Antimonate Reduction Coupled to Anaerobic Ammonium Oxidation in Paddy Soil: Process Evidence and a Putative Syntrophic Microbial Model

The coupling of metal(loid) (e.g., Fe(III) and As(V)) reduction with anaerobic ammonium oxidation (anammox) is emerging as a critical process impacting the fate of N and metal(loid)s. Despite the chemical analogs of As and Sb, Sb(V) reduction exhibits different thermodynamics from As(V) reduction, which may constrain its coupling with anammox (termed “Sbammox”) and impose stricter limitations on the metabolic pathway. To determine the occurrence and mechanism of Sbammox, Sb-contaminated paddy soil was used to establish the microcosms. Using 15 N isotope tracing, we confirmed the existence of Sbammox with the synchronous 15 N–N 2 and Sb(III) productions and their concurrent suppressions by the inhibitor acetylene (C 2 H 2 ). In contrast to the single-species-driving Asammox and Feammox, a tripartite syntrophic consortium was proposed to mediate Sbammox by DNA-stable isotope probing (SIP) combined with amplicon sequencing and metagenomic analysis. In this consortium, Ramlibacter and Candidatus Brocadia are proposed as the candidate Sb(V) reducer and ammonium oxidizer, respectively, with Geobacter hypothesized to mediate interspecies electron transfer. This distinct microbial strategy suggests that the specific thermodynamic constraints of Sb(V) necessitate a cooperative strategy rather than a solitary metabolic pathway. These findings are essential for understanding the divergent biogeochemical behaviors of As and Sb and underscore a critical dual risk in exacerbating nitrogen loss and Sb toxicity in agro-ecosystems.