Merlins Micro World: Xenobiotics- Novel pollutants

· Foreign compounds introduced into the biosphere
· Manmade
· differ in their structure from natural organic compounds

Common examples include halo and nitro substituted organic compounds, (used as solvents, propellants, refrigerants, plastics, detergents, explosives, pestciides, etc

· Xenobiotics are usually resistant to biodegradation
· If completely resistant to biodegradation, termed as recalcitrant compounds
· Some of these are degraded in a co-metabolic mode or give rise to bound & polymeric residues in the soil-these are of environmental concern
· Some of the more recalcitrant xenobiotics are subject to biomagnification
· In the food web that begin with microorganisms, they accumulate in the higher life forms, and cause serious damage to the top level carnivores

Microbial infallibility

Given favourable environmental conditions, all natural organic compounds degrade, o else, large scale accumulations would be present in the biosphere

· This concept of microbial infallibility, introduced by Alexander (1965), explains the biodegradative capacity of microorganisms. It says, if environmental conditions are favourable, no natural organic compound is totally resistant to biodegradation

· Evolution of various natural biopolymers have been slow and gradual. There was enough time for parallel evolution of microbial catabolism to degrade these substrates. However, developments in synthetic chemistry and large scale production led to a variety of xenobiotic compounds that end up in the environment

· Once in the environment, they have different fates. Most synthetic compounds which are similar to natural compounds are degraded by the microorganisms. Xenobiotics with molecular structures and chemical bonds not recognized by the existing microbial enzymes, either resist degradation or are metabolized incompletely

· This results in the accumulation of xenobiotics in the environment. Microbial infallibility is thus, defeated by human interventions

Recalcitrance

Recalcitrance is the resistance to biodegradation. It is the structure- induced stability of a xenobiotic molecule-mainly due to 'unusual' chemical bonds and/or substituents, which block the attack by microbial catabolic enzymes. Type, number and position of bonds and substituents affect the xenobiotic character.

Typical features of recalcitrant organic compounds.

• High molecular mass

• Low solubility in water

• Condensed polycyclic structures such as benzene and pyridine rings

• Presence of substituted halogen and nitro structures

• artificial/synthetic bonds and substituents.

Because of these properties, recalcitrant compounds may fail to induce degradative microbial enzymes or the microorganisms may find it difficult to access the xenobiotics due to insolubility. Some such compounds or their metabolic products may be toxic to the microorganism. All these contribute to the resistance to biodegradation.

The extent of biodegradation and the rate at which it occurs depend on the chemical structure and concentration of the compound being degraded, the type and number of microorganisms present, and the physicochemical properties of the environment.

'Biodegradation' involves the breakdown of organic compounds, usually by microorganisms, into biomass and water, carbon dioxide, and minerals.

The complete breakdown of an organic compound into inorganic components is termed 'mineralization'.

Degradation of an organic compound to a less complex organic compound is referred to as 'incomplete (partial) biodegradation'.

'Biotransformation' is the metabolic modification of a compound, resulting in the loss or alteration of some characteristic properties of the original compound, (with no/minor loss of molecular complexity). Biotransformation may affect the solubility, mobility in the environment, or toxicity of the organic compound.

A microbial population growing on one compound may accidentally transform a contaminating chemical that it cannot use as carbon and energy source. This is 'co-metabolism'. The organisms do not benefit from the co-metabolic process. Co-metabolic transformation may result in a minor modification of the molecule, or incomplete or even complete degradation.

The products of partial biodegradation, or biotransformation, or co-metabolic conversion of a xenobiotic may be less harmful as the original compound, or they may be as hazardous or even more hazardous as the original compound.

In natural environments, the products of bioconversion processes may be further transformed or degraded by other microorganisms, eventually leading to complete degradation by the microbial consortium.

Co-metabolic processes, and biodegradation by microbial consortia are thus of enormous ecological importance.

Persistent xenobiotics and metabolic dead-end products can accumulate in the environment as part of the soil humus, or enter the food chain leading to biomagnification. Adsorption to soil and sediments as well as entrapment in the pores of the soil are major causes for their persistence.