Earth Surface Science Institute seminar: Organic carbon transformation facilitated by Fe redox processes in minerals

Professor Hailiang Dong (Miami University) will present his research, part of the Earth Surface Science Institute seminar series.


Mineral–microbe interactions affect the biogeochemical cycling of iron and carbon in soils and sediments. Soil organic matter (SOM) is often associated with Fe-bearing minerals, forming mineral-OM complexes. Past studies have shown that microbial reduction of Fe (hydr)oxides results in destabilization of such complexes and mineral/OM transformation. However, much less is known about the role of clay minerals in such process, despite their ubiquity in soils and sediments. We have been studying the impacts of microbially-mediated Fe redox cycling on SOM transformation.

To study the impact of microbial Fe(III) reduction on SOM transformation, we synthesized ternary complexes of clay minerals, ferrihydrite, and SOM, and then microbially reduced such complexes using metal-reducing bacteria. Results showed that mineral-bound SOM served as electron donor to couple with reduction of structural Fe(III) in ferrihydrite and clays. As a result, mineral-bound SOM was transformed to microbial products, but compounds with aromatic structures, carboxyl groups and large molecular weight were more resistant to transformation, likely due to their strong binding with minerals.

To study the impact of microbial Fe(II) oxidation on SOM transformation, we pre-reduced Fe-bearing clay minerals using chemical and biological methods, and subsequently equilibrated with SOM. Upon oxidation in air without light, reactive oxygen species (ROS) were generated from reduced clays through Fenton reactions. ROS oxidized SOM to small-molecular-weight compounds, bleached chromophores and increased oxygen-containing functional groups of SOM. These changes resemble the photochemical transformation of SOM under light irradiation. This light-independent degradation of SOM may be responsible for the increased bio-availability of SOM as carbon substrates for microorganisms under redox-fluctuating conditions.


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