Impact of Fixed Nitrogen Availability on Dehalococcoides mccartyi Reductive Dechlorination Activity

Biostimulation to promote reductive dechlorination is widely practiced, but the value of adding an exogenous nitrogen (N) source (e.g., NH₄ ⁺) during treatment is unclear. This study investigates the effect of NH₄ ⁺ availability on organohalide-respiring Dehalococcoides mccartyi (Dhc) growth and reductive dechlorination in enrichment cultures derived from groundwater (PW4) and river sediment (TC) impacted with chlorinated ethenes. In PW4 cultures, the addition of NH₄ ⁺ increased cis-1,2-dichloroethene (cDCE)-to-ethene dechlorination rates about 5-fold (20.6 ± 1.6 versus 3.8 ± 0.5 μM Cl^- d^-1), and the total number of Dhc 16S rRNA gene copies were about 43-fold higher in incubations with NH₄ ⁺ ((1.8 ± 0.9) × 10⁸ mL^-1) compared to incubations without NH₄ ⁺ ((4.1 ± 0.8) × 10⁷ mL^-1). In TC cultures, NH₄ ⁺ also stimulated cDCE-to-ethene dechlorination and Dhc growth. Quantitative polymerase chain reaction (qPCR) revealed that Cornell-type Dhc capable of N₂ fixation dominated PW4 cultures without NH₄ ⁺, but their relative abundance decreased in cultures with NH₄ ⁺ amendment (i.e., 99 versus 54% of total Dhc). Pinellas-type Dhc incapable of N₂ fixation were responsible for cDCE dechlorination in TC cultures, and diazotrophic community members met their fixed N requirement in the medium without NH₄ ⁺. Responses to NH₄ ⁺ were apparent at the community level, and N₂-fixing bacterial populations increased in incubations without NH₄ ⁺. Quantitative assessment of Dhc nitrogenase genes, transcripts, and proteomics data linked Cornell-type Dhc nifD and nifK expression with fixed N limitation. NH₄ ⁺ additions also demonstrated positive effects on Dhc in situ dechlorination activity in the vicinity of well PW4. These findings demonstrate that biostimulation with NH₄ ⁺ can enhance Dhc reductive dechlorination rates; however, a "do nothing" approach that relies on indigenous diazotrophs can achieve similar dechlorination end points and avoids the potential for stalled dechlorination due to inhibitory levels of NH₄ ⁺ or transformation products (i.e., nitrous oxide).