Abstract
ABSTRACTNitrification is a key process of the biogeochemical nitrogen cycle and of biological wastewater treatment. The second step, nitrite oxidation to nitrate, is catalyzed by phylogenetically diverse, chemolithoautotrophic nitrite-oxidizing bacteria (NOB). Uncultured NOB from the genus “Candidatus Nitrotoga” are widespread in natural and engineered ecosystems. Knowledge about their biology is sparse, because no genomic information and no pure “Ca. Nitrotoga” culture was available. Here we obtained the first “Ca. Nitrotoga” isolate from activated sludge. This organism, “Candidatus Nitrotoga fabula,” prefers higher temperatures (>20°C; optimum, 24 to 28°C) than previous “Ca. Nitrotoga” enrichments, which were described as cold-adapted NOB. “Ca. Nitrotoga fabula” also showed an unusually high tolerance to nitrite (activity at 30 mM NO2−) and nitrate (up to 25 mM NO3−). Nitrite oxidation followed Michaelis-Menten kinetics, with an apparent Km (Km(app)) of ~89 µM nitrite and a Vmax of ~28 µmol of nitrite per mg of protein per h. Key metabolic pathways of “Ca. Nitrotoga fabula” were reconstructed from the closed genome. “Ca. Nitrotoga fabula” possesses a new type of periplasmic nitrite oxidoreductase belonging to a lineage of mostly uncharacterized proteins. This novel enzyme indicates (i) separate evolution of nitrite oxidation in “Ca. Nitrotoga” and other NOB, (ii) the possible existence of phylogenetically diverse, unrecognized NOB, and (iii) together with new metagenomic data, the potential existence of nitrite-oxidizing archaea. For carbon fixation, “Ca. Nitrotoga fabula” uses the Calvin-Benson-Bassham cycle. It also carries genes encoding complete pathways for hydrogen and sulfite oxidation, suggesting that alternative energy metabolisms enable “Ca. Nitrotoga fabula” to survive nitrite depletion and colonize new niches.
Highlights
Nitrification is a key process of the biogeochemical nitrogen cycle and of biological wastewater treatment
Nitrification is critical for the removal of excess nitrogen from sewage in wastewater treatment plants (WWTPs), whereas in agriculture, it contributes to the loss of nitrogen from fertilized soils [1]
Like Nitrospira, “Ca. Nitrotoga” members can be versatile nitrite-oxidizing bacteria (NOB) whose metabolic flexibility may explain their competitive success in dynamic environments such as WWTPs
Summary
Nitrification is a key process of the biogeochemical nitrogen cycle and of biological wastewater treatment. A novel “reciprocal feeding” interaction of NOB from the genus Nitrospira with ammonia oxidizers was described, where the NOB initiate nitrification by releasing ammonia from urea or cyanate [4, 6] Another surprise was the discovery of photolithoautotrophic NOB that use nitrite as an electron donor for anoxygenic photosynthesis [7] and most likely evolved independently of the chemolithoautotrophic NOB [8]. A core paradigm of nitrification research stated that ammonia and nitrite oxidation are always catalyzed by distinct organisms, which cooperate by cross-feeding This long-standing opinion was contradicted by the discovery of complete nitrifiers (comammox organisms) in the NOB genus Nitrospira, which perform both steps of nitrification [9, 10]. The known phylogenetic diversity of NOB has been expanded by the description of several new NOB lineages: the genus Nitrolancea in the Chloroflexi [14], the candidate genus “Candidatus Nitromaritima” in the Nitrospinae [15], and the candidate genus “Candidatus Nitrotoga” in the Betaproteobacteria, family Gallionellaceae [16]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
