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Sulfur autotrophic denitrification utilizes elemental sulfur as the electron donor for nitrate removal from aquatic environments. Organic carbon could stimulate the conversion of sulfur and facilitates the S0-based denitrification process in the mix-trophic. In this study, the co-cultured system of sulfur reducer (Geobacter sulfurreducens) and Thiobacillus denitrificans was used to investigate that how organic carbon could boost the S0-based denitrification. The results showed that the rate of S0-based denitrification was improved with C/N ratio of 0.13 and this improvement continued even after the acetate was exhausted. Sulfur probe test and Raman analysis suggested that reduced sulfur species (S x 2−) were formed with the addition of organic carbon. The S x 2− could recombine with element sulfur and the bioavailability of S0 would be improved, as a result, the rate of S0-based denitrification increased as well. Nitrate reduction rate could further increase with the C/N ratio of 0.88, but it would decrease significantly when the C/N ratio increased to 1.50 as the high concentration of generated S2−. Our results provided explanations that why organic carbon addition would improve the bioavailability of S0 which could further promote the S0-dominant denitrification process. Unlabelled Image • Organic carbon can boost the S0-based denitrification of co-culture system. • Geobacter sulfurreducens could convert S0 to reduced sulfur species (S x 2−). • Sulfur bioavailability could be improved by the sulfur conversion process. [ABSTRACT FROM AUTHOR]