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

Nitrous oxide (NO) and NO (nitrous acid (HONO) + nitric oxide (NO) + nitrogen dioxide (NO)) 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 NO and NO emissions Yet, little is known about HONO and NO 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 NO and NO 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 production by the comammox bacterium compared to AOB; ii) highlight the role of the NO reductase in the biological formation of NO based on results from NHOH inhibition assays and its stimulation during archaeal and bacterial ammonia oxidations; iii) postulate that the lack of hydroxylamine (NHOH) and NO transformation enzymatic activities may lead to a buildup of NHOH/NO which can abiotically react to NO ; iv) collectively confirm restrained NO and NO emission by comammox bacteria, an unneglectable consortium of microbes in global atmospheric emission of reactive nitrogen gases.