LAUSR

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.