Natural organic matter coating changes the role of iron oxides in carbon preservation in marine sediments

Yunru Chen, Heidi Taubner, Verena B. Heuer, Jenny Altun, Rianne van Kaam, Lars Robben, Wolfgang Bach, Lea C. Wunder, Graciana Willis Poratti, Sabine Kasten, Matthias Zabel, Kai-Uwe Hinrichs, Michael W. Friedrich

Preservation of organic carbon (OC) in marine and terrestrial depositional settings is enhanced by the association between OC and reactive iron oxides. However, microbially mediated iron reduction in anoxic marine sediments may lead to the reductive dissolution of iron oxides and consequently to the remobilization of iron-associated organic matter. The underlying mechanism remains poorly studied. Here, we studied the interaction between ferrihydrite (Fh) and 13C-acetate using incubations with marine surface sediments. We found that when iron reduction was inhibited, adding pure Fh protected 51% more acetate from degradation compared to the incubation without Fh addition. In contrast, adding natural organic matter (NOM)-coated Fh protected 102% more acetate, likely due to a larger surface area from smaller aggregate sizes. When iron reduction was occurring, we observed significant differences. While only a limited amount of Fh-associated 13C-acetate was remobilized from uncoated Fh, 76% of the Fh-associated 13C-acetate was remobilized from NOM-coated Fh and remineralized by microorganisms. These results suggest that natural organic matter coating strongly alters the aggregate size of iron oxides and increases their surface area, enhancing their capability to adsorb organic molecules. However, much of these adsorbed molecules is subsequently remobilized through microbially mediated iron reduction. Therefore, both aggregate structure and microbial processes dynamically controlled the fate of iron-associated organic matter, which need to be considered when assessing the role of iron oxides in long-term carbon preservation.