Context. The discovery of complete ammonia oxidation (comammox) bacteria has challenged the paradigm of the classical two-step nitrification. Comammox bacteria are widely distributed in wastewater treatment plants, accounting for 0.02-0.57% of the bacterial community.They exhibited relatively high affinity to ammonia and oxygen, which were determining factors for their ecological niches.
Gap. Contributions of comammox bacteria to nitrification and the niche differentiation of comammox bacteriaand other N-cycling microorganisms.
Aims.This study aims to identify theabundance and activity of comammox bacteria, as well as their ammonia-dependent cooperation with anammox bacteria in a low-DO nitrifying biofilm reactor.
Findings."Candidatus Nitrospira nitrosa" like comammox bacteria were the dominant nitrifiers, accounting for up to 14.4% ofthe total community. With elevated influent ammonium concentrationfrom 7 to 33 mg N/L, total nitrogen removal increased from 1.8% to 34.0%, where "Ca. Kuenenia stuttgartiensis"like anammox bacteriacontributed up to 27.4% of ammonium removal in the reactor. 19.4%-42.9% of the nitrite produced by comammox bacteria was diffused to extracellular space, which was then utilized by anammox bacteria. Compared with incoula, nitrite transportergene (focA) and anammox metabolism genes(hzo, hdh) were enriched by 344.4-352.5 times in Phase II.Comammox bacteria lacked the synthetic pathwaysfor a few essential amino acids and cofactors, e.g., serine,whereas heterotrophicGammaproteobacteriaandCandadatus Kueneniawere potential providers of these micro-nutrients. Tetrahydrofolate, an important intermediate for CO2 assimilation by anammox bacteria, can be supplied by comammox bacteriain return.
Utilization. Due to the oxygen sensitivity and low growth activity of anammox bacteria, partial nitrification-Anammox (PN/A) systems are normally operated at around 1 mg O2/L, and MBBR is a commonly applied configuration. The dominance of comammox bacteria herein indicated their competitive advantagesover conventional AOB/AOA/NOB. Cooperative interactionsbetween comammox and anammox bacteria could facilitatethe integrated PN/Aprocess. However,thelimited nitrite supply by comammox bacteria also sets a ceiling for total nitrogen removal by such systems.
