Anaerobic conditions are not uncommon in the environment and can develop in water or saturated sediment/soil environments. However, even in well-aerated solid phase systems, there are interfacial microenvironments with little or no oxygen. In all of these environments, anaerobiosis occurs when the rate of oxygen consumption by microorganisms is greater than the rate of oxygen diffusion through either air or water. In the absence of oxygen, organic compounds can be mineralized through anaerobic respiration, in which an electron acceptor other than oxygen is used. The series of alternative electron acceptors in the environment includes iron, nitrate, manganese, sulfate, and carbonate, which are listed in order from most oxidizing to most reducing. This progression means they are usually utilized in this order because the amount of energy generated for growth depends on the oxidation potential of the electron acceptor. Since none of these electron acceptors are as oxidizing as oxygen, growth under anaerobic conditions is never as efficient as growth under aerobic conditions (Fig. 22). As shown in Fig. 22, aerobic conditions refer to specific potential facultative and obligate anaerobes which metabolize over a spectrum of redox potentials.
Anaerobic degradation pathways have not been as extensively studied as aerobic degradation of organic compounds.Interestingly,many compounds that
are easily degraded aerobically, such as saturated hydrocarbons, are far more difficult to degrade anaerobically. However, in at least one group of compounds, those that are highly halogenated, the halogen substituents are removed more rapidly under anaerobic conditions. However, once dehalogenation has occurred, the remaining molecule behaves more typically; that is, it is generally degraded more rapidly and extensively under aerobic conditions. As a consequence of this sequential process, bioremediation technologies have been developed that utilize sequential anaerobic-aerobic treatments to optimize degradation of highly halogenated compounds.
Continue reading here: Aliphatic Hydrocarbons
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