Ferrous iron accelerates the population-level recovery of Methanocorpusculum and enhances Dehalococcoides-mediated dechlorination of chloroethenes.

Bioremediation of chloroethenes by Dehalococcoides species is a widely applied strategy for subsurface detoxification because it is environmentally friendly and cost-effective; however, field applications are often limited by slow and incomplete remediation, particularly due to delayed population-level recovery of key microorganisms. To identify a practical approach to overcome this limitation, this study investigated how ferrous iron [Fe(II)] modulates the co-occurrence of Dehalococcoides and methanogens to accelerate dechlorination of chloroethenes. Tetrachloroethene dechlorination was evaluated using two buffering systems: carbonate-buffered cultures representing subsurface-relevant conditions and phosphate-buffered cultures representing well-controlled conditions. Quantitative PCR and GC analyses showed that Fe(II) addition at 20 mg L-1 accelerates population-level recovery of the methanogen Methanocorpusculum, resulting in complete dechlorination 28% earlier than the 4 mg L-1 Fe(II) condition in the carbonate-buffered system. In contrast, higher Fe(II) concentrations (80-160 mg L-1) inhibited Methanocorpusculum growth and showed a tendency toward slower progression of dechlorination. Strong positive correlations (R = 0.736-0.955) between the 16S rRNA gene copy numbers of Dehalococcoides and Methanocorpusculum across both cultures indicate consistent co-occurrence of the two taxa. These results suggest that Fe(II) primarily accelerates chloroethene dechlorination by shortening population-level methanogenic recovery. Managing Fe(II) levels to balance methanogenic growth and inhibition provides insights into the role of Fe(II) in regulating population-level recovery and dechlorination dynamics of chloroethenes.