Anammox coupled with arsenate reduction

Given the active As(V) reduction and ammonium oxidation in rice paddies, it is proposed that a novel biogeochemical process, anaerobic ammonium oxidation coupled with As(V) reduction (Asammox), may occur in As-contaminated rice paddies. In this study, Asammox was observed in microcosms inoculated from the As-contaminated paddy soil using 15N isotope tracer analysis. Compared with the treatment amended by 15N-urea only, the production of 15N-labeled N2 (i.e., 30N2 and 29N2) was significantly greater in the treatments amended by As(V) and 15N-urea. In addition, the abundances of the genes encoding arsenate reductase (arrA) and hydrazine synthase (hzsB) were also significantly higher in this treatment than that in the treatment with 15N-urea only. Moreover, putative Asammox bacteria, taxonomically affiliated with Halomonas, Pelagibacterium, and Chelativorans, were identified by DNA-stable isotope probing. In addition, bacteria associated with Ca. Brocadia were the most dominant Anammox bacteria and may interact with Asammox bacteria together to oxidize ammonium. It is proposed that bacteria responsible for Asammox and Anammox co-occurred in these cultures, suggesting that the N loss may be attributed to the contribution of Asammox and Anammox bacteria. The observation of the Asammox, a novel biogeochemical process, and identification of bacteria responsible for this biogeochemical process expand the fundamental understanding of both N and As biogeochemical cycling.