This study explored the response of NO3
–-N bioreduction to Cr(VI) stress, including reduction
efficiency
and the pathways involved (denitrification and dissimilatory nitrate
reduction to ammonium (DNRA)). Different response patterns of NO3
–-N conversion were proposed under Cr(VI)
suppress (0, 0.5, 5, 15, 30, 50, and 80 mg/L) by evaluating Cr(VI)
dose dependence, toxicity accumulation, bioelectron behavior, and
microbial community structure. Cr(VI) concentrations of >30 mg/L
rapidly
inhibited NO3
–-N removal and immediately
induced DNRA. However, denitrification completely dominated the NO3
–-N reduction pathway at Cr(VI) concentrations
of <15 mg/L. Therefore, 30 and 80 mg/L Cr(VI) (R4 and
R6) were selected to explore the selection of the different
NO3
–-N removal pathways. The pathway
of NO3
–-N reduction at 30 mg/L Cr(VI)
exhibited continuous adaptation, wherein the coexistence of denitrification
(51.7%) and DNRA (13.6%) was achieved by regulating the distribution
of denitrifiers (37.6%) and DNRA bacteria (32.8%). Comparatively,
DNRA gradually replaced denitrification at 80 mg/L Cr(VI). The intracellular
Cr(III) accumulation in R6 was 6.60-fold greater than in
R4, causing more severe oxidant injury and cell death.
The activated NO3
–-N reduction pathway
depended on the value of nitrite reductase activity/nitrate reductase
activity, with 0.84–1.08 associated with DNRA activation and
1.48–1.57 with DNRA predominance. Although Cr(VI) increased
microbial community richness and improved community structure stability,
the inhibition or death of nitrogen-reducing microorganisms caused
by Cr(VI) decreased NO3
–-N reduction
efficiency.