Potential roles of anaerobic ammonium oxidation (anammox) in overlying water of rivers with suspended sediments Sibo Zhang . Xinghui Xia . Ting Liu . Lingzi Xia . Liwei Zhang . Zhimei Jia . Yayuan Li Received: 27 May 2016 / Accepted: 20 January 2017 / Published online: 9 February 2017 Ó Springer International Publishing Switzerland 2017 Abstract Anaerobic ammonium oxidation (anam- mox) is believed to be an important sink for fixed inorganic nitrogen in terrestrial and aquatic ecosys- tems, and many studies have reported that macroscale oxic–anoxic interfaces, such as riparian zones, were hotspots of anammox reaction. However, no research has linked microscale interfaces with the anammox process in natural environments. This study provides evidence for the presence of anammox bacteria and potential anammox activity on the suspended sedi- ment (SPS) in the oxic water of the Yellow River. The anammox bacteria in the overlying water were mainly attached to SPS. The abundance of anammox bacteria in the overlying water was positively correlated with SPS concentration (R 2 = 0.97, P \ 0.01), with abun- dance ranging from 9.5 9 10 2 to 1.5 9 10 4 hydrazine synthase gene copies per g of SPS. Phylogenic analysis of anammox bacteria revealed that the SPS phase was dominated by Candidatus Brocadia. Candidatus Scalindua genera was detected in this study with a conductivity of 1100 lS cm -1 . More- over, 15 NH þ 4 -amended anaerobic incubation of the overlying water showed that the average potential anammox activity was 0.076 nmol-N L -1 day -1 . The 15 N labeling simulation experiments demonstrated the occurrence of anammox in the oxic water of the Yellow River. This study suggests that the anammox process at the SPS–water interface might be a non- negligible pathway for the loss of fixed nitrogen in natural freshwaters, but this remains to be determined in further studies. Keywords Anammox Á Oxic–anoxic interface Á Suspended sediment Á Freshwater Á Nitrogen loss Introduction Anaerobic ammonium oxidation (anammox) is the biological conversion of ammonium and nitrite into dinitrogen gas (NH þ 4 þ NO À 2 ! N 2 þ 2H 2 O) (Kartal et al. 2011). So far, known anammox species are divided into five genera, ‘Brocadia’, ‘Kuenenia’, ‘Anammoxoglobus’, ‘Jettenia’, and ‘Scalindua’. None of these anammox bacteria have been obtained as classical pure cultures, and so all have the taxonomical status of ‘Candidatus’ (Kartal et al. 2013). At present, the anammox process is identified as an important pathway of global nitrogen loss, responsible for up to Responsible Editor: Breck Bowden Electronic supplementary material The online version of this article (doi:10.1007/s10533-017-0297-x) contains supple- mentary material, which is available to authorized users. S. Zhang Á X. Xia (&) Á T. Liu Á L. Xia Á L. Zhang Á Z. Jia Á Y. Li School of Environment, Beijing Normal University/State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China e-mail: xiaxh@bnu.edu.cn 123 Biogeochemistry (2017) 132:237–249 DOI 10.1007/s10533-017-0297-x