The advantages and benefits of aerobic cometabolism
Aerobic metabolism yields the energy-transferring molecule TPA at a much higher rate than anaerobic metabolism. The higher energy yield produces a greater viable population. The oxygenase enzymes involved in the cometabolism of chlorinated solvents are not induced by the presence of contaminants, so a high microbial population has a proportionately high yield of oxygenase enzymes. Aerobic cometabolism of chlorinated solvents converts the contaminant to an organic acid and ultimately complete mineralization1. Vinyl chloride is not generated as a byproduct. Overall aerobic cometabolism is a fast and complete remediation strategy for chlorinated solvents.
Aerobic cometabolism of chlorinated solvent is not as widely recognized as anaerobic reductive dechlorination. While bioremediation is most effective and sustainable when the method is compatible with the existing site conditions, aerobic cometabolism has unique benefits derived from the advantages of aerobic respiration. These advantages and benefits can reduce costs and risks associated with soil and ground water remediation.
The basic advantage of aerobic cometabolism is the total amount of energy generated. During metabolism adeosine triphosphate (ATP) is generated to release free energy that will be used for cellular reactions. For example from one molecule of glucose, aerobic respiration produces 36 molecules of ATP and anaerobic respiration generates only two.
The difference in the amount of energy generated from metabolizing a molecule of glucose, for example, can be attributed to the individual steps of aerobic and anaerobic respiration. Regardless whether an organism is aerobic or anaerobic, metabolism begins with glycolisis. Gylcolysis is defined as the splitting of a glucose molecule into two molecules of pyruvate. During this process, electron carriers and a total of two ATP are produced.
In aerobic respiration, pyruvate is processed by the Krebs cycle. This cycle generates several by-products including carbon dioxide, electron carriers, ATP and acetyl coenzyme A. The electron carriers from glycolysis and the Krebs cycle are used in the electron transport chain, which will result in a net total of 36 molecules of ATP.
After glycolysis, anaerobic organisms utilize fermentation for metabolism. The electron carriers from glycolysis are oxidized and reused in glycolysis. While some aerobic organisms may utilize fermentation if an oxygen source is depleted, anaerobic organisms are generally fastidious and can only metabolize nutrients without the presence of oxygen.
The energy produced by metabolism and stored in ATP is used to fuel all cellular reactions. In bioremediation, energy is not only used to breakdown contaminants, but the energy is also used to increase the number of cells that can be involved in the remediation.
For aerobic organisms the cell growth rate is proportional to the carbon material metabolized. In general, aerobic organisms convert 20% to 50% of the carbon in a sugar to cellular carbon and the rest is converted to CO2. Pseudomanads, convert about 50% of the metabolized carbon to cellular carbon. The conversion rate is proportional to the amount of ATP generated by the metabolic pathways available to the cell2.
Among anaerobic organisms, the carbon in the fermentation substrate is not converted to cellular carbon2. The fermentable material serves only as a source of energy. The cellular carbon is derived from other sources, and the energy derived from fermentation fuels the reaction. Thus the cellular growth is proportional to the ATP yield from fermentation.
As the ATP yield from fermentation is lower in anaerobic organisms than in aerobic, the population yield is less. Aerobic metabolism yields a higher cellular population that is involved in the bioremediation. Thus, cleanup goals are achieved much faster by aerobic bioremediation at contaminated sites.
Oxygenase Enzymes Produced
Constitutive enzymes are those that have continuous gene expression as the production of the enzymes is not initiated by an environmental factor such as the presence of a toxin. Some of the oxygenase enzymes involved in the aerobic cometabolism of chlorinated solvents, such as toluene-o-xylene monooxygenase (ToMO), are constitutive enzymes3. Since the enzymes are not induced by contaminants they can be useful in the metabolism of a wide range of organic chemicals. They have been found to be capable of simultaneously degrading mixtures of tetrachloroethylene, trichloroethylene, dichloroethylenes, vinyl chloride, chloroaromatic compounds and chloroethanes4,5.
The enzymes degrade the chlorinated solvents by forming an unstable epoxide6. The chlorinated epoxy ethanes are completely mineralized. The end products of this degradation route are CO2 and water. Vinyl chloride is not formed as an intermediate, thus reducing the potential increased toxicological risk associated with vinyl chloride.
Furthermore, since the enzymes do not require the presence of a contaminant for their growth, the organisms with this capability may be grown in a bioreactor without the presence of the contaminant or an analogous compound under strict QA/QC to produce high yields for bioaugmentation.
Aerobic cometabolism has advantages over anaerobic reductive dehalogenation for the bioremediation of chlorinated solvents. Fundamentally aerobic organisms can grow higher populations, which increases the opportunity for cell to contaminant molecule contact and produces high concentrations of the reactive enzymes involved. Aerobic cometabolism of chlorinated solvents completely mineralizes the contaminant. Under the right conditions, aerobic cometabolism is a fast and complete remediation strategy for chlorinated solvents.
1. Nelson, M. J., Montgomery, S. O., Mahaffey, W. R., and Pritchard, P. H. 1987. Biodegradation of trichloroethylene and involvement of an aromatic biodegradative pathway. Applied Environmental Microbiology 53(5): 949-954.
2. Stanier, R., Ingraham, J., Wheelis, M., and Painter, P. 1986. The Microbial World, Prentice-Hall, Englewood Cliffs, New Jersey.
3. Ryoo, D., Shim, H., Canada, K., Barbieri, P., and Wood, T.K. 2000. Aerobic degradation of tetrachloroethylene by toluene-o-xylene monooxygenase of Pseudomonas Stutzeri OX1. Nature Biotechnology 18: 775-778.
4. Shim, H., Ryoo, D., Barbieri, P., and Wood, T.K. 2001. Aerobic degradation of mixtures of tetrachloroethylene, trichloroethylene, dichloroethylene, and vinyl chloride by toluene-o-xylene monooxygenase of Pseudomonas stutzeri OX1. Applied Microbiol. Biotechnol 56: 265-269.
5. Wackett, L.P., and Gibson, D. T. 1988. Biotransformation of trichloroethylene by toluene dioxygenase in whole-cell studies with Pseudomonas putida F1. Appl. Environmental Microbiology. 54: 1703-1708.
6. Van Hycklama Vlieg, J. E. T., de Koning, W., and Janssen, D.B. 1996. Transformation kineticsof chlorinated ethenes by Methylosinos trichosporium OB3b and detection of unstable epoxides by on-line gas chromatography. Appl. Environmental Microbiology 62 (9): 3304-3312.