Discovery of a Novel Microorganism that Completely Detoxifies Chloroethenes to Ethene
Assoc Prof JZ He (Department of Civil and Environmental Engineering)
Industrial discharges of chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) pose a serious threat to human health and the environment due to their potential carcinogenic effects and persistence in the environment. Efforts to remediate these contaminants often result in the accumulation of byproducts, such as
trans-1,2-dichloroethene (trans-DCE),
cis-1,2-dichloroethene (cis-DCE) or the carcinogen vinyl chloride (VC). Microorganisms that could detoxify both TCE and
trans-DCE completely to ethene still remain elusive. In this project, an anaerobic TCE-dechlorinating culture, designated as
Dehalococcoides sp. strain 11a (Figure 1A), was enriched from the outlet of an industrial wastewater treatment station. Strain 11a demonstrates rapid and consistent dechlorination of VC, all dichloroethene (DCE) isomers, and TCE metabolically to ethene for the first time. The discovery of new
Dehalococcoides species provides further insights into the genomes of these important chloroethene-respiring bacteria and the evolutionary history of this genus.
The unique capacity of Dehalococcoides sp. strain 11a in rapid dechlorination of TCE,
trans-DCE, and VC makes it an excellent candidate for bioremediation of chloroethene-contaminated sites. In particular, strain 11a could dechlorinate VC at a rate of (407.1±2.3) µmol L-1 day-1, and up to 258 nmol chloride removal per min per mg of protein, which was 2-3 folds higher than previous
Dehalococcoides species. Strain 11a demonstrates its bioremediation potential in that complete detoxification of PCE to ethene in groundwater sample was achieved within 10 days when co-cultured with a PCE-to-cis-DCE dechlorinating bacteria,
Sulfurospirillum multivorans (Figure 1B). In order to understand the functional reductases (RDases) that carry the detoxification process, comparative genomics using
Dehalococcoides genus-targeting microarrays have been applied to analyze the genomes of the recently isolated
Dehalococcoides strains. The incongruence that is found between the functional activity of
Dehalococcoides strains and the core genome phylogeny of each subgroup is likely driven by the horizontal transfer of the key RDase-encoding genes. Targeting the presence and expression of these RDase genes can be useful for field diagnostic purposes and the results of this study further support their use in bioremediation projects. In order to facilitate the application of RDases as biomarkers, future work will have to focus on determining the functions of the large pool of uncharacterized RDases.
In addition to this new Dehalococcoides sp. strain 11a, we have also successfully cultivated and characterized another chloroethene-dechlorinating culture,
Dehalococcoides sp. strain ANAS2. The cultivation of both 11a and ANAS2 enhances our understanding of the evolution of this unusual microbial group -
Dehalococcoides species. This study also provides promising application of
Dehalococcoides in successful bioremediation of chloroethene-contaminated sites.
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