In order to grow, the microorganisms, in addition to an organic carbon that serve as a source of carbon and energy, require inorganic nitrogen, phosphorus, and an electron acceptor. The electron acceptor is O2 for aerobes, but it may be nitrate, sulfate, CO2, ferric iron, or organic compounds for specific bacteria. Many microorganisms also require low concentrations of growth factors, for example, amino acids, vitamins, or other molecules. Absence of any of these essential nutrients from an environment prevents growth of microorganisms adversely affecting the biodegradation of toxic chemicals.
Soils, sediments, and natural waters may contain sufficient organic carbon, but most of the carbon exists in complex form and only a small portion is available to support microbial growth. Under these circumstances, it is likely that availability of carbon may become a limiting factor for sustained microbial growth, even if the other nutrients are present in abundant quantities. However, if an easily degradable toxic chemical is introduced in such an environment at high concentrations, then after some time supply of other nutrients may also be exhausted and they become a limiting factor in microbial growth. For example, after oil spill, the environment surrounding oil becomes high in carbon concentration, resulting in high microbial growth. This may lead to depletion of inorganic nitrogen and phosphorus.
Addition of nitrogen and phosphorus to groundwater contaminated with gasoline stimulates growth and degradation of gasoline by bacteria. Similar approaches have shown positive results in seawater after crude oil spill where enhanced degradation of oil was observed after addition of inorganic nitrogen and phosphorus fertilizers. In some environments, adding just nitrogen or phosphorus but not both enhances degradation. However, there are cases when microbial growth is simultaneously limited by both nutrients.
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