Impact of Fixed Nitrogen Availability on Reductive Dechlorination Activity.

Biostimulation to promote reductive dechlorination is widely practiced, but the value of adding an exogenous nitrogen (N) source (e.g., NH) during treatment is unclear. This study investigates the effect of NH availability on organohalide-respiring () 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 NH increased -1,2-dichloroethene (cDCE)-to-ethene dechlorination rates about 5-fold (20.6 ± 1.6 versus 3.8 ± 0.5 μM Cl d), and the total number of 16S rRNA gene copies were about 43-fold higher in incubations with NH ((1.8 ± 0.9) × 10 mL) compared to incubations without NH ((4.1 ± 0.8) × 10 mL). In TC cultures, NH also stimulated cDCE-to-ethene dechlorination and growth. Quantitative polymerase chain reaction (qPCR) revealed that Cornell-type capable of N fixation dominated PW4 cultures without NH, but their relative abundance decreased in cultures with NH amendment (i.e., 99 versus 54% of total ). Pinellas-type incapable of N fixation were responsible for cDCE dechlorination in TC cultures, and diazotrophic community members met their fixed N requirement in the medium without NH. Responses to NH were apparent at the community level, and N-fixing bacterial populations increased in incubations without NH. Quantitative assessment of nitrogenase genes, transcripts, and proteomics data linked Cornell-type and expression with fixed N limitation. NH additions also demonstrated positive effects on in situ dechlorination activity in the vicinity of well PW4. These findings demonstrate that biostimulation with NH can enhance 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 NH or transformation products (i.e., nitrous oxide).