Evidence for the occurrence of Feammox coupled with nitrate-dependent Fe (II) oxidation in natural enrichment cultures

Feammox is a newly discovered process of anaerobic ammonium oxidation driven by Fe(III) reduction. Nitrate-dependent Fe(II) oxidation (NDFO) is the coupling of Fe(II) oxidation and nitrate reduction to produce N-2 under anaerobic conditions. It has not been reported whether the coupling of the two reactions exists in natural enrichment. In this study, enrichment culture experiments were carrired out to prove the occurrence of Feammox with NDFO. The results indicated that the nitrogen and iron cycle were formed during natural enrichment cultures, including Fe(III) reduction and NH4+-N was oxidation to NO3--N, NO2--N and N-2, Fe(III) and Fe(II) were cyclically formed, and Fe(II) was oxidized with NO3--N reduced to N-2. The removal efficiencies of ammonium nitrogen and total nitrogen in the incubation were about 92.9% and 20% respectively. Organic carbon experiments indicate that sodium acetate can promote the initial NO3--N removal and a low concentration of organic carbon limited the NDFO process because iron-oxidizing bacteria are mixotrophic microorganisms. The added 9,10-anthraquinone-2,6-disulfonate (AQDS) in the later stage can promote NDFO to remove nitrate, thereby increasing the TN removal efficiency to 50%. N-15-isotope tracer incubations provided direct evidence for the occurrence of Feammox coupled to NDFO, with rates producing N-30(2) of Feammox (0.024-0.0288 mg N.L-1.d(-1)) and NDFO (0.0465-0.0833 mg N.L-1.d(-1)) in three groups (Wetland/Wheat soil/Sediment). 16S rRNA sequencing further demonstrated that Pseudomonas, Rhodanobacter, Acinetobacter and Thermomonas were the dominant generas among the enrichment cultures, and these bacteria belonged to FeOB and FeRB, which may further promote Feammox coupled to NDFO in the cultivation system.

Automated summary

This study proved the coexistence of Feammox and NDFO in natural enrichment, forming nitrogen and iron cycles. Sodium acetate and 9,10-anthraquinone-2,6-disulfonate (AQDS) were found to promote the reaction. 16S rRNA sequencing revealed Pseudomonas, Rhodanobacter, Acinetobacter and Thermomonas as dominant genera in the enrichment cultures.