Organohalides such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent toxic groundwater contaminants. Remediation of organohalide-contaminated sites has high priority, and efficient and cost-effective remedies are needed to prevent environment and human exposure through contaminated water. Bacterial organohalide-respiration plays a major role in organohalide detoxification. Dehalococcoides mccartyi (Dhc) are key mediators in bioremediation, since only Dhc strains have been documented in complete detoxification of chlorinated ethenes to benign ethene. Dhc depends on other microorganisms in the environment for essential growth requirements (e.g., hydrogen and vitamins). For successful implementation of the reductive dechlorination to remediate contaminated sites, microbial interactions controlling Dhc reductive dechlorination must be elucidated. The overall objective of this research was to address the key gaps in the scientific understanding of the controls over Dhc reductive dechlorination activity, including Dhc corrinoid-related interactions with other microorganisms. Detailed hydrogeological and microbial characterization of mixed chlorinated solvent contaminated Third Creek site (Knoxville, TN) attributed an important role to the creek sediment, where organohalide-respiring bacteria (e.g., Dhc and Dehalobacter) co-exist, for detoxification of contaminants. Different chlorinated solvent-amendments affected Dhc strain selection and non-dechlorinating microbial composition in enrichment cultures derived from Third Creek sediment. Corrinoid-auxotroph Dhc require corrinoid cofactor for the reductive dehalogenase enzyme systems. Microorganisms including Acetobacterium, Clostridium, Geobacter, and methanogens were identified as corrinoid-producers in the enrichment cultures. 5,6- dimethyl-benzimidazole cobamide (DMB-Cba) was the most abundant corrinoid in enrichment cultures to support Dhc reductive dechlorination. Different lower base-amendments affected Dhc reductive dechlorination rates and extents. Lower base-amendments to enrichment cultures caused a shift from production of DMB-Cba to production of corrinoids with the amended lower bases, some of which caused lower dechlorination rates. In addition, different Dhc strains became abundant with different lower base-amendment in cultures, demonstrating the role of corrinoid in Dhc strain selection. Lastly this research demonstrated that different geochemical conditions and corresponding microbial populations determined the composition and concentration of bioavailable corrinoid pools; thus directly controlling Dhc reductive dechlorination activity. The findings of this research are relevant to environmental remediation practitioners and provide valuable information for improving bioremediation strategies to achieve successful contaminated-site cleanup.