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When low to moderate concentrations of oil are thoroughly dispersed within aerobic water, it is unlikely that a lack of oxygen is a limiting factor. However, in cases where pools of oil accumulate in soils and sediments, in regions of intense decomposition and in some groundwater, the development of anoxic conditions can severely limit the rate of biodegradation. MINERAL NUTRIENTS Petroleum is rich in carbon and energy, but deficient in those mineral nutrients ( nitrogen, phosphorus and iron) necessary for microbial growth. Consequently, petroleum degrading microorganisms must obtain their essential mineral nutrients ' elsewhere . Those projects in which macro- and micro-nutrients are carefully monitored have shown improved degradation rates. Nutrients are often supplied in the form of compost or commercial fertilizer. ADDITIONAL FACTORS In addition to the above, two processes are carefully considered in the biodegradation (microbial metabolism) of petroleum hydrocarbons . These are sparing and co-oxidation. Both processes can, and probably most frequently do, occur within a petroleum spill . For example, acetate, an intermediate product in hydrocarbon biodegradation, has been found to reduce the utilization of hexadecane. The basis for this sparing effect is not well defined, but it certainly occurs for other hydro- carbons . Such phenomena do not alter the metabolic pathways of degradation, but determine whether the enzymes necessary for metabolic attack on a particular hydrocarbon are active, or even produced. These sparing effects have a marked influence on the persistence of particular hydrocarbons within a petroleum mixture and so significantly impact the degradation process . Co-oxidation is a process in which compounds which otherwise would not be degraded can be attacked by enzymes due to the abilities of the individual microorganisms producing those enzymes to utilize other hydrocarbons within the petroleum mixture . The complexities of petroleum compounds provide an excellent chemical environment in which co-oxidation can occur. Many complex branched and cyclic hydrocarbons are degraded as a result of the co-oxidation process . It has been found, for instance, that the degradation of hydrocarbons within a high-octane gasoline was not in agreement with the degradation of individual hydrocarbons comprising the mixture. III .