Bioremediation of Chlorinated Ethenes: pH Effects, Novel Dechlorinators and Decision-Making Tools

Chlorinated solvents have been widely used in different areas of modern society. Usage of these chlorinated solvents was not necessarily accompanied with proper handling and disposal of these hazardous compounds, which caused a variety of environmental problems and continues to affect human health. Remediation of chlorinated ethenes contaminated sites has high priority for state regulators and site owners. Among the available treatment technologies, bioremediation shows great promise as a cost-effective corrective strategy for a variety of environmental pollutants. Prerequisites are that the microbiology involved in contaminant degradation and geochemical factors, such as pH, are understood, so that bioremediation technologies can be confidently implemented. The aims of this dissertation work were 1) to enrich and isolate PCE dechlorinators under low pH conditions, 2) to investigate how pH fluctuations affect the microbial community of a PCE-to-ethene consortium, 3) to determine the pH tolerance of Dehalococcoides mccartyi (Dhc), 4) to identify a non-Dehalococcoides type microorganism responsible for reductive dechlorination of vinyl chloride, 5) to identify and characterize a novel vinyl chloride reductase gene and 6) to develop an Excel-based tool to guide remedial practitioners to select suitable remediation strategies. Only one enrichment culture out of total sixteen sites samples showed PCE dechlorination activity at pH 5.5 and stoichiometric conversion to cDCE occurred after repeated transfers. The analysis of 16S rRNA gene sequencing data revealed the genera Desulfovibrio, Sulfurospirillum, and Megasphaera were most abundant in pH 5.5 enrichment. Two PCE-dechlorinating isolates (strains PLC-TCE and PLC-DCE) were obtained from a pH 5.5 enrichment, and identified as members of the genus Sulfurospirillum. Experiments with a Dhc-containing consortium demonstrated that exposure time affected Dhc ability to recover reductive dechlorination activity following low pH exposure. Low pH conditions affected Dhc strains differently, and Dhc strains carrying the vcrA gene responsible for reductive dechlorination of the human carcinogen vinyl chloride (VC) were least tolerant to low pH. Enrichment and isolation efforts led to the discovery of a Dehalogenimonas (Dhgm) species capable of respiring chlorinated ethenes, including VC. These research findings advance understanding of the microbial reductive dechlorination process and will improve the implementation of in situ bioremediation.