Metabolite Analysis as Direct Proof of Biodegradation: Experience from Monitored Natural Attenuation (MNA) Projects
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Abstract:
Signature metabolites provide direct geochemical indication that in-situ biodegradation of released organic compounds (e.g., oil and its refined products) is occurring. Experience shows that monitored natural attenuation site conditions are often more complex than in theory and often require a more profound comprehension of the governing natural attenuation processes. Frequently, there is lack of direct proof that contaminant degradation (mainly through biodegradation) is occurring. Advanced tools are emerging that aim to provide answers about whether contaminants of concern are actually (bio)degraded and to the extent. Signature metabolite analysis provides direct proof of mineral oil hydrocarbon biodegradation and is among these advanced tools. Yet, during the previous 15 years, metabolite analysis has only been used sporadically in research projects. The target metabolites consist of aromatic acids such as benzoates and benzylsuccinates and uniquely indicate in-situ biodegradation of individual contaminants of concern. Three case studies have been summarized to share practical experience with signature metabolite analysis for contaminants that include benzene, toluene, ethyl benzene, and xylenes; trimethylbenzenes; and polycyclic aromatic hydrocarbons. The summarized case study involving jet fuel-contamination was the first reported field study in which aromatic acid homologs were formed by microbial metabolism of C4 through C7 benzene. Signature metabolite analysis can be used to improve understanding of natural attenuation processes to close data gaps with respect to the general degradation mechanisms. Direct evidence for biodegradation (e.g., metabolite identity and concentration; microbial identity and quantity; daughter product degradation ratios, stable carbon isotope ratios) facilitates remediation planning and management; provides information useful to scientists and engineers that must determine the mechanisms that produced observed environmental conditions; and provides information for stakeholders involved in environmental cleanup litigation and cost appropriation.Humin
Degradation
Polycyclic aromatic hydrocarbon
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The effect of concentration on the biodegradation of synthetic organic chemicals by natural microbial communities was investigated by adding individual 14 C-labeled organic compounds to stream water at various initial concentrations and measuring the formation of 14 CO 2 . The rate of degradation of p -chlorobenzoate and chloroacetate at initial concentrations of 47 pg/ml to 47 μg/ml fell markedly with lower initial concentrations, although half or more of the compound was converted to CO 2 in 8 days or less. On the other hand, little mineralization of 2,4-dichlorophenoxyacetate and 1-naphthyl- N -methylcarbamate, or the naphthol formed from the latter, occurred when these compounds were present at initial concentrations of 2 to 3 ng/ml or less, although 60% or more of the chemical initially present at higher concentrations was converted to CO 2 in 6 days. It is concluded that laboratory tests of biodegradation involving chemical concentrations greater than those in nature may not correctly assess the rate of biodegradation in natural ecosystems and that low substrate concentration may be important in limiting biodegradation in natural waters.
Degradation
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Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous persistent environmental contaminants generated by natural combustion processes and human activities. PAHs are considered hazardous because of cytotoxic, mutagenic, and carcinogenic effects. Sixteen individual PAH compounds have been identified as priority pollutants by the United States Environmental Protection Agency (U.S. EPA). All substances originated in to the environment by either biogenic or anthropogenic sources. Anthropogenic compounds describe synthetic compounds, and compound classes as well as elements and naturally occurring chemical entities which are mobilized by man's activities. In the marine environment, the fate of pollutants is largely determined by biogeochemical process. Some of these chemical changes enhance the toxicity of the pollutants. Other chemical changes cause the degradation or immobilization of pollutants and, as a result, act to purify the waters. Possible fates for PAHs, released into the environment, include volatilization, photo-oxidation, chemical oxidation, bioaccumulation and adsorption on soil particles, leaching, and microbial degradation. Elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) have been found in mangrove sediments due to anthropogenic compounds.
Volatilisation
Biogeochemical Cycle
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Isotope Analysis
Degradation
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Hexadecane
Phytane
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Microbial degradation of pyrene was studied in soils in the presence and absence of earthworms (Eisenia foetida) to demonstrate an integrated innovative strategy for bioremediation of sites lightly polluted by polycyclic aromatic hydrocarbons. Desorption of pyrene and soil microbial respiration were measured to elucidate the mechanism of enhanced microbial degradation. The results showed that both soil properties and contact time could influence pyrene biodegradation. The introduction of E. foetida enhanced pyrene removal significantly both in freshly spiked and aged soils. The percentage pyrene removal in the presence of E. foetida was 45.5–91.0% after 14 d of incubation, which were 2.1 to 2.8 times greater than those without the worms. The enhanced pyrene removal is attributed to both enhanced microbial degradation and uptake by the worms. Microbial degradation of pyrene increased by 1.2 to 1.6 times in the presence of the worms. Overall, the introduction of live worms could improve both pyrene bioavailability and microbial activity, which leads to enhanced microbial degradation of pyrene.
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