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low to moderate concentrations of oil are thoroughly dispersed in 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 (bio)degradation Mineral Nutrients Petroleum is rich in carbon and energy , but deficient in those mineral nutrients necessary for microbial growth (primarily nitrogen, phosphorus , iron, and potassium) Consequently, petroleum degrading microorganisms must obtain these essential materials elsewhere Those pro3ects in which macro- and micro-nutrients are carefully monitored and supplied appropriately have shown improved degradation rates Nutrients are often supplied as a component of organic matter ( such as compost ) or as custom formulations of commercial fertilizers Additional Factors In addition to the above, two interesting processes can, and probably most often do, occur in the degradation of petroleum hydrocarbons These are sparing and co- oxidation . 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 hydrocarbons , as well Co-oxidation is a process in which compounds which otherwise would not be degraded can be attached by enzymes due to the abilities of the individual microorganisms producing the 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, and many complex branched and cyclic hydrocarbons are degraded as a result It has been found, for instance , that the degradation of hydrocarbons within a gasoline mixture occurred at a rate different from the degradation of individual compounds comprising the mixture Phenomena such as these 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 A number of aerobic soil biodegradation protocols have been found to offer excellent degradation potentials for petroleum contaminated soils These are III .