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).