Enhanced Coupling of Fe(II) Oxidation and Nitrate Reduction Benefits Chemoautotrophic Carbon Fixation in Estuarine and Coastal Sediments

Fe(II) has exerted a profound impact on chemoautotrophic carbon fixation (CCF); however, how NO₃^–mediates Fe(II) oxidation and the CCF process in estuarine and coastal sediments remains poorly understood. Here, the coupling mechanisms of microbially mediated NO₃^–-reducing Fe(II) oxidation and the CCF process were examined through anoxic incubation experiments. Three field sites of Xitan, Dongtan, and Luchaogang spanning the environmental gradients of salinity, NO₃^–, and Fe(II) in the Yangtze Estuary were investigated. Microbial NO₃^–reduction was significantly enhanced in the presence of Fe(II), with accompanying yields of NO₂^–and N₂O. The CCF rates increased by 4.3–31% under Fe(II) addition alone and increased by 69–156% under the combination of Fe(II) and NO₃^–, indicating that Fe(II) oxidation enhanced by NO₃^–reduction favors the chemoautotrophic process. Secondary bacteria-iron mineral complexes were produced during microbial NO₃^–reduction, facilitating cell encrustation formation and carbon preservation. Fe(II) and NO₃^–were observed to increase diversities and abundances of denitrifying and carbon-fixing communities, which were biological factors mediating the favored denitrification and CCF rates. In addition, the enhanced networks and connections of denitrifying and carbon-fixing communities suggested that Fe(II) could be a crucial intermediator linking denitrification and chemoautotrophic processes. These results highlighted that the enhanced coupling of Fe(II) oxidation and NO₃^–reduction benefits the chemoautotrophic process, which plays a crucial role in simultaneously alleviating nitrogen pollution and carbon emissions in estuaries and coastal environments.