Iron(III) (oxyhydr)oxides can represent the dominant microbial electron acceptor under anoxic conditions in many aquatic environments which makes understanding the mechanisms and processes regulating their dissolution and transformation particularly important. In a previous laboratory-based study, it has been shown that 0.05 mM of thiosulfate can reduce 6 mM of ferrihydrite indirectly via enzymatic reduction of thiosulfate to sulfide by the sulfur-reducing bacterium Sulfurospirillum deleyianum followed by abiotic reduction of ferrihydrite coupled to re-oxidation of sulfide. Thiosulfate, elemental sulfur, and polysulfides were proposed as re-oxidized S-species functioning as electron shuttles. However, the exact electron transfer pathway remained unknown. Here, we present a detailed analysis of the sulfur species involved. Apart from thiosulfate, also substoichiometric amounts of sulfite, tetrathionate, sulfide, or polysulfides initiated ferrihydrite reduction. The share of thiosulfate produced during abiotic ferrihydrite-dependent re-oxidation of sulfide was about 10% of total sulfur at maximum. The main abiotic oxidation product was elemental sulfur attached to the iron mineral surface, which indicates that direct contact between the microorganisms and ferrihydrite is necessary for maintaining the iron reduction process. Polysulfides were not detected in the liquid phase. Minor amounts were found associated either with microorganisms or the mineral phase. The abiotic oxidation of sulfide in the reaction with ferrihydrite was identified as rate-determining. Cysteine, added as sulfur source and reducing agent, also lead to abiotic ferrihydrite reduction and therefore should be eliminated when sulfur redox reactions are investigated. Overall, we could demonstrate the large impact of small amounts of intermediate sulfur species on biogeochemical iron transformations.
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