Degradation by co-metabolism

Degradation by co-metabolism starts immediately and follows a first-order kinetic progress. A plot of log concentration against time will follow a straight line, whereas degradation when adaptation must first occur follows a somewhat more complicated pattern. There will be a lag period with slow degradation, followed by a more or less logarithmic phase. At very low... 

On the other hand, the presence of quickly mineralized substrates in higher concentrations may facilitate an initial transformation of the xenobiotic molecule by co-metabolism. The co-metabolized substance may then be available for further degradation and mineralization. Thus, the presence of other substrates may increase the possibilities for a substance to be degraded. 

In some cases, microorganisms can transform a contaminant, but they are not able to use this compound as a source of energy or carbon. This biotransformation is often called co-metabolism. In co-metabolism, the transformation of the compound is an incidental reaction catalyzed by enzymes, which are involved in the normal microbial metabolism.33 A well-known example of co-metabolism is the degradation of (TCE) by methanotrophic bacteria, a group of bacteria that use methane as their source of carbon and energy. When metabolizing methane, methanotrophs produce the enzyme methane monooxygenase, which catalyzes the oxidation of TCE and other chlorinated aliphatics under aerobic conditions.34 In addition to methane, toluene and phenol have been used as primary substrates to stimulate the aerobic co-metabolism of chlorinated solvents. 

Fortuitous or co-metabolic biodegradation may account for a significant portion of the removal of xenobiotics in the environment.24 Numerous examples of co-metabolic activity have been described for pure substrates,22 but co-metabolism has been very difficult to demonstrate in mixed-substrate, mixed-culture systems, because products of the co-metabolic reactions of one species may be degraded by another.24 To encourage co-metabolism, easily degradable co-substrates should be included in the leachate prior to biological treatment. Fatty acids, which often occur in landfill leachates, may fulfill this requirement. 

The pollutant degradation by WRF is a co-metabolic process in which additional C and N sources are required. This capacity represents an advantage respect bacteria as it prevents the need to internalize the pollutant, thus avoiding toxicity problems and permitting to attack low-soluble compounds. 

Two processes of microbial degradation must be emphasized in our understanding the fate of chemicals in the environment, metabolism via mineralization or co-metabolism. The former is specifically for process carried by bacterial and support the growth of the microorganisms while the latter one involves the presence of a second source of carbon and energy in which the microorganisms actually use these for growth, but also... 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

Bioinfiltration is limited by the ability of soil microorganisms to degrade the contaminants of concern, since bacteria cannot metabolize or co-metabolize contaminants at toxic concentrations. Indigenous microbial populations require sufficient time to adapt to contaminants. 

Now we consider situations in which transformation of the organic compound of interest does not cause growth of the microbial population. This may apply in many engineered laboratory and field situations (e.g., Semprini, 1997 Kim and Hao, 1999 Rittmann and McCarty, 2001). The rate of chemical removal in such cases may be controlled by the speed with which an enzyme catalyzes the chemical s structural change (e.g., steps 2, 3 and 4 in Fig. 17.1). This situation has been referred to as co-metabolism, when the relevant enzyme, intended to catalyze transformations of natural substances, also catalyzes the degradation of xenobiotic compounds due to its imperfect substrate specificity (Horvath, 1972 Alexander, 1981). Although the term, co-metabolism, may be used too broadly (Wackett, 1996), in this section we only consider instances in which enzyme-compound interactions limit the overall substrate s removal. Since enzyme-mediated kinetics were characterized long ago by Michaelis and Menten (Nelson and Cox, 2000), we will refer to such situations as Michaelis-Menten cases. 

In such a case, the organisms have an enzyme capable of oxidizing Q. Only degradation of Q allows the bacteria to obtain carbon and energy sufficient to maintain and multiply the population. Due to imperfect substrate specificity, the same enzymatic reaction takes place with BQ too. Similar co-metabolism by enzymes intended for natural substrates has been reported for many xenobiotic compounds like chlorinated solvents (Semprini, 1997), chlorophenols (Kim and Hao, 1999), and chlorobiphenyls (Kohler et al., 1988). 

Under aerobic conditions TNT can be mineralized by a range of bacteria and fungi, often co-metabolically with the degradation of a more degradable substrate There is even evidence that some plants are able to deaminate TNT reductively. 

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