Comparison of sulfur cyclings coupled with anaerobic ammonium oxidation in response to different river remediations

The evolution from ‘malodor’ to ‘algae bloom’ in remediated urban rivers has received an ever-increasing attention due to relatively high nitrogen (N) loadings. As pivotal autotrophic processes, anaerobic ammonium oxidation (anammox) and sulfur-driven denitrification (SDD) represent promising candidates for N removal in remediated rivers with low C/N ratios. However, how and to what extent SDD is coupled with anammox remained largely unknown. Our investigations of four remediated urban rivers revealed that SDD was more prevalent than anammox, with Thiobacillus (0.13 %–2.51 %) dominating over Candidatus_Brocadia (0–0.02 %). The injection of Ca(NO₃)₂ greatly enhanced the coupling of SDD and anammox (SDDA) in anaerobic environments, achieving a maximum total nitrogen (TN) removal of 93.89 %. Metagenomic and metatranscriptomic analyses identified Thiobacillus, Thermomonas, and Candidatus_Brocadia as key microbial players, with their activities increased by 0.060 %, 0.015 %, and 0.498 %, respectively. Differently, Sulfurisoma, Dechloromonas, and Candidatus_Scalindua emerged as key players in Na₂S₂O₃-group, while Sulfurisoma, Sulfurimonas, and Candidatus_Scalindua played pivotal roles in FeS₂-group. Additionally, river simulations revealed that Na₂S₂O₃-group showed the strongest SDDA coupling, supported by the highest abundances of soxB (0.14 %), narG (0.05 %), nirS (5.92 %), and hzsB (6.14 %). The FeS₂-group demonstrated moderate coupling, whereas the Ca(NO₃)₂-group displayed the weakest performance. Moreover, Na₂S₂O₃-group also exhibited excellent TN removal (87.58 %) in real river scenarios, indicating its potential as one promising N removal strategy for practical application. This study contributes to the understanding of S-N cyclings in river ecosystems and provides insights into manipulating N-reduction for possible application in remediated urban rivers.