The bacterium Dehalococcoides mccartyi is known for its specialized diet of toxic, man-made chemicals that have contaminated soil and groundwater. Its discovery offered a natural solution to a widespread pollution problem. The existence of an organism that thrives on substances harmful to most other life forms has opened new avenues for environmental cleanup.
Dehalococcoides breaks down industrial solvents for its survival. This metabolic capability makes it useful in environmental remediation efforts. The bacterium’s function has led to its application in treating sites contaminated with chemicals from industrial processes, providing insight into nature’s capacity to adapt to synthetic compounds.
The Unique Metabolism of Dehalococcoides
The metabolic process that allows Dehalococcoides mccartyi to survive is a form of anaerobic respiration called organohalide respiration. Similar to how humans breathe oxygen, this bacterium “breathes” chlorinated solvents as part of its respiratory process. This function is driven by specialized enzymes known as reductive dehalogenases.
To generate energy, the bacterium requires an electron donor, usually hydrogen (H₂), and an electron acceptor. The chlorinated solvent acts as the electron acceptor. As electrons are transferred from the hydrogen to the chlorinated compound, the bacterium gains energy, facilitating the removal of chlorine atoms from the pollutant molecule.
Role in Environmental Cleanup
Dehalococcoides mccartyi’s metabolism makes it useful for bioremediation, which uses living organisms to remove contaminants. The bacterium is used in a strategy called bioaugmentation, where cultures are introduced into a contaminated site to enhance pollutant breakdown. This method has been used for over two decades to address chemical spills and long-term contamination.
Cleanup efforts target sites polluted with chlorinated solvents, such as former dry-cleaning facilities, industrial plants, and military bases. At these locations, chemicals like tetrachloroethene (PCE) and trichloroethene (TCE) often contaminate groundwater aquifers. These dense chemicals sink deep into the subsurface, becoming persistent pollution sources that are difficult to remove conventionally.
Injecting Dehalococcoides cultures into a contaminated area initiates the natural degradation of these substances. The bacteria spread through the subsurface and break down the solvents into non-toxic compounds. This in-situ (in-place) treatment avoids the need for excavation or pumping and treating large volumes of groundwater.
Essential Growth Conditions
For Dehalococcoides mccartyi to clean up a site, specific environmental conditions must be met. The primary requirement is an anaerobic environment, one completely devoid of oxygen. Oxygen is toxic to these bacteria because it inhibits the reductive dehalogenase enzymes responsible for breaking down chlorinated solvents. Creating and maintaining oxygen-free conditions is a first step in any bioremediation project using this microbe.
The bacterium also needs a steady supply of an electron donor, which is hydrogen gas (H₂). Dehalococcoides does not produce its own hydrogen, relying on other microorganisms in the environment to generate it. These microbes ferment organic carbon, releasing hydrogen as a byproduct that fuels the dechlorination process.
The microbe has specific nutritional needs, including vitamin B12 (cobalamin). Vitamin B12 is a component of the reductive dehalogenase enzymes that perform the dechlorination reaction. Without enough vitamin B12, the bacteria cannot synthesize these enzymes and their metabolic activity stops. Bioremediation projects often add acetate as a carbon source and other nutrients to ensure the microbial community thrives.
The Complete Dechlorination Process
The breakdown of chlorinated solvents by Dehalococcoides mccartyi occurs in a stepwise sequence. The process begins with tetrachloroethene (PCE), a solvent with four chlorine atoms. The bacterium removes one chlorine atom at a time, transforming PCE into trichloroethene (TCE), then into dichloroethene (DCE), and finally into vinyl chloride (VC).
The final step is the conversion of vinyl chloride into ethene, a harmless, non-toxic gas. The ability to complete this step is what makes certain strains of Dehalococcoides effective for remediation. This transformation from a toxic solvent into a benign hydrocarbon represents a full detoxification of the contaminant.
The dechlorination process must proceed all the way to ethene because some intermediate products are more hazardous than the original pollutant. Vinyl chloride (VC) is a known human carcinogen and is more toxic than PCE or TCE. If environmental conditions are unsuitable, the process can stall and lead to an accumulation of VC. Ensuring the right conditions are present for the bacteria to complete the sequence is a goal of any bioremediation plan.