Dynamics of ferrihydrite-bound organic carbon during microbial Fe reduction

The dynamics of iron (Fe)-bound organic carbon (OC) during dissimilatory microbial Fe(III) reduction has the potential to play an important role in regulating the biogeochemical cycling of carbon (C) in permanently or transiently anoxic soils and sediments. In this study, we investigated the release and transformation of ferrihydrite (Fh)-bound OC during microbial reduction of Fe by Shewanella putrefaciens strain CN32 under a fixed Fe concentration of 13 mM and varying C/Fe molar ratios. We found that reduction of Fe and reductive release of OC was dependent on the C/Fe molar ratio, with high C/Fe ratio enhancing both reduction of Fe and release of OC. For Fh-OC co-precipitates with C/Fe ratio of 3.7, 54.7% of Fh-bound OC was released to solution phase when 25.1% of Fe was reduced. The presence of OC inhibited the transformation of Fh to more crystalline Fe phases both in the bulk and on the surface. Upon reduction, Fh-bound OC became more concentrated on the surface of Fh-OC co-precipitates, and surface components were enriched with carboxylic functional groups. Reduction increased the lability of Fh-bound OC for Fh-OC co-precipitate with C/Fe ratio of 3.7, and aromatic OC was preferentially retained within the co-precipitates. Our results indicate that microbial reduction altered the quantity and composition of OC released from Fh-OC co-precipitates, depending on the C/Fe ratio and associations between Fe and OC. Assuming higher availability of released OC compared to original Fh-bound OC, reduction of Fh can likely lead to enhanced degradation of OC and result in a shorter residence time for OC in soils and sediments.