Bioremediation

Accidental spills of petroleum or other hydrocarbons on land and water are regrettable but frequent occurrences. Such spills can range in size from a few gallons that may be spilled during refueling to enormous spillages of millions of tons as occurred to both the sea and land during the Gulf War of 1991. Once spilled, petroleum and its various refined products can be persistent environmental contaminants. However, these organic chemicals can also be metabolized by certain microorganisms, whose processes transform the toxins into simpler compounds, ultimately to carbon dioxide, water, and other inorganic chemicals.

Numerous attempts have been made to increase the rates by which microorganisms break down spilled hydrocarbons. In some cases, specially prepared concentrates of bacteria that are highly efficient at metabolizing hydrocarbons have been "seeded" into spill areas in an attempt to increase the rate of degradation of the spill residues. Although this technique has sometimes been effective, it commonly is not. This is because the indigenous microbial communities of soils and aquatic sediments contain many species of bacteria and fungi that are capable of utilizing hydrocarbons as a metabolic substrate. After a spill, the occurrence of large concentrations of hydrocarbons in soil or sediment stimulates rapid growth of those microorganisms. Consequently, seeding of microorganisms that are metabolically specific to hydrocarbons does not always make much of a difference to the overall rate of degradation.

More important, however, is the fact that the environmental conditions under which spill residues occur are almost always highly sub-optimal for their degradation by microorganisms. Most commonly, the rate of microbial breakdown of spilled hydrocarbons is limited by the availability of oxygen or of certain nutrients such as nitrate and phosphate. Therefore, the microbial breakdown of spilled hydrocarbons on land can be greatly enhanced by occasionally tilling the soil to keep conditions aerated and by fertilizing with nitrogen and phosphorus while keeping conditions moist but not wet. Thus, bioremediation systems for dealing with soils contaminated by spilled gasoline or petroleum can be based on simple tillage and fertilization.

Similarly, petroleum refineries may utilize a bioremediation process called land farming, in which oily wastes are spread onto land, which is then tilled and fertilized until microbes reduce the residue concentrations to an acceptable level.

After some petroleum spills, more innovative approaches may prove to be useful. For example, it is difficult to fertilize aquatic habitats, because the nutrients simply wash away and are therefore not effective for very long. In the case of the Exxon Valdez spill in Alaska in 1989, research demonstrated that nutrients could be applied to soiled beaches as an oleophilic (that is, oilseeking), nitrogen and phosphorus-containing fertilizer. Because of its oleophilic nature, the fertilizer adhered to the petroleum residues and was able to significantly enhance the rate of oil degradation by the naturally occurring community of microorganisms. This treatment was applied to about 73 mi (118 km) of oiled beach and proved to be successful in speeding up the process of degradation of the residues by increasing the rate of oxidation by about 50%. No attempts were made in this case to "seed" the microbial community with species specifically adapted to metabolizing hydrocarbons. It was believed that hydrocarbon-specific microbes were naturally present in the beach sediment and that their activity and that of species with broader substrate tolerances only had to be enhanced by making the ecological conditions more favorable, that is, by fertilizing.