co-metabolism is important for the degradation of many environmental contaminants particularly chlorinated pesticides solvents (e.g. trichloroethylene,), polychlorinated biphenyls, and many polyaromatic hydrocarbons (Fournier et al. 1996). Giri and Rai (2012), studied Biodegradation of endosulfan isomers in broth culture and soil microcosm by Pseudomonas fluorescens. After 15 days incubation, maximum 92.80% α and 79.35% β endosulfan isomers were degraded in shake flask culture at 20 mg/L concentration, followed by 50 and 100 mg/L, while the corresponding values in static condition were 69.15 and 51.39%, respectively.