Evidences of the electrogenic sulfur oxidation in constructed wetlands

The sulfur redox cycling, mainly involving sulfide oxidation and sulfate reduction, remains a crucial factor that regulates the treatment performance of constructed wetlands (CWs). However, anoxic environments normally prevail in the CW systems, harboring vast reduced sulfur and sulfur minerals, where the occurrence and mechanism of anoxic sulfide oxidation remain unknown. In this study, CW microcosms filled with quartz sand (Qtz) and pyrite (Pyt) were established to investigate the anoxic sulfur oxidation under the bioelectrochemical manipulations. As a result, the δ³⁴S-sulfate increased from 8.75 ± 0.29‰ in the influent to 51.74 ± 16.21‰ (Pyt) and 46.12 ± 25.95‰ (Qtz) at the anoxic zone under the open-circuit condition, and to 34.50 ± 6.99‰ (Pyt) and 42.53 ± 19.59‰ (Qtz) at that under the close-circuit conditions. This suggested the concurrent sulfate reduction and electrogenic sulfide oxidation in the systems. Based on a modified isotopic fractionation model, the electrogenic sulfide oxidation were further calculated (i.e., up to 87.58 and 265.13 mgS·m^-2·d^-1 in Qtz and Pyt, respectively). Results of fluorescence in situ hybridization and metagenomic analyses demonstrated the occurrence of filamentous bacteria belonging to Desulfobulbaceae, with relative abundances of 0.32 ± 0.00% and 0.88 ± 0.25% in the anoxic zones of Qtz and Pyt, respectively. Pyrite was found to upregulate the functional genes encoding microbial transformation of elemental sulfur, sulfide, and thiosulfate. Interspecies network analyses revealed mutual relationships between the filamentous bacteria and microorganisms involved in sulfur, iron, and carbon transformations. Together, this study provided new insights to the electrogenic sulfide oxidation in CWs.