Nitrosomonas europaea is one of the most abundantly found and widely studied ammonia oxidizing bacteria in natural and engineered environments. While significant work exists on the aerobic oxidative pathways of N. europaea related to ammonia and hydroxylamine oxidation
to nitrite, relatively less is understood about reductive N pathways, which relate to denitrification and the production of gaseous intermediates such as nitric and nitrous oxides. In this study, expression of four genes involved in both oxidative and reductive N transformations was simultaneously
studied at batch growth under different dissolved oxygen concentrations. Notably, amoA and hao expression increased with increasing dissolved oxygen limitation suggesting a novel competitive strategy of N. europaea to survive oxygen limitation. nirK expression and
production of NO were strongly correlated suggesting that NO was derived from nitrite reduction. nirK expression was triggered more by high nitrite concentrations than by oxygen limitation. The possibility of denitrification by ammonia oxidizing bacteria and production of gaseous nitrogen
oxides is being increasingly documented from biological nitrogen removal systems. This study provides a mechanistic basis for (partially) attributing these observed gaseous N fluxes to AOB activity at the functional gene expression level.
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