Dynamics of Ferrihydrite-Bound Organic Carbon during Microbial Reduction.

Dynamics of iron (Fe)-bound organic carbon (OC) upon the microbial reduction of Fe play an important role in regulating the biogeochemical cycles of C. In this study, we investigated the release and transformation of ferrihydrite (Fh)-bound OC upon the microbial reduction. We found that the reduction of Fe and reductive release of OC was highly dependent on the C/Fe molar ratio. For Fh-OC complexes with C/Fe of 3.7, 54.7% of Fh-bound OC was released to solution phase, when 25% of Fe was reduced. Presence of OC inhibited the transformation of Fh to crystalline Fe oxide both in the bulk and on the surface. Upon reduction, Fh-bound OC was more enriched on the surface of Fh-OC complexes, and concentrated with carboxylic functional group for the surface component. Interactions between Fe and OC were weakened by the reduction, indicating by the decreased thermal resistance. Reduction increased the lability of Fh-bound OC, with more aromatic OC retained with the complexes. Our results indicate that microbial reduction significantly released, mobilized and transformed Fh-bound OC, depending on the OC/Fe ratio and associations between Fh and OC. Assuming higher availability of released and mobile OC compared to original Fh-bound OC, reduction of Fh can lead to substantial degradation of OC and its short residence time.