Those aspects of microbiology in which crude oil, refined petroleum products, or pure hydrocarbons serve as nutrients for the growth of microorganisms or are altered as a result of their activities. Applications of petroleum microbiology include oil pollution control, enhanced oil recovery, microbial contamination of petroleum fuels and oil emulsions, and conversion of petroleum hydrocarbons into microbial products.

Many species of bacteria, fungi, and algae have the enzymatic capability to use petroleum hydrocarbons as food. Biodegradation of petroleum requires an appropriate mixture of microorganisms, contact with oxygen gas, and large quantities of utilizable nitrogen and phosphorus compounds and smaller amounts of other elements essential for the growth of all microorganisms. Part of the hydrocarbons are converted into carbon dioxide and water and part into cellular materials, such as proteins and nucleic acids. The requirement for a mixture of different microorganisms arises from the fact that petroleum is composed of a wide variety of different groups of hydrocarbons, whereas any specific microorganism is highly specialized with regard to the type of hydrocarbon it can digest. The bacterial genera that contain the most frequently isolated hydrocarbon degraders are Pseudomonas, Acinetobacter, Flavobacterium, Brevibacterium, Corynebacterium, Arthrobacter, Mycobacterium, and Nocardia. The fungal genera that contain oil utilizers include Candida, Cladosporium, Rhodotorula, Torulopsis, and Trichosporium. Biodegradation

Oil pollution results from natural hydrocarbon seeps, accidental spills, and intentional discharge of oily materials into the environment. Once the oil is released and comes into contact with water, air, and the necessary salts, microorganisms present in the environment begin the natural process of petroleum biodegradation. If this process did not occur, the world's oceans would soon become completely covered with a layer of oil. The reason that oil spills become a pollution problem is that the natural microbial systems for degrading the oil become temporarily overwhelmed. Water pollution

The largest potential application of petroleum microbiology is in the field of enhanced oil recovery. Microbial products, as well as viable microorganisms, have been used as stimulation agents to enhance oil recovery from petroleum reservoirs. Xanthan, a polysaccharide produced by Xanthomonas campestris, is used as a waterflood thickening agent in oil recovery. Emulsan, a lipopolysaccharide produced by a strain of Acinetobacter calcoaceticus, stabilizes oil-in-water emulsions. A number of other microbial products are being tested for potential application in enhanced oil recovery processes. Field tests have indicated that injection of viable microorganisms with their nutrients into petroleum reservoirs can lead to enhanced oil recovery, presumably due to production of carbon dioxide gas, acids, and surfactants.

A variety of valuable materials, such as amino acids, carbohydrates, nucleotides, vitamins, enzymes, antibiotics, citric acid, long-chain dicarboxylic acids, and biomass, can be produced by microbial processes using petroleum hydrocarbons as substrates. The main advantage of using hydrocarbons as substrates is their lower cost. Also, certain products, such as tetradecane-1,14-dicarboxylic acid, a raw material for preparing perfumes, are synthesized in higher yields on hydrocarbon than on carbohydrate substrates.

The most active area of research and development in petroleum microbiology since the mid-1960s has been in the large-scale production and concentration of microorganisms for animal feed and human food. Dried microbial cells are collectively referred to as single-cell protein. In spite of its advantages, single-cell protein has not yet played a significant role in providing protein for animal feed or human consumption. However, many scientists are optimistic about its potential. The ability of microorganisms to utilize petroleum also has its detrimental aspects, particularly with respect to the deterioration of petroleum fuels, asphalt coatings, and oil emulsions used with cutting machinery. All hydrocarbons become contaminated if they come into contact with water during storage. Bacterial physiology and metabolism