Anaerobic ammonium oxidation (anammox) bacterial diversity, abundance, and activity in sediments of the Indus Estuary

Anaerobic ammonium oxidation (anammox) is an important bioprocess for nitrogen removal and has been studied in estuarine environments. However, knowledge on anammox bacterial community dynamics and related controlling factors remains limited in these ecosystems. In this study, the community compositions, abundance, and activity of anammox bacteria in the surface sediments from the Indus Estuary were investigated along a salinity gradient, considering the links between the anammox bacterial community dynamics and environmental variables. The potential importance of anammox was also estimated for nitrogen removal. High anammox bacterial diversity was detected in the sediments of the Indus Estuary, including Kuenenia, Brocadia, Scalindua, Jettenia, and a novel anammox-like cluster. Kuenenia was identified as the dominant anammox bacteria in most samples. Anammox bacterial diversity was significantly correlated with sediment NO₃⁻, while the distribution of anammox bacterial community was significantly related to temperature and sediment sulfide (P < 0.05). The anammox bacterial abundance based on the 16S rRNA gene varied between 1.64 × 10⁶ copies g⁻¹ and 8.21 × 10⁸ copies g⁻¹, and was significantly correlated with sediment Fe(II). Based on an ¹⁵N isotope-tracing technique, potential anammox rates were found in the range 0.01–0.32 μmol N kg⁻¹ h⁻¹, and were controlled mainly by salinity, Fe(II), and TOC. It was estimated that the anammox bacteria contributed about 21.9% to the total nitrogen loss, on average. These results show the importance of anammox bacteria for nitrogen transformation and removal in estuarine and coastal environments.