Xenobiotics
Xenobiotics are man-made chemical compounds that are very difficult to degrade. These compounds are made by synthetic organic chemicals and are stranger to the biosphere. They accumulate in the environment and cause harmful effects on the living system.

Xenobiotics include chemically synthesized compounds such as pesticides, polyethylene, polystyrene and PVC. Some compounds are recalcitrance i.e., not easily biodegradable due to the extensive branching of the molecule or introduction of halogen, nitro or sulphonyl group.

DDT and halogenated aliphatic and aromatic compounds are most important xenobiotics because they have high potential to damage the environment. Although, many such compounds are highly beneficial to the humans but their long term presence in the environment cause deleterious effects.

Recalcitrant toxic pesticides such as BHC, PCBs, and DDT are not easily biodegraded and their concentration goes on increasing in the soil and water with time. For example, there is a continuous increase in the concentration of DDT at successive trophic levels in food chain. Model compounds of DDT such as diphenymethane and dichlorodiphenylmethane are biodegrade by various microbes. Pseudomonas putida converts DDT to several compounds under anaerobic conditions. In aerobic conditions, ring cleavage yield p-chlorophenylacetic acid that in turn served as a substrate for an arthrobactor species.

Biodegradation
The term means transformation of a chemical compound from highly complicated form (organic) to simple (inorganic) form through biological means. If we say a compound is biodegradable, it means that it can be converted into various inorganic forms or can be mineralized i.e., possible to convert into carbon dioxide and water.

Basis of biodegradation
Microbes play the most important role in the process of biodegradation. Certain abiotic mechanisms and photo-oxidation also play an important role in the degradation of certain organic chemicals but such transformations are generally incomplete because these processes cannot convert the compound into inorganic form

Before the industrialization, biosphere of earth remained constant because of more or less balanced biosynthesis and biodegradation reactions. During the course of evolution, a vast variety of chemical compounds are biosynthesized in nature and microbes were exposed to these compounds. For this millions of year's exposure, they have developed the capacity and mechanism to attack these compounds. Out of several chemical compounds synthesized by chemists, many have structural features and bonding similar to that of natural compounds, so can be biodegraded

Biodegradation of aromatic compounds
Sources of aromatic compounds in the environment include degradation of lignin in plants, use of detergents, pesticides, drugs and dyes etc. Several polycyclic aromatic hydrocarbons (PAH) released from industrial processes are carcinogenic. Variety of microbes including bacteria, fungi, yeast can act on these compounds and degrade them. Benzene, toluene, xylene, ethylbenzene are degraded by bacteria. Presence of methyl, chloro, nitro, amino and sulphonyl group in benzene ring cause recalcitrancy of the compound. Catechol and protocatechuate are the intermediates in the catabolism of benzene, phenol, vanillin, shikimate.

Both are degraded by two different mechanisms:-
1) Ortho cleavage (intradiol fission):- catechol is cleaved between two phenolic hydroxyl groups generating cis-cis muconic acid. This muconic acid is converted into oxo-adipic acid, which is finally degraded into acetyl CoA and succinate, which then enters into TCA cycle.
2) Meta fission (extradiol cleavage):- catechol is cleaved to 2-hydroxy-muconic semialdehyde which is converted to acetaldehyde and pyruvate via 4-hydroxy-2-oxo valerate.

Biodegradation of pesticides
In soil, herbicides and pesticides are degraded at different rates. As example,
1) Diquat and paraquat are photolysed to α-picolinate and N-methyl-isonicotinate, respectively, which are then degraded by microbes. Binding of herbicides to soil particles retard their biodegradation.
2) Parathion hydrolase from Pseudomonas diminuta is encoded by plasmids, which hydrolyse parathion to p-nitrophenol, the latter is then degraded by other microbes.

Biodegradation of chloroaromatic compounds
Chloroaromatic compounds are the pollutants of major concern and are toxic, resist to biodegradation. Chlorobenzene, dichlorobenzene and trichlorobenzene are not easily biodegraded in biological treatment system. However, they are degraded by some soil microorganisms and also co-metabolically by strains of Pseudomonas putida.

Chlorophenols and chlorocatechol are the intermediate compounds in the biodegradation of chlorobenzenes, various pesticides and other chloroaromatic compounds.

2, 4-dichlorophenol is readily degraded by many pseudomonas, achromobactor and arthrobactor. Polychlorinated phenols, especially pentachlorophenol (PCP) used in wood preservation, as fungicide and herbicide. PCP inhibits anaerobic digestion of sludge. Strains of pseudomonas, arthrobactor, flavobacterium use PCP as sol.

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