This study describes the enrichment of anammox bacteria in a column simulating oxygen limited flooded paddy soils, which are important man-made ecosystems that receive substantial amounts of fixed nitrogen. The upper 50 cm of the paddy soil, containing a high amount of ammonium [1.6–10.4 mmol N kg (dry weight)−1], was selected as the inoculum for anammox enrichment. After 18 months of incubation with freshwater from the paddy soil ecosystem, the enrichment culture consumed approximately 4 mmol ammonium l−1 day−1 and 5 mmol nitrite l−1 day−1. The maximum specific anammox activity of the culture was 35.7 μmol N g (dry weight)−1 h−1. Fluorescence in situ hybridization indicated that anammox cells constituted 50% ± 10% of the enrichment culture. The phylogenetic analyses of 16S rRNA and the diagnostic hydrazine synthase (hzsA) genes showed that two dominant anammox species were enriched from paddy soil. The enriched Candidatus Anammoxoglobus-like organisms showed a 16S rRNA gene similarity of 97.5–99.2% to Candidatus Anammoxoglobus propionicus and the Candidatus Jettenia-like organisms showed 92.1–93.1% 16S rRNA gene identity to Candidatus Jettenia asiatica. Real-time quantitative PCR of hzsA gene suggested that up to 1010 copies g (dry weight)−1 of soil anammox bacteria were present in the enrichment culture.
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