Anaerobic ammonium oxidation coupled to arsenate reduction, a novel biogeochemical process observed in arsenic-contaminated paddy soil

Anaerobic ammonium oxidation represents an important pathway of N loss, which can be coupled with reduction of nitrite and metal(loid)s (e.g., Fe(III) and Mn(IV)). Similar to Fe(III) and Mn(IV), As(V) is also an active metal(loid) and ammonium oxidation coupled with As(V) reduction is thermodynamically feasible. However, little is known about this potential process. In this study, anaerobic ammonium oxidation coupled with As(V) reduction, designated as Asammox, was observed in cultures inoculated from As-contaminated paddy soil using ¹⁵N isotope tracer analysis. Compared with the treatment amended with ¹⁵N-urea only, the production of ¹⁵N-labeled N₂ (i.e., ³⁰N₂ and ²⁹N₂) was significantly greater in the treatment amended with As(V) and ¹⁵N-urea. Furthermore, the abundances of the genes encoding for arsenate reductase (arrA) and hydrazine synthase (hzsB) were significantly higher in the treatment amended with As(V) and ¹⁵N-urea than those in the treatment amended with ¹⁵N-urea only. In addition, putative Asammox bacteria affiliated with Halomonas, Pelagibacterium, and Chelativorans were identified by DNA-stable isotope probing. Members of Ca. Brocadia were the most dominant Anammox bacteria in the soil cultures and may interact with Asammox bacteria in ammonium oxidation, suggesting that the N loss may be attributed to the contribution of Asammox and Anammox in the As-contaminated soil. The observation of Asammox, a novel biogeochemical process, and identification of bacteria responsible for this biogeochemical process expands the fundamental understanding of both N and As biogeochemical cycling. In addition, this study provides a proof-of-concept for investigating anaerobic ammonium oxidation coupled with metal(loid)s reduction by combining stable isotope probing and isotope tracer microcosm incubations.