Elemental sulfur facilitates co-metabolism of Cr(VI) and nitrate by autotrophic denitrifiers in constructed wetlands

The antagonistic microbial reduction of chromate and nitrate poses significant challenges for their simultaneous removal in autotrophic denitrifying systems. This study explored the treatment performance and mechanisms for the simultaneous removal of chromate (0, 2, and 10 mg·L^-1) and nitrate (20 mg·L^-1) in constructed wetland (CW) microcosms with elemental sulfur additions at 0-, 5-, and 25-times background levels. Results showed that sulfur-amended microcosms achieved chromate and nitrate removal up to 76.46 % and 28.12 % higher, respectively, than 0S groups. Notably, the nitrate removal rate constant was higher in the presence of chromate than its absence, exclusively in the 25S groups. In a sediment core assay, the 25S groups also exhibited substantially facilitating effect of chromate removal potential in the presence of nitrate. Metagenomic analyses revealed upregulations of denitrification and sulfur oxidation-related functional genes along with sulfur supplementation. Autotrophic denitrifiers including Dechloromonas, Thiobacillus, Sulfuricella, and Sulfuritalea made significant contributions to chromate and nitrate removal rates, as well as functional genes encoding sulfur, nitrogen, and chromium transformation, in response to sulfur addition. These findings shed first light on the co-metabolism of chromate and nitrate by sulfur-based autotrophic denitrifiers, emphasizing their pivotal role in denitrification systems, e.g., CWs, with chromate inputs.