Abstract
Nitrous oxide (N2O) and nitric oxide (NO) are atmospheric trace gases that contribute to climate change and affect stratospheric and ground-level ozone concentrations. Ammonia oxidizing bacteria (AOB) and archaea (AOA) are key players in the nitrogen cycle and major producers of N2O and NO globally. However, nothing is known about N2O and NO production by the recently discovered and widely distributed complete ammonia oxidizers (comammox). Here, we show that the comammox bacterium Nitrospira inopinata is sensitive to inhibition by an NO scavenger, cannot denitrify to N2O, and emits N2O at levels that are comparable to AOA but much lower than AOB. Furthermore, we demonstrate that N2O formed by N. inopinata formed under varying oxygen regimes originates from abiotic conversion of hydroxylamine. Our findings indicate that comammox microbes may produce less N2O during nitrification than AOB.
Highlights
Nitrous oxide (N2O) and nitric oxide (NO) are atmospheric trace gases that contribute to climate change and affect stratospheric and ground-level ozone concentrations
NO and N2O production by Ammonia oxidizing bacteria (AOB), AOA, and nitrite oxidizing bacteria (NOB) has been intensively studied and several key genes used by these nitrifiers for the production and consumption of these gases have been identified[8,11,34,35,36,37,38] (Figs. 1 and 2)
As a first step to close this knowledge gap, we updated previous comparative genomic analyses of comammox genomes[26,27] and mined them for key genes that might be involved in NO and N2O metabolism (Fig. 2). These analyses included N. inopinata, the only comammox organism available as pure culture[33], as well as genomes from two comammox enrichments and 15 comammox metagenome-assembled genomes (MAGs) from environmental samples retrieved via metagenomics[23,26,29,30,39]
Summary
Nitrous oxide (N2O) and nitric oxide (NO) are atmospheric trace gases that contribute to climate change and affect stratospheric and ground-level ozone concentrations. Instantaneous NO concentrations were below our detection limit (~0.25 nM) for the closely related non-comammox Nitrospira, N. moscoviensis, during NO2− oxidation (Supplementary Fig. 4).
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