Selective Enrichment of Comammox in a Moving Bed Biofilm Reactor with Sufficient Oxygen Supply.

The recent discovery of comammox (complete ammonia oxidation) has upended the long-held nitrification paradigm. Although comammox have been identified in wastewater treatment systems, the conditions for their dominance over canonical ammonia oxidizers remain unclear. Here, we report the dominance of comammox in a moving bed biofilm reactor (MBBR) fed with synthetic mainstream wastewater. Integrated 16S rRNA gene amplicon sequencing, fluorescence in situ hybridization (FISH), and metagenomic sequencing methods demonstrated the selective enrichment of comammox bacteria when the MBBR was operated at a dissolved oxygen (DO) concentration above 6 mg O/L. The dominance of comammox over canonical ammonia oxidizers (i.e., ) was attributed to the low residual ammonium concentration (0.02-0.52 mg N/L) formed in the high-DO MBBR. Two clade A comammox were identified, which are phylogenetically close to Nitrospira nitrosa. Interestingly, cryosectioning-FISH showed these two comammox species spatially distributed on the surface of the biofilm. Moreover, the ammonia-oxidizing activity of comammox -dominated biofilms was susceptible to the oxygen supply, which dropped by half with the DO concentration decrease from 6 to 2 mg O/L. These features collectively suggest a low apparent oxygen affinity for the comammox -dominated biofilms in the high-DO nitrifying MBBR.