Nitrosospira spp. can produce nitrous oxide via a nitrifier denitrification pathway

Summary Nitrous oxide (N 2 O) emission from soils is a major contributor to the atmospheric loading of this potent greenhouse gas. It is thought that autotrophic ammonia oxidizing bacteria (AOB) are a significant source of soil‐derived N 2 O and a denitrification pathway (i.e. reduction of NO 2 – to NO and N 2 O), so‐called nitrifier denitrification, has been demonstrated as a N 2 O production mechanism in Nitrosomonas europaea. It is thought that Nitrosospira spp. are the dominant AOB in soil, but little information is available on their ability to produce N 2 O or on the existence of a nitrifier denitrification pathway in this lineage. This study aims to characterize N 2 O production and nitrifier denitrification in seven strains of AOB representative of clusters 0, 2 and 3 in the cultured Nitrosospira lineage. Nitrosomonas europaea ATCC 19718 and ATCC 25978 were analysed for comparison. The aerobically incubated test strains produced significant ( P < 0.001) amounts of N 2 O and total N 2 O production rates ranged from 2.0 amol cell −1 h −1 , in Nitrosospira tenuis strain NV12, to 58.0 amol cell −1 h −1 , in N. europaea ATCC 19718. Nitrosomonas europaea ATCC 19718 was atypical in that it produced four times more N 2 O than the next highest producing strain. All AOB tested were able to carry out nitrifier denitrification under aerobic conditions, as determined by production of 15 N‐N 2 O from applied 15 N‐NO 2 – . Up to 13.5% of the N 2 O produced was derived from the exogenously applied 15 N‐NO 2 – . The results suggest that nitrifier denitrification could be a universal trait in the betaproteobacterial AOB and its potential ecological significance is discussed.