Ammonia availability due to chloramination can promote the growth of
nitrifying organisms, which can deplete chloramine residuals and result in operational
problems for drinking water utilities. In this study, we used a metagenomic
approach to determine the identity and functional potential of microorganisms involved
in nitrogen biotransformation within chloraminated drinking water reservoirs.
Spatial changes in the nitrogen species included an increase in nitrate concentrations
accompanied by a decrease in ammonium concentrations with increasing
distance from the site of chloramination. This nitrifying activity was likely driven
by canonical ammonia-oxidizing bacteria (i.e., Nitrosomonas) and nitrite-oxidizing
bacteria (i.e., Nitrospira) as well as by complete-ammonia-oxidizing (i.e., comammox)
Nitrospira-like bacteria. Functional annotation was used to evaluate genes associated
with nitrogen metabolism, and the community gene catalogue contained
mostly genes involved in nitrification, nitrate and nitrite reduction, and nitric oxide
reduction. Furthermore, we assembled 47 high-quality metagenome-assembled genomes
(MAGs) representing a highly diverse assemblage of bacteria. Of these, five
MAGs showed high coverage across all samples, which included two Nitrosomonas,
Nitrospira, Sphingomonas, and Rhizobiales-like MAGs. Systematic genome-level analyses
of these MAGs in relation to nitrogen metabolism suggest that under ammonialimited
conditions, nitrate may be also reduced back to ammonia for assimilation.
Alternatively, nitrate may be reduced to nitric oxide and may potentially play a role
in regulating biofilm formation. Overall, this study provides insight into the microbial
communities and their nitrogen metabolism and, together with the water chemistry
data, improves our understanding of nitrogen biotransformation in chloraminated
drinking water distribution systems.