Nitrifiers

Nitrifiers are microorganisms that perform nitrification, the biological oxidation of reduced nitrogen compounds to nitrate. They are mainly chemolithoautotrophs that harvest energy from the oxidation of ammonia or nitrite and fix CO2 as their carbon source. The process operates in two main steps carried out by different groups: ammonia-oxidizing organisms convert ammonia to nitrite, and nitrite-oxidizing organisms convert nitrite to nitrate. In recent years, complete ammonia oxidation (comammox) organisms have been identified that can perform both steps within a single organism, expanding the traditional view of nitrification as a two-step process.

Nitrifiers include ammonia-oxidizing bacteria (AOB) such as Nitrosomonas and Nitrosospira, ammonia-oxidizing archaea (AOA) found in diverse environments, and nitrite-oxidizing bacteria (NOB) such as Nitrobacter and Nitrospira. Comammox bacteria, primarily some Nitrospira lineages, can oxidize ammonia all the way to nitrate. Genetic markers such as amoA and nxrA are commonly used to identify and study these groups in environmental samples.

Nitrification is an aerobic process, requiring sufficient oxygen, and is influenced by temperature, pH, and the availability of ammonia and nitrite. It plays a central role in the nitrogen cycle by transforming ammonium into nitrate, which affects plant nutrient availability and groundwater mobility. In wastewater treatment, nitrification is exploited to remove nitrogen, typically in aerated reactors, and can be coupled with denitrification for complete nitrogen removal. Nitrifiers can be inhibited by low oxygen, extreme pH, high ammonia levels, or toxic compounds, and under some conditions may contribute to nitrous oxide emissions. The concept was established in the late 19th century with foundational work by Winogradsky.