Chemoautotrophic Carbon Fixation Favors Iron-Bound Organic Carbon Formation in Estuarine and Coastal Sediments

Chemoautotrophic carbon fixation (CCF) is crucial for carbon sequestration; however, how the CCF process mediates carbon preservation remains poorly understood. Here, we investigated the CCF process, iron-bound organic carbon (Fe-OC), and chemoautotrophs through FeS-amended and in situ field samples in estuarine and coastal sediments. The CCF rates and Fe-OC concentrations were substantially enhanced by FeS addition and were significantly higher under anaerobic conditions than under aerobic conditions. Sulfide rather than Fe²⁺ facilitated CCF rates, while more Fe-OC formation was caused by Fe²⁺. More humic substances, aromatic protein, and high diffraction intensity in the presence of FeS indicated that iron oxides protect organic carbon against microbial degradation. The enhanced abundance of functional genes in relation to nitrification, sulfide, and iron oxidation, as well as cbbL and cbbM genes synchronously, favored the CCF process and Fe-OC formation. The CCF rates and Fe-OC concentrations were also higher in Fe²⁺─and sulfide-enriched field sediments, and a positive feedback relationship was observed between Fe-OC formation and the CCF process. These results highlighted that the chemoautotrophic process favors Fe-OC formation, in which the enhanced Fe-OC may restrain microbial degradation of organic carbon, thereby facilitating carbon retention and preservation. This preservation mechanism provided a robust pathway for carbon sequestration in estuarine and coastal wetlands, representing a negative climate warming feedback loop.