Tide-driven biogeochemical gradient shapes the vertical profile of dark carbon fixation in intertidal wetlands

Dark carbon fixation (DCF) is recognized as a vital carbon sink process in the biogeochemical cycle of coastal wetlands. Tidal hydrodynamic dynamics in intertidal wetlands significantly affect the physicochemical conditions, yet their impacts on DCF activities and the associated chemoautotrophs remain unclear. Here, we investigated the vertical distribution patterns of DCF rates and the dynamics of chemoautotrophs in intertidal sediments using ¹⁴C isotope tracing and metagenomic techniques. Results showed that DCF rates peaked in the subsurface layer (10–35 cm depth, 110.13 ± 28.71 mmol C m⁻³·d⁻¹), but were relatively lower in the surface layer (0–10 cm, 20.19 ± 5.93 mmol C m⁻³·d⁻¹) and the deep layer (80–100 cm, 13.79 ± 1.28 mmol C m⁻³·d⁻¹). Higher DCF activities in the subsurface layer might be attributed to more sufficient reducing substrates such as sulfide and ferrous iron (Fe²⁺), as well as the redox gradient shaped by tidal fluctuations. Metagenomic analysis highlighted that the Wood-Ljungdahl (WL) cycle was the predominant carbon fixation pathway for chemoautotrophs in intertidal sediments. The estimated DCF flux measured in the present study (50.96 mmol C m⁻² d⁻¹) was significantly higher than previous reported rates obtained exclusively from the surface sediments. In conclusion, this study enhances our understanding of the complex DCF process and provides scientific support for a more accurate evaluation of the carbon sink potential in intertidal wetlands.