Tidal pumping facilitates dissimilatory nitrate reduction in intertidal marshes

Intertidal marshes are alternately exposed and submerged due to periodic ebb and flood tides. The tidal cycle is an important factor controlling the biogeochemical processes of these ecosystems. Intertidal sediments are important hotspots of dissimilatory nitrate reduction and interacting nitrogen cycling microorganisms, but the effect of tides on dissimilatory nitrate reduction, including denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium, remains unexplored in these habitats. Here, we use isotope-tracing and molecular approaches simultaneously to show that both nitrate-reduction activities and associated functional bacterial abundances are enhanced at the sediment-tidal water interface and at the tide-induced groundwater fluctuating layer. This pattern suggests that tidal pumping may sustain dissimilatory nitrate reduction in intertidal zones. The tidal effect is supported further by nutrient profiles, fluctuations in nitrogen components over flood-ebb tidal cycles, and tidal simulation experiments. This study demonstrates the importance of tides in regulating the dynamics of dissimilatory nitrate-reducing pathways and thus provides new insights into the biogeochemical cycles of nitrogen and other elements in intertidal marshes.
Vertical profiles of dissimilatory nitrate-reduction activities in intertidal marshes. I. Schematic of the sampling cores at low tide (a) and high tide (b) in intertidal marshes. II. The vertical distribution patterns of potential denitrification (DNF) rates (a), anammox (ANA) rates (b), DNRA rates (c), and their relative contributions to total nitrate reduction in April (d) and October (e), respectively. SWI: Sediment-water interface; SCL: SWI controlled layer in which sediment is primarily affected by overlying tidal water over tidal cycles; TL: Transition layer; GCL: Groundwater controlled layer in which sediment is primarily affected by groundwater fluctuation over tidal cycles. Sediment-layer identification (SCL, TL, and GCL) is based on the depth distributions of sediment water content (Supplementary Fig. 2). Error bars indicates s.d. (n = 3).
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