ABSTRACTCorrinoids are essential cofactors of reductive dehalogenases inDehalococcoides mccartyi, an important bacterium in bioremediation, yet sequencedD. mccartyistrains do not possess the complete pathway forde novocorrinoid biosynthesis.Pelosinussp. andDesulfovibriosp. have been detected in dechlorinating communities enriched from contaminated groundwater without exogenous cobalamin corrinoid. To investigate the corrinoid-related interactions among key members of these communities, we constructed consortia by growingD. mccartyistrain 195 (Dhc195) in cobalamin-free, trichloroethene (TCE)- and lactate-amended medium in cocultures withDesulfovibrio vulgarisHildenborough (DvH) orPelosinus fermentansR7 (PfR7) and with both in tricultures. Only the triculture exhibited sustainable dechlorination and cell growth when a physiological level of 5,6-dimethylbenzimidazole (DMB), the lower ligand of cobalamin, was provided. In the triculture, DvH provided hydrogen while PfR7 provided corrinoids to Dhc195, and the initiation of dechlorination and Dhc195 cell growth was highly dependent on the growth of PfR7. Corrinoid analysis indicated that Dhc195 imported and remodeled the phenolic corrinoids produced by PfR7 into cobalamin in the presence of DMB. Transcriptomic analyses of Dhc195 showed the induction of the CbiZ-dependent corrinoid-remodeling pathway and BtuFCD corrinoid ABC transporter genes during corrinoid salvaging and remodeling. In contrast, another operon annotated to encode a putative iron/cobalamin ABC transporter (DET1174-DET1176) was induced when cobalamin was exogenously provided. Interestingly, a global upregulation of phage-related genes was observed when PfR7 was present. These findings provide insights into both the gene regulation of corrinoid salvaging and remodeling in Dhc195 when it is grown without exogenous cobalamin and microbe-to-microbe interactions in dechlorinating microbial communities.