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.