Hydrocarbons are catabolized or broken down into simpler substances by microorganisms using aerobic respiration, anaerobic respiration, and fermentation. In general, aerobic degradation processes are more often used for biodegradation because the degradation process is more rapid and more complete, and problematic products such as methane and hydrogen sulfide are not produced. However, anaerobic degradation is important for dehalogenation.

In aerobic respiration, organic molecules are oxidized to carbon dioxide (CO2), water, and other end products using molecular oxygen as the terminal electron acceptor. Oxygen is also incorporated into the intermediate products of microbial catabolism through oxidase enzyme action, making these products more susceptible to further biodegradation. Microorganisms metabolize hydrocarbons by anaerobic respiration in the absence of molecular oxygen using inorganic substrates as terminal electron acceptors. Naturally occurring aerobic bacteria can decompose natural and synthetic organic materials to harmless or stable forms by mineralizing them to CO2 and water. Some anthropogenic compounds appear refractory to biodegradation by naturally occurring microbial populations because of environmental influences, lack of solubility, and the absence of required enzymes, nutrients or other factors. However, properly selected or engineered micro-

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