Intertidal marshes are exposed and submerged alternately due to periodic ebb and flood tides,and the tidal cycle is important in controlling biogeochemical processes in these habitats.The intertidal sediments have been identified as important hotspots of interacting nitrogen cycle microorganisms including the recently discovered anammox bacteria.Nevertheless,the tidal effects on anammox in these habitats remained largely unexplored.Here we used isotope-tracing and molecular approaches simultaneously to show that anammox bacterial abundance and activity were enhanced at sediment-tidal water interface and at tidal-induced groundwater fluctuating layers.