Earth’s surface became contaminated with natural and xenobiotic toxic chemicals. Examples include polluted aquifers and other water bodies, mining areas, and industrial sites contaminated with petrochemical products, pesticides, heavy metals, radionuclides and many of the xenobiotics
The basic principle of bioremediation involves utilizing the activity of microorganisms naturally present in the soil and water, or selected organisms inoculated into the environment, to biodegrade or detoxify contaminating compounds in situ.
In the majority of cases a microorganisms will be involved in the biodegradation of the contaminant, rather than a single species.
To optimize the process, the growth of indigenous microorganisms is necessary. It can be achieved by the addition of key nutrients such as nitrogen and phosphorus, which are normally present in growth-limiting concentrations.
This enables the natural microbial flora to develop and metabolize the contaminant. Alternatively, known biodegraders of the contaminant that have been identified, isolated and their activities optimized can be used as an inoculant.
For example, a recent addition to the growing list of microorganisms able to sequester or reduce metals is Geobacter metallireducens.
This bacterium can remove uranium, a radioactive waste, from drainage waters in mining operations and from contaminated groundwaters.
However, the most radiation-resistant bacterium known is Deinococcus radiodurans; this organism is also being developed to help clean up soil and water contaminated by solvents, heavy metals and radioactive waste.
A genetically engineered strain of D. radiodurans has been produced which can detoxify mercury (genes derived from Escherichia coli) and degrade toluene (genes derived from Pseudomonas putida) in radioactive environments.
The use of genetically engineered microorganisms has been investigated in other areas of bioremediation.
However, legislative problems, public concern regarding the release of genetically modified organisms into the environment and possible genetic instability have restricted their application.
Nevertheless, due to comparatively low cost and generally positive environmental impact, bioremediation offers an attractive alternative or adjunct to conventional clean-up technologies.
Its implementation has been successful at many sites, particularly those contaminated with petroleum products.
However, it is not always the technology of choice, as accurate predictions of degradation rate are rarely possible.