N 2 O and NO y production by the comammox bacterium Nitrospira inopinata in comparison with canonical ammonia oxidizers.

Nitrous oxide (N ₂ O) and NO _y (nitrous acid (HONO) + nitric oxide (NO) + nitrogen dioxide (NO ₂ )) are released as byproducts or obligate intermediates during aerobic ammonia oxidation, and further influence global warming and atmospheric chemistry. The ammonia oxidation process is catalyzed by groups of globally distributed ammonia-oxidizing microorganisms, which are playing a major role in atmospheric N ₂ O and NO _y emissions _. Yet, little is known about HONO and NO ₂ production by the recently discovered, widely distributed complete ammonia oxidizers (comammox), able to individually perform the oxidation of ammonia to nitrate via nitrite. Here, we examined the N ₂ O and NO _y production patterns by comammox bacterium Nitrospira inopinata during aerobic ammonia oxidation, in comparison to its canonical ammonia-converting counterparts, representatives of the ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Our findings, i) show low yield NO _y production by the comammox bacterium compared to AOB; ii) highlight the role of the NO reductase in the biological formation of N ₂ O based on results from NH ₂ OH inhibition assays and its stimulation during archaeal and bacterial ammonia oxidations; iii) postulate that the lack of hydroxylamine (NH ₂ OH) and NO transformation enzymatic activities may lead to a buildup of NH ₂ OH/NO which can abiotically react to N ₂ O ; iv) collectively confirm restrained N ₂ O and NO _y emission by comammox bacteria, an unneglectable consortium of microbes in global atmospheric emission of reactive nitrogen gases.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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