A New Perspective on Microbes Formerly Known as Nitrite-Oxidizing Bacteria

Nitrite-oxidizing bacteria (NOB) catalyze the second step of nitrification, nitrite oxidation to nitrate, which is an important process of the biogeochemical nitrogen cycle. NOB were traditionally perceived as physiologically restricted organisms and were less intensively studied than other nitrogen-cycling microorganisms. This picture is in contrast to new discoveries of an unexpected high diversity of mostly uncultured NOB and a great physiological versatility, which includes complex microbe–microbe interactions and lifestyles outside the nitrogen cycle. Most surprisingly, close relatives to NOB perform complete nitrification (ammonia oxidation to nitrate) and this finding will have far-reaching implications for nitrification research. We review recent work that has changed our perspective on NOB and provides a new basis for future studies on these enigmatic organisms.

TrendsNitrite-oxidizing bacteria (NOB) are key players in the biogeochemical nitrogen cycle. They are a phylogenetically diverse guild with pronounced ecological niche specialization and they differ from each other in fundamental physiological and molecular traits.NOB are involved in complex symbioses with ammonia-oxidizing and heterotrophic microorganisms. In a new type of interaction called reciprocal feeding, NOB recruit ammonia oxidizers to use urea or cyanate as energy source.NOB are surprisingly versatile and can switch between nitrite oxidation and alternative metabolisms such as H2 or formate oxidation. Thus, NOB may have diverse ecological functions within and beyond the nitrogen cycle.The unexpected discovery of complete ammonia oxidizers (‘comammox’) in the NOB genus Nitrospira will have broad implications for future research on nitrification and the nitrogen cycle.