Enhanced resilience to oxygen exposure and toxicity of chlorinated solvents in immobilized Dehalococcoides mccartyi

Members of Dehalococcoides mccartyi (Dhc) are strictly anaerobic and play crucial roles in the restoration of many industrial sites impacted by chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). In situ bioremediation with Dhc involves intricate procedures intended to minimize oxygen intrusion, and achieving optimal dechlorination performance in aquifers near the dense non-aqueous phase liquids source zone is challenging. Here, we respectively embedded Dhc strain 195 and the biomass of a Dhc-containing, PCE-dechlorinating consortium in poly(vinyl alcohol)-alginate hydrogel beads. The ethene-forming potential was well-retained in immobilized Dhc following a prolonged oxygen exposure spanning from 12 hours to 7 days, with dechlorination rates ranging from 54.6 ± 4.2-101.9 ± 13.5 µM Cl^- released day^-1. In contrast, suspended strain 195 and the Dhc-containing biomass exposed to oxygen for a shorter duration were completely deactivated, or suffered a substantial reduction in dechlorination potential. Cell immobilization also significantly improved the ability of Dhc to tolerate the toxic effects of chlorinated solvents. When exposed to 300 mg L^-1 TCE or free-phase PCE, an immobilized Dhc inoculum enabled more rapid recovery of dechlorination activity with shorter lag phases and up to 2.1-fold higher dechlorination rate compared to the use of their suspended counterparts. Our results demonstrate the effectiveness of cell immobilization for shielding Dhc from various environmental stresses (e.g., oxygen exposure).