The halocarbons, both halogenated compounds and halogenated solvents, are widespread pollutants found in air, water, soil, and sediment; they are recalcitrant molecules resistant to mineralization because the carbon-halogen bond is so stable. The stability and chemical inertness of many halogenated compounds is part of their appeal in many industrial processes, however it also makes them slow to degrade once they are released into the environment. In general, the more halogens there are on a molecule, the slower the degradation process. The focus for bioremediation of halocarbons is to exploit ability of organisms that have the natural capacity to mineralize these toxic contaminants into benign molecules
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Halogenated compounds have been used for a variety of purposes for hundreds of different industrial processes over the last 50 years. However they present a danger to human health and include such known toxins and potential carcinogens as dioxins, pesticides and PCBs. One prevalent example of a halogenated organic compound is the widely used pesticide DDT, which has been shown to bioaccumulate in animal fat tissue, disrupt hormone function, and damage ecosystems. PCB’s, polychlorinated biphenyls, are another type of halogenated organic compound that have a variety of industrial applications as coolants, lubricants, plasticizers, and dies. PCB’s are toxic chemicals and exposure has been shown to cause cancer in laboratory studies. PCB’s may also adversely effect human health by contributing to neurological, immune system, reproductive system, and other organ damage (see the EPA for more detail on PCB health effects ).
A major problem with halogenated compounds is that they belong to a class of molecules known as POP’s, persistant organic pollutants, which tend to biodegrade very slowly. It was originally thought that there were no natural sources of halogenated compounds in the environment, and hence no organisms that had evolved to exploit them. However, it has recently been shown that this is not the case (organisms as well as volcanic eruptions can produce these compounds,), and that natural production of chlorinated phenols may actually be greater than anthropogenic sources. Since these compounds have in fact existed for millions of years, there are naturally occurring strains of bacteria have evolved to break down halogenated compounds, thus opening up the possibility for bioremediation treatment of contaminated sites.