Impact of the microbial community on chlorinated ethene degradation: laboratory, modeling, and field investigation

Chlorinated ethenes are common contaminants in subsurface environments worldwide. These compounds frequently form groundwater contaminant plumes that can threaten drinking water abstraction wells and are difficult to remediate. Degradation of chlorinated ethenes by organohalide-respiring bacteria (OHRB) via reductive anaerobic dechlorination occurs naturally in anoxic environments and can be used as a cost-effective, non-invasive way to address this problem. In an ideal scenario, OHRB use chlorinated ethenes as electron acceptors and sequentially reduce tetrachloroethene (PCE) to trichloroethene (TCE), cis-1,2-dichloroethene (cDCE), vinyl chloride (VC), and ultimately the non-toxic ethene. However, this degradation process does not always reliably reach its non-toxic endpoint, and instead toxic daughter products can accumulate. Most studies on OHRB communities to investigate this problem are conducted on simplified consortia, where the other community members are selected for due to their role in supporting OHRB; however, in the environment, OHRB are only one part of a complex and varied microbial community. The impact of other processes and other functional guilds in the subsurface microbial community may play a role in the slow or incomplete dechlorination observed in chlorinated ethene plumes.
Various methods, including laboratory experiments, biogeochemical modelling, and field investigations, can be used to explore the impact of the microbial community on chlorinated ethene dechlorination. This thesis presents a unique combination of all of these investigation methods to advance our understanding of the impact of the microbial community on chlorinated ethene dechlorination by OHRB in contaminant plumes.
Three different laboratory experimental investigations were conducted in the context of this thesis: one to study intra-guild interactions of multiple OHRB strains with each other, and two to study inter-guild interactions, between OHRB, sulfate-reducing bacteria (SRB), and Fe(III)-reducing bacteria (FeRB). In each experimental configuration, the included guilds used the same electron donor, which was supplied in excess. Since chlorinated ethenes at high concentration are inhibitory/toxic to the OHRB, cultivation in the laboratory is best done with the inclusion of a liquid organic layer between the aqueous and gaseous phases that allows for the concentration of chlorinated ethene in the aqueous medium to be controlled.
A biogeochemical modelling tool was developed to interpret such laboratory experiments. In order to accurately quantify chlorinated ethene dechlorination by OHRB, the model included microbial growth and decay of all guilds, microbial reduction of other electron acceptors by the SRB and FeRB guilds, aqueous speciation of compounds in the growth medium, kinetic and equilibrium geochemical reactions, removal of aqueous and gaseous samples, partitioning of volatile/organic compounds, and kinetic mass transfer between the phases. The processes occurring in the batch experimental systems were able to be quantitatively interpreted using the modeling tool based on a rich dataset comprised of complementary analyses: chloride, Fe(II), sulfate, dechlorination products, and microbial community composition. Intra-guild investigations showed that two different strains of OHRB that used the same electron acceptor were able to coexist in the same consortium due to their differing kinetic characteristics. Inter-guild investigations with OHRB, SRB, and FeRB show that SRB have an impact on reductive dechlorination that is moderated by FeRB. In the case of a PCE-dechlorinating OHRB consortium, SRB inhibited dechlorination. In the case of a cDCE and VC-dechlorinating OHRB consortium, an added SRB stimulated cDCE and VC dechlorination over a base case with another SRB strain indigenous to the consortium.
Just as investigation of OHRB communities in the laboratory is strengthened by complementary datasets, investigation of OHRB in situ requires collection of multiple lines of evidence. One line of evidence alone is usually not enough to ascertain whether degradation occurs in the subsurface, and if so, by what mechanism and to what extent. An investigation of a chlorinated ethene plume was undertaken using lines of evidence such as analysis for specific degraders and reductive dehalogenase genes, 16S rRNA gene amplicon sequencing analysis, and 13C and 37Cl dual isotopes, among others. By combining information from the various lines of evidence and quantifying first order degradation rate constants, it was determined that cDCE degradation occurs via both biotic and abiotic mechanisms and that monitored natural attenuation is, at present, sufficient to manage the contamination plume.
In conclusion, this PhD project has furthered our understanding of how the microbial community impacts dechlorination of chlorinated ethenes. The project has also demonstrated that the combination of various investigative methods and complementary data types yields in-depth conceptual and quantitative understanding of the abiotic and biotic processes occurring in chlorinated ethene degrading systems.