Aerobic and Anaerobic Biodegradation Potentials of Microorganisms

Abstract

Microorganisms represent essential components of the global carbon cycle. In addition, it appears that most xenobiotic industrial chemicals can be degraded by microorganisms, either by a combination of cometabolic steps, often yielding partial degradation, or by serving as growth substrate which is accompanied by mineralization of at least part of the molecule. Using a number of examples, including aromatic, chloroaromatic, aliphatic, and chloroaliphatic compounds, I have presented some principles on the degradation. The great influence of some environmental conditions on the degradation, such as the presence or absence of oxygen, the availability of other electron acceptors such as nitrate or sulfate, has been discussed with special emphasis on the type of reactions and the rates of degradation that occur.

While aerobic microorganisms use oxidative reactions, the degradation by anaerobic bacteria takes place by reductive types of reactions. The oxidative sequences of aromatic and chloroaromatic compounds in aerobic bacteria yield central intermediates with a diphenolic structure. These compounds are then cleaved by enzymes that use molecular oxygen. In contrast, the anaerobes degrade aromatic compounds by reductive conversions and the central intermediates ready for hydrolytic ring cleavage bear a 1,3-dioxo structure.

Aerobic bacteria and fungi, especially ligninolytic ones, were shown to use mechanistically different catabolic pathways and enzymes. The ligninolytic fungi convert oxygen to hydrogen peroxide which is then used for the formation of an aryl cation radical undergoing spontaneous rearrangements and degradation.

The broad variety of mechanisms which brings about dechlorination is another important part of this work. Although the diversity of the compounds discussed is very large, the strategy of the organisms used in the degradation includes various analogous reactions.

Another important aspect of discussion is the different degree of degradation. While most research is done on organisms that are able to use the respective compound as the growth substrate, i.e., carbon dioxide and biomass result, the cometabolic potential of microorganisms should not be neglected. Cometabolism takes place very much in nature and brings about some modification of a target compound.

In general, anoxic microbial degradation seems to be of greater relevance in nature than earlier expected. It is remarkable that some chlorinated compounds such as chlorobenzoates, chlorophenols, or tetrachloroethene may function as a physiologically functional electron acceptor in a type of anaerobic respiration, which leads to non-chlorinated or lower chlorinated products.

Since various compounds will not be degraded totally by one type of organism the complementary potential of anaerobic and aerobic populations in combination is thought to be a method to bring about complete mineralization.

Finally, the possibility of enhancing the degradative potential of aerobic organisms in the laboratory, i.e., artificial evolution of enzymes and pathways, by different genetic approaches, is discussed.

Haloaliphatic and haloaromatic compounds as electron acceptors with dechlorination: dehalorespiration, Sequential anaerobic-aerobic processes, Enhancing the degradative potential by in vivo and in vitro techniques