Dissolved organic matter, a complex mixture of carbon-containing molecules found in soils, rivers, lakes, wetlands, and sediments, plays a central role in how carbon moves through the environment. It can feed microbes, bind pollutants, influence nutrient availability, and affect how much carbon is stored or released as carbon dioxide. Yet scientists are still working to understand why some dissolved organic matter is quickly consumed by microbes, while other portions persist much longer.
A new study published in Carbon Research offers fresh insight into this question by showing how iron oxide minerals can reshape dissolved organic matter before microbes begin to break it down. The research focuses on goethite, a common iron oxide mineral found in soils and aquatic environments, and reveals that mineral adsorption does not simply remove organic matter from water. Instead, it selectively sorts organic molecules, changing what remains available for microbial degradation.
"Our study shows that minerals and microbes should not be treated as separate controls on dissolved organic matter," said the study's corresponding authors. "Iron oxides can first filter the molecular composition of organic matter, and this filtering process then influences which microbes become active and how fast carbon is transformed."
The researchers extracted dissolved organic matter from forest soil and exposed it to goethite under two pH conditions, 4.5 and 6.5. They then incubated the original and mineral-fractionated samples with native soil microbes for 63 days. To track what happened, the team combined ultraviolet-visible spectroscopy, fluorescence spectroscopy, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry, and 16S rRNA gene sequencing.
The results showed that goethite preferentially adsorbed aromatic, high-molecular-weight compounds, including lignin-like, tannin-like, and condensed aromatic molecules. These compounds are often more resistant to microbial degradation. In contrast, more biodegradable components, such as proteins, aliphatics, and some low-molecular-weight molecules, were enriched in the remaining solution. This effect was stronger at lower pH.
That mineral-driven sorting had major consequences for biodegradation. Dissolved organic matter fractionated at pH 6.5 showed the greatest overall degradation, with dissolved organic carbon loss reaching about 63.1 percent by Day 63. The pH 4.5 fractionated sample, however, degraded more rapidly at first, reaching about 52.4 percent loss by Day 49, before declining later as the easily degradable pool was depleted. The authors suggest that this later decline reflected microbial cell death and release of intracellular materials back into the solution.
The study also revealed a clear sequence in microbial feeding behavior. Microbial communities first consumed protein-like and lipid-like compounds, then shifted toward quinone-like molecules, and later made greater use of humic-like substances such as lignins. Different bacterial groups were linked to different types of organic matter. Gammaproteobacteria and Actinobacteria were mainly associated with degradation of labile protein-like and lipid-like fractions, while Alphaproteobacteria, Acidimicrobiia, Planctomycetes, and related groups became more important as humic-like compounds accumulated.
These findings are important because iron oxides are widespread in natural and engineered environments. By changing which organic molecules stay dissolved and which are removed, minerals may influence whether carbon is rapidly respired by microbes, transported through water, or stabilized for longer periods.
The study provides a more detailed molecular picture of how mineral surfaces and microbial communities work together to regulate carbon cycling. It may help improve predictions of carbon fate in iron-rich soils, wetlands, sediments, and water treatment systems, especially under changing pH conditions.
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Journal reference: Liang, Y., Liu, T., Cen, Z. et al. Iron oxide fractionation alters the biodegradability of dissolved organic matter: molecular dynamics and microbial interactions. Carbon Res. 5, 28 (2026).
https://doi.org/10.1007/s44246-026-00272-6
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About Carbon Research
The journal Carbon Research is an international multidisciplinary platform for communicating advances in fundamental and applied research on natural and engineered carbonaceous materials that are associated with ecological and environmental functions, energy generation, and global change. It is a fully Open Access (OA) journal and the Article Publishing Charges (APC) are waived until Dec 31, 2025. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon functions around the world to deliver findings from this rapidly expanding field of science. The journal is currently indexed by Scopus and Ei Compendex, and as of June 2025, the dynamic CiteScore value is 15.4.
