Community dynamics and activity of nirS-harboring denitrifiers in sediments of the Indus River Estuary

Denitrification is an important pathway for reactive nitrogen removal from aquatic ecosystems. In this study, the biodiversity, abundance, and activity of cytochrome cd₁-type nitrate reductase gene (nirS)-harboring denitrifiers in the sediments of the Indus River Estuary were examined by molecular and isotope-tracing techniques. Results showed that the nirS-harboring denitrifier communities showed significant geographical variations along the estuarine salinity gradient. Real-time quantitative PCR showed that the abundance of nirS-harboring denitrifiers ranged from 5.3 × 10⁶ to 2.5 × 10⁸ copies g⁻¹, without significant spatiotemporal variation. The potential rates of denitrification varied from 0.01 to 6.27 μmol N kg⁻¹ h⁻¹ and correlated significantly to TOC and Fe(II) (P < 0.05). On the basis of ¹⁵N isotope-tracing experiments, the denitrification process contributed 18.4–99.4% to the total nitrogen loss in the sediments of the Indus River Estuary. This study provides novel insights into the microbial mechanism of nitrogen removal process in estuarine ecosystems.