Anaerobic Microbial Degradation of Ferrihydrite-Adsorbed Organic Carbon:DOC and Fe(II) Release Mediated By Dissimilatory Iron Reduction.

C-mineral association, formed between iron oxide and dissolved organic carbon (DOC) occurs ubiquitously in natural environments, and this association is all the more important due to its control on transportation and sequestration of organic carbon. However, little is known about the stability of C-mineral association under reducing conditions and its implication on carbon turnover and cycling. In this study, anaerobic incubation experiments were conducted to investigate the biodegradability of ferrihydrite (Fe-oxide)-adsorbed organic carbon with well-known Fe-reducing bacterium, Shewanella oneidensis MR-1. Bulk dissolved organic carbon (DOC) were extracted from forest humified layer (HDOC) and forest litter layer (LDOC). Reductive dissolution of Fe(II) from C-ferrihydrite association was observed with both HDOC and LDOC, indicating that C-mineral association was inclined to decomposition by iron-reducing microorganisms. Despite same starting C concentration in both DOC, Fe(II) reduced by HDOC was 14% and 28% of LDOC,respectively. Meanwhile, an increase of 13% and 11% in DOC concentrations in ambient solution were observed with HDOC and LDOC. Our DOC spectroscopic data corroborated the field observations that dissimilatory iron reduction leads to a shift in DOC  properties towards more chemically recalcitrant C. This study directly linked DOC release with iron dissolution and thus suggested the anaerobic microbial iron reduction is an importance mechanism for Fe dissolution and C mobilization, and hence play an important role in carbon transportation and storage, especially in sedimentary and wetland system.