Sulfur in humin as a redox-active element for extracellular electron transfer

• Sulfur is a redox-active element in humin for extracellular electron transfer. • Oxidization states of sulfur in all humin ranged widely from −2 to +6. • Sulfur with oxidation states from −2 to +5 in the organic fraction supports humin function. • Sulfur plays important role in microbial network in geosphere. • Distribution of sulfur oxidation states in humin reflects the source environment. Humin (HM), an insoluble humic substance that occurs as an organo-mineral complex, mediates extracellular electron transfer (EET) among microorganisms in the geosphere. Although sulfur (S)-containing functional groups have been posited as possible redox-active groups in HM, there has been no direct evidence in this regard. Here, we characterized the redox-active S in HM from different natural sources using synchrotron radiation-based X-ray absorption spectroscopy in partial fluorescence yield mode. The S content in HM is less than 0.9%, with the oxidation states ranged widely from −2 to +6. About 9% to 19% of total S in HM was redox variable, corresponding to 1.2 × 10 −5 to 17.1 × 10 −5 Eq/g -HM redox capacity. The redox capacity of HM driven by S is comparable to or larger than the electron donating capacity of HM observed in anaerobic microbial dechlorination of pentachlorophenol, suggesting that S is one of the significant redox-active elements in HM. These findings provide new insights into the role of S cycling in biogeochemical processes and microbial energy networks.