Naturally occurring bacteria found in soil, groundwater, water and sediments may be a key for environmental pollutant removal. This so-called biodegradation is a part of natural attenuation, which according to the US Environmental Protection Agency could be defined as "The effect of naturally occurring physical, chemical and biological processes, or any combination of those processes to reduce the load, concentration, flux or toxicity of polluting substances without human intervention". The effectiveness of biodegradation depends on the type of contaminant present as well as on the complex environmental conditions that determine microbial community structure, electron donor availability ( the electron donors could be defined as releasing an electron during cellular respiration, resulting in the release of energy. Microorganisms, such as bacteria, obtain energy in the electron transfer processes ) and degradation reactions in the subsurface. Under natural conditions, biodegradation of many contaminants is often a slow process and long timeframes may be required to achieve a remediation objective. Because of this, scientists lately focus more and more on stimulating the growth of that part of the site-specific microbial community that demonstrates the largest degradation potential. Here the idea is to study natural biodegradation processes occurring at a contaminated site in the lab to understand the microbial community structure, kinetics and determine the maximum degradation potential. Once these aspects are sufficiently understood and the site´s hydrogeology has been characterized, a growth-stimulating medium can be injected at certain locations to enhance biodegradation.
Although many studies have been carried over several decades, to improve our understanding of natural attenuation processes there is still a great deal to be learned regarding the mechanisms governing natural attenuation processes and their ability to address different types of contamination problems.
For example, chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE), represent a common class of contaminants used for degreasing in the dry cleaning, electronic manufacturing and machine maintenance industries. Where these contaminants had been used excessively in the past they often have leaked into the subsurface from storage tanks and machinery and have formed large plumes in the groundwater that often shows chlorinated ethene concentrations, which have been proven unhealthy.
The bacterium Dehalococcoides mccartyi is an organic halide-respiring anaerobic bacterium that uses chlorinated compounds for its dehalogenation activity. In other words, this bacterium is specialized to grow with halogenated compounds such as chlorinated aliphatic hydrocarbons as electron acceptor via a respiratory process, i.e. an electron-consuming process, and in most of the cases hydrogen is used as the final electron donor.
The removal of chlorinated ethenes by Dehalococcoides mccartyi via dehalogenation follows the sequential degradation of PCE to TCE to the dichloroethene isomers (cis-DCE, trans-DCE, 1,1-DCE), then to vinyl chloride (VC) and finally to ethene/ethane. However, a complete dechlorination always depends on the environmental conditions and other limiting factors. During the last two decades, several studies have been carried out based on the addition of electron donors for achieving a complete removal of chlorinated compounds, nevertheless, no convincing conclusions have yet been drawn.
As a precursor to an in-situ stimulation of the natural biodegradation potential, Uwe Schneidewind and his colleagues from VITO evaluated the dechlorination reaction occurring at an aquifer contaminated with TCE and its daughter products, discharging into the Zenne River, Belgium, in their article titled " Kinetics of dechlorination by dehalococcoides mccartyi using different carbon sources ".
Sediment material was collected from three locations of the aquifer as well as from the riverbed and used in microcosm studies (measurement of microbial activity). The growth of the microbial community was stimulated by using different carbon sources (i.e. external supply of energy in the form of food) such as lactate (C3H6O3) or molasses (C6H12NNaO3S) and the dechlorination reaction in each microcosm was monitored. Afterwards, the observed reactions were modelled using first order, Michaelis-Menten and Monod kinetics.
Reductive dechlorination of TCE took place only when external carbon sources were added to microcosms, and occurred concomitant with a pronounced increase in the Dehalococcoides mccartyi cell count as determined by 16S rRNA gene-targeted qPCR (i.e. growth measurements and bioremediation monitoring method). This indicates that native dechlorinating bacteria are present in the aquifer of the Zenne site and that the oligotrophic nature of the aquifer prevents a complete degradation to ethene. The type of carbon source, the cell number of D. mccartyi or the reductive dehalogenase genes, however, did not unequivocally explain the observed differences in degradation rates or the extent of dechlorination. Results point to the role of the supporting microbial community but it remains to be verified how the complexity of the microbial (inter)actions should be represented in a model framework.
Schneidewind, U., Haest, P., Atashgahi, S., Maphosa, F., Hamonts, K., Maesen, M., Calderer, M., Seuntjens, P., Smidt, H., Springael, D., & Dejonghe, W. (2014). Kinetics of dechlorination by Dehalococcoides mccartyi using different carbon sources Journal of Contaminant Hydrology, 157 , 25-36 DOI: 10.1016/j.jconhyd.2013.10.006