ABSTRACT
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References.................................................................................................... 549
Natural microbial communities possess an immense metabolic diversity (see
Chapter 2). This metabolic potential is essential for global nutrient cycling,
facilitating the breakdown of a wide assortment of organic compounds in the
environment. Current thinking dictates that, eventually, all naturally occurring
organic compounds may ultimately be degradable by microorganisms under
favorable conditions. The argument is that, despite the existence of complex
organic compounds-such as lignin and phenolics-in the environment, these
compounds have not continuously accumulated throughout history. There are,
however, concerns with anthropogenic inputs into soil due to the large-scale
exploration of fossil fuels and the production of synthetic organic compounds,
such as pesticides. Such compounds can be released into the soil environment
(accidentally or purposely) from either a point source or a non-point source. In
particular, many of the synthetic compounds are xenobiotic, having novel
chemical structures that are recalcitrant to microbial degradation because
natural microorganisms have not had time to adapt their enzymes to react to
and degrade those compounds. Often these compounds are recalcitrant
because they are:
1. Halogenated, sulphonated or substituted with nitro groups
2. Too toxic or present at inhibitory concentrations
3. Unavailable to degradative microorganisms (due to low solubility
in water or by being tightly adsorbed to the soil matrix)
Because of the persistent nature of some of these pollutants, they can
accumulate in organisms at lower trophic levels and “biomagnify” up to
higher trophic levels of the food chain a process called biomagnification. This
problem was highlighted in the book Silent Spring by Rachel Carson [1] in the
1960s for DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane), a pesticide
that accumulated in large predatory animals and birds to levels four to six
orders of magnitude higher than what was introduced into the environment.
