BayCEER Workshop 2026

08. October 2026

Microbial Organic Matter Redirects Sulfur Fate and Suppresses Pyrite Formation during Sulfidation of Ferric Oxyhydroxides

Xiaoqiao Tang¹, Kerstin Hockmann², Martin Obst³, Laurel ThomasArrigo⁴, Mareike Lacina⁴, Fatih Sekerci⁵, Muammar Mansor⁵, Andreas Kappler⁵, Stefan Peiffer¹

¹ Department of Hydrology, BayCEER, University of Bayreuth, 95447 Bayreuth, Germany
² Applied Geochemistry, Institute of Earth and Environmental Sciences, University of Freiburg, 79104 Freiburg, Germany
³ Experimental Biogeochemistry, BayCEER, University of Bayreuth, 95447 Bayreuth, Germany
⁴ Environmental Chemistry, Institute of Chemistry, University of Neuchâtel, 2000 Neuchâtel, Switzerland
⁵ Geomicrobiology, Department of Geosciences, University of Tübingen, 72076 Tübingen, Germany

P 10 in Posters

Pyrite is the thermodynamically stable end product of iron and sulfur cycling and represents a major sink for both elements in reducing environments. Pyrite can rapidly form through the ferric-hydroxide-surface pathway during ferric oxyhydroxide sulfidation under ambient conditions. However, ferric oxyhydroxides in natural environments are frequently associated with microbially derived organic matter, and its influence on abiotic pyrite formation remains poorly understood.

Here, we investigated the influence of microbial organic matter on sulfidation pathways by reacting synthetic biogenic ferric oxyhydroxides, natural biofilms, and pure synthetic ferrihydrite with aqueous sulfide under anoxic conditions at pH 6. Iron and sulfur transformations over three weeks were characterized using wet chemistry, Raman micro-spectroscopy, X-ray diffraction and iron X-ray absorption spectroscopy.

The results show that pyrite formed rapidly within 5–7 days in the pure ferrihydrite system, accompanied by ferrihydrite transformation into lepidocrocite and goethite. In contrast, no pyrite formation was observed in systems containing microbial organic matter; instead, sulfide was stabilized as elemental sulfur, primarily in term of orthorhombic sulfur (S₈). Additionally, in the biogenic ferric oxyhydroxide system, phosphate introduced from the cultivation medium was sequestered as vivianite, while the presence of microbial organic matter inhibited ferrihydrite transformation into more crystalline secondary minerals.

Our findings demonstrate that microbial organic matter redirects iron and sulfur transformation pathways, suppressing abiotic pyrite formation by stabilizing intermediate sulfur species and regulating iron mineral evolution. This study highlights the importance of microbial organic matter in controlling pyritization pathways and iron-sulfur cycling in natural environments.