Ammonia-oxidizing archaea involved in nitrogen removal.

Ammonia oxidation is critical to global nitrogen cycling and is often thought to be driven only by ammonia-oxidizing bacteria. The recent finding of new ammonia-oxidizing organisms belonging to the archaeal domain challenges this perception. Two major microbial groups are now believed to be involved in ammonia oxidation: chemolithotrophic ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Candidatus "Nitrosopumilus maritimus", the first isolated ammonia-oxidizing archaeon from a tropical marine aquarium tank, representative of the ubiquitous marine group 1 Crenarchaeota, contains putative genes for all three subunits (amoA, amoB, and amoC) of ammonia monooxygenase, the key enzyme responsible for ammonia oxidation. In this article, important concepts of the nitrogen cycle, ammonia oxidation processes, ammonia-oxidizing organisms, and their physiology are described. AOA are found to thrive in various habitats including hot/thermal springs, marine and fresh waters, soils, and wastewater treatment systems, where they may outnumber their counterpart, AOB. Various molecular tools have been applied to study AOB and AOA and determine their abundance and community structure changes from natural and engineered systems. The presence of AOA in activated sludge opens new opportunities for elucidating its role of ammonia removal in wastewater treatment plants and wetlands. Several significant questions related to AOA research have been raised to evoke reader involvement for broadening future studies.