DNRA in intertidal sediments of the Yangtze Estuary

Dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in regulating the fate of reactive nitrogen in estuarine and coastal ecosystems. In this work, intertidal sediments of the Yangtze Estuary were collected in January and August 2015. Potential rates of DNRA and associated functional gene were investigated with nitrogen isotope-tracing and molecular techniques. The measured DNRA rates ranged from 0.14 to 5.57 μmol ¹⁵N kg⁻¹ h⁻¹ in the intertidal sediments. DNRA rates were tightly related to abundance of nrfA gene (p < 0.001), demonstrating that fermentation reaction may be the dominant pathway of DNRA in the study area. Redundancy analysis (RDA) showed a relationship between DNRA and organic matter and NO₂ ⁻, suggesting that these substrates stimulated the metabolism of DNRA microorganisms. On the other hand, the correlation between abundances of nrfA gene and Fe²⁺ and sulfide in the RDA analysis implied that oxidation of both Fe²⁺ and sulfide can enhance fermentative DNRA by providing extra free energy. DNRA converted approximately 2.29 × 10⁵ t of nitrate to ammonia annually in the sampling area of the Yangtze Estuary, and most of the converted ammonium was retained in the estuarine ecosystem. DNRA may further contribute to eutrophication in the Yangtze Estuary and also in the other hypereutrophic estuaries.