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Anaerobic ammonium oxidation (anammox) is an important process in the nitrogen cycle and anammox bacteria have been widely detected in many natural environments.However,their presence in riverine ecosystems has long been overlooked,and their contribution to the nitrogen cycling in natural and river sediments is currently unknown.This study investigated the diversity,composition,distribution and abundance of anammox bacteria in the nutrient gradient sediments of the Dongjiang River by molecular ecology analysis.Both the selected 16S rRNA and hzo gene molecular biomarkers indicated the ubiquitous occurrence of anammox bacteria in sediment samples and the main phylotypes are closely related to the known anammox bacterial genera Candidatus Brocadia,Candidatus Jettenia and Candidatus Anammoxoglobus.The cluster A of 16S rRNA gene sequences showed distant identity (<94%) to the known Candidatus Jettenia species,inferring this cluster of anammox bacteria to be a new species.These suggest a higher diversity of anammox bacteria in riverine than marine ecosystems and could be a consequence of the larger variety of suitable niches in the river sediments.Moreover,as opposed to marine habitats where Candidatus Scalindua dominates anammox guilds,Jettenia and Brocadia appear to be the most common representatives in riverine ecosystem.The abundance of anammox bacteria in the sediments showed a heterogeneous distribution and positive correlation with the nitrite concentration along the river,while the diversity of anammox bacteria was negatively correlated with the nitrite content.Redundancy analysis (RDA) indicated that the ratio of the total nitrogen (TN) to the total carbon (TC),and the concentrations of nitrate and nitrite in the sediment showed significant impact on the composition,structure and distribution of the sediment anammox bacterial community.Thus,this study extended a better understanding of anammox bacterial diversity and their ecological distribution in the riverine ecosystem.