Microbial Reduction of Mineral-Organic Matter Complexes on a Molecular Scale

Researchers took a detailed look at anaerobic microbial transformations of organic matter complexed with ferrihydrite, a common iron mineral.

The Science

In soil, common clay minerals and iron oxide form complexes with carbon-containing organic matter. These complexes stabilize organic matter, thus reducing carbon dioxide emissions from soil. However, in environments without oxygen, microbes can reduce the ferric iron atoms, destabilizing the organic matter complexes. To better understand the stability of these complexes in anoxic environments, a team of scientists studied how the clay component of these three-part complexes influenced molecular transformations of the iron mineral and organic matter in the presence of a well-known iron-reducing microbe.

The Impact

Mineralization of organic matter could account for more than 40%of total anaerobic oxidation in forest soils, but transformations of clay-iron mineral-organic matter complexes have not been well characterized on the molecular level. This work provides more detail about conditions that release organic matter from these complexes and make it available for microbial transformations.


Researchers at China University of Geosciences, along with researchers at the Environmental Molecular Science Laboratory (EMSL) and China University of Petroleum, studied how the presence of clay influences the release of organic matter from complexes with an iron-based mineral.

Previous studies have not looked at the molecular-level interactions between complexes containing these three components, particularly in an oxygen-free environment in the presence of a known iron-reducing microbe. The researchers used EMSL’s time-of-flight secondary ion mass spectrometry (ToF-SIMS) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to determine molecular-level changes in organic matter before and after microbial reduction in the presence of hydrogen gas. The work was part of EMSL’s Biogeochemical Transformations Integrated Research Platform.

The researchers found that clay initially decreased the amount of organic matter associated with the iron mineral; then in a second reduction phase, the clay increased the amount of organic matter released from these complexes. They also identified microbial metabolic products from the reduction of organic matter bound in these complexes. Future work could examine how multiple species of microbes affect the stability of these clay-iron mineral-organic matter complexes.

Principal Investigator(s)

Hailiang Dong
China University of Geosciences


This work was supported by the National Natural Science Foundation and National Key Research and Development Program of China. A portion of the research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a U.S. Department of Energy (DOE) Office of Science user facility, sponsored by the Office of Science’s Office of Biological and Environmental Research.


Zeng, Q., Huang, L., Ma, J. et al. “Bio reduction of ferrihydrite-montmorillonite-organic matter complexes: Effect of montmorillonite and fate of organic matter.” Geochemica et Cosmochimica Acta 276, 327–344 (2020). [DOI:10.1016/j.gca.2020.03.011]