Abstract
Nitrous oxide (N2O) is a key climate change gas and nitrifying microbes living in terrestrial ecosystems contribute significantly to its formation. Many soils are acidic and global change will cause acidification of aquatic and terrestrial ecosystems, but the effect of decreasing pH on N2O formation by nitrifiers is poorly understood. Here, we used isotope-ratio mass spectrometry to investigate the effect of acidification on production of N2O by pure cultures of two ammonia-oxidizing archaea (AOA; Nitrosocosmicus oleophilus and Nitrosotenuis chungbukensis) and an ammonia-oxidizing bacterium (AOB; Nitrosomonas europaea). For all three strains acidification led to increased emission of N2O. However, changes of 15N site preference (SP) values within the N2O molecule (as indicators of pathways for N2O formation), caused by decreasing pH, were highly different between the tested AOA and AOB. While acidification decreased the SP value in the AOB strain, SP values increased to a maximum value of 29‰ in N. oleophilus. In addition, 15N-nitrite tracer experiments showed that acidification boosted nitrite transformation into N2O in all strains, but the incorporation rate was different for each ammonia oxidizer. Unexpectedly, for N. oleophilus more than 50% of the N2O produced at pH 5.5 had both nitrogen atoms from nitrite and we demonstrated that under these conditions expression of a putative cytochrome P450 NO reductase is strongly upregulated. Collectively, our results indicate that N. oleophilus might be able to enzymatically denitrify nitrite to N2O at low pH.
Highlights
ammonia-oxidizing bacterium (AOB) at the lowest pH tested, the presence of the nitrite-oxidizing bacterium (NOB) in the cultures caused a significant decrease of N2O yields (28% and 48%, respectively) (P < 0.001), suggesting that the accumulation of nitrite in the experiments without addition of NOB contributed to the increased N2O production under these conditions (Supplementary Fig. S1)
The NH2OH concentrations in the abiotic controls were selected based on the recently published data that showed maximum concentrations of extracellular NH2OH of
At pH 5.5, we observed significantly increased transcription of two cytochrome P450-like genes (MY3_00641 and MY3_01637) of strain N. oleophilus MY3 compared to transcription of housekeeping genes such as those encoding the 16S rRNA, AmoA, and enzymes required for Relateve gene expression cDNA copy
Summary
Indications for enzymatic denitrification to N2O at low pH in an ammonia-oxidizing archaeon. Herbold2,6 ● Michael Wagner 2,6,7 ● Sung-Keun Rhee 1
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