Transcriptomic response ofNitrosomonas europaeatransitioned from ammonia- to oxygen-limited steady-state growth

Abstract

AbstractAmmonia-oxidizing microorganisms perform the first step of nitrification, the oxidation of ammonia to nitrite. The bacteriumNitrosomonas europaeais the best characterized ammonia oxidizer to date. Exposure to hypoxic conditions has a profound effect on the physiology ofN. europaea, e.g. by inducing nitrifier denitrification, resulting in increased nitric and nitrous oxide production. This metabolic shift is of major significance in agricultural soils, as it contributes to fertilizer loss and global climate change. Previous studies investigating the effect of oxygen limitation onN. europaeahave focused on the transcriptional regulation of genes involved in nitrification and nitrifier denitrification. Here, we combine steady-state cultivation with whole genome transcriptomics to investigate the overall effect of oxygen limitation onN. europaea. Under oxygen-limited conditions, growth yield was reduced and ammonia to nitrite conversion was not stoichiometric, suggesting the production of nitrogenous gases. However, the transcription of the principal nitric oxide reductase (cNOR) did not change significantly during oxygen-limited growth, while the transcription of the nitrite reductase-encoding gene (nirK) was significantly lower. In contrast, both heme-copper containing cytochromecoxidases encoded byN. europaeawere upregulated during oxygen-limited growth. Particularly striking was the significant increase in transcription of the B-type heme-copper oxidase, proposed to function as a nitric oxide reductase (sNOR) in ammonia-oxidizing bacteria. In the context of previous physiological studies, as well as the evolutionary placement ofN. europaea’ssNOR with regards to other heme-copper oxidases, these results suggest sNOR may function as a high-affinity terminal oxidase inN. europaeaand other AOB.ImportanceNitrification is a ubiquitous, microbially mediated process in the environment and an essential process in engineered systems such as wastewater and drinking water treatment plants. However, nitrification also contributes to fertilizer loss from agricultural environments increasing the eutrophication of downstream aquatic ecosystems and produces the greenhouse gas nitrous oxide. As ammonia-oxidizing bacteria are the most dominant ammonia-oxidizing microbes in fertilized agricultural soils, understanding their response to a variety of environmental conditions is essential for curbing the negative environmental effects of nitrification. Notably, oxygen limitation has been reported to significantly increase nitric oxide and nitrous oxide production during nitrification. Here we investigate the physiology of the best characterized ammonia-oxidizing bacterium,Nitrosomonas europaea, growing under oxygen-limited conditions.