Research advances of ammonia oxidation microorganisms in wastewater: metabolic characteristics, microbial community, influencing factors and process applications

Ammonia oxidation carried out by ammonia-oxidizing microorganisms (AOMs) is a central step in the global nitrogen cycle. Aerobic AOMs comprise conventional ammonia-oxidizing bacteria (AOB), novel ammonia-oxidizing archaea (AOA), which could exist in complex and extreme conditions, and complete ammonia oxidizers (comammox), which directly oxidize ammonia to nitrate within a single cell. Anaerobic AOMs mainly comprise anaerobic ammonia-oxidizing bacteria (AnAOB), which can transform NH₄⁺-N and NO₂^--N into N₂ under anaerobic conditions. In this review, the unique metabolic characteristics, microbial community of AOMs and the influencing factors are discussed. Process applications of nitrification/denitrification, nitritation/denitrification, nitritation/anammox and partial denitrification/anammox in wastewater treatment systems are emphasized. The future development of nitrogen removal processes using AOMs is expected, enrichment of comammox facilitates the complete nitrification performance, inhibiting the activity of comammox and NOB could achieve stable nitritation, and additionally, AnAOB conducting the anammox process in municipal wastewater is a promising development direction.

Functional microorganisms and enzymes related nitrogen cycle in the biofilm performing simultaneous nitrification and denitrification

Simultaneous nitrification and denitrification (SND) is a potential energy-saving process in wastewater treatment while the nitrogen removal mechanism is still unclear due to the lack of information about the functional microbes and enzymes. Sequencing batch biofilm reactors were implemented to achieve efficient SND. Eight nitrogen removal related microorganisms out of the top abundant 20 microbial community and reference species were used to construct a phylogenetic tree. Functional enzymes and modules analysis were investigated to reveal the SND pathway: in the aerobic part of the biofilm, ammonia oxidation was catalyzed by complete ammonia oxidizers while in the inner anoxic part, denitrification, dissimilatory nitrate reduction (DNRA) and nitrogen fixation (NF) cooperated to stimulate nitrate removal. These results provide a practical aeration control strategy to achieve SND and indicate that DNRA and NF are important nitrogen removal pathways that should not be ignored in the SND mechanism.

Comammox bacterial abundance, activity, and contribution in agricultural rhizosphere soils

The newly identified complete ammonia oxidation (comammox), which is capable of oxidizing ammonia directly to nitrate, has complemented our knowledge of nitrification in the global nitrogen (N) cycle. However, understanding the contribution and ecological roles of comammox in complex soil environments is still in its infancy. Here, the community structure and function of comammox and the interactions with other ammonia oxidation processes in rhizosphere and non-rhizosphere soils of four different crop fields (maize, cotton, soybean, and millet) were investigated in summer and winter. The only identified comammox species Candidatus Nitrospira nitrificans was widely distributed in all sampled soils. Comammox bacterial abundance was lower than that of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). The measured comammox potential rate ranged from 0.01 ± 0.002 to 0.40 ± 0.02 mg N kg^-1 d^-1, contributing <19.2 and 22.1% to ammonia oxidation in summer and winter, the remainder being due to AOA and AOB. The potential rate and community composition of comammox bacteria were significantly different on a temporal scale, while crop species and soil types (rhizosphere and non-rhizosphere) showed no obvious influences. In terms of oxidation rates, AOA (1.2 ± 0.7 mg N kg^-1 d^-1) dominated the ammonia oxidation in agricultural soils over AOB (0.31 ± 0.1 mg N kg^-1 d^-1) and comammox (0.2 ± 0.1 mg N kg^-1 d^-1). Both anammox bacterial abundance and activity were below the detection limits, indicating a negligible contribution of anammox in agricultural rhizosphere soils. The identification of comammox bacterial abundance and activity in situ enriches our knowledge of nitrification in agricultural systems.