three major oxidative mechanisms to consider: photo-oxidation, auto-oxida- 

 tion, and microbial oxidation. Betancourt and McLean (1973) examined the 

 oxidative degradation of the No. 6 fuel oil from the Awow spill, noting that 

 the original cargo from the Awow contained 2.28% sulfur and that after 20 

 months of onshore weathering the sulfur content had dropped to 1.45%. Since 

 the sulfur-containing components of oil are generally not considered to be 

 volatile, the conclusion one arrives at is that sulfur-containing compounds 

 are preferentially oxidized during weathering. 



Many investigators have contributed to our knowledge of microbial degra- 

 dation of oil, such phenomena having been the subject of several investiga- 

 tions since the 1920' s. Numerous organisms have been identified that can use 

 hydrocarbons as both an energy and as a carbon source, but the process of 

 biodegradation of oil is extremely complex. It may seem surprising at first 

 to read of the variety of hydrocarbons that can serve as "food" for micro- 

 organisms, from methane to asphalt, but were it not for the adaptability of 

 these organisms oil from natural seepages might well have covered the earth's 

 surface to a depth of several centimeters by now. 



Microorganisms are able to convert hydrocarbons into more soluble alco- 

 hols, organic acids, and into CO2 and water via enzyme-catalyzed reactions. 

 For aerobic metabolism of a simple hydrocarbon, the oxidation reaction is 

 represented by the reaction: 



Cn'H2n + (3/2)n02 ^ nC02 + nH20. 



If the organism consumed all the available hydrocarbon for energy purposes, 

 the ratio of CO2 produced to O2 consumed should equal 0.67. This ratio, 

 called the respiratory quotient, was examined in 1942 by Stone et al. for 

 mixed soil bacteria. They found values of the respiratory quotient as low as 

 0.12 for heavy fractions of crude oil, and as high as 0.64 for refined motor 

 oil. Also in 1942, Johnson et al. measured respiratory quotients for Bac - 

 terium alphaticum of 0.47 for heptane, 0.48 for octane, and 0.63 for nonane 

 and dodecane. Anaerobic oxidation of hydrocarbons can occur with either 

 nitrate or sulfate taking the place of oxygen as the electron donor. Anaero- 

 bic oxidation of hydrocarbons is usually incomplete, leaving the hydrocarbon 

 in some partly oxidized state rather than as CO2 and water. 



The biological conversion of a pure hydrocarbon to a more water-soluble 

 alcohol or acid assists in the removal of petroleum hydrocarbons from the sea 

 surface. Since most of the hydrocarbon-utilizing organisms are not lipoidal, 

 the microbial oxidation of an oil slick is restricted to the aqueous side of 

 the water/oil interface. The increased solubility of the partly oxidized 

 intermediates by this process allows for transfer of some of the oil away 

 from the interfacial region. Beerstecher (1954) , based on measurements at 

 room temperature, reported rates of crude oil oxidation of 1.2 to 1.5 g 

 m~^d~^, and for refined oil of about 0.5 g m~2d~l . 



There have been a number of investigations into the effect of the 

 molecular properties of the substrate on the rate of microbial oxidation of 

 petroleum hydrocarbons. In general, it can be said that the longer-chain 



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