Microbial Nitrogen Metabolism in Chloraminated Drinking Water Reservoirs

Ammonia availability due to chloramination can promote the growth of nitrifying organisms, which can deplete chloramine residuals and result in opera-tional problems for drinking water utilities. In this study, we used a metagenomic approach to determine the identity and functional potential of microorganisms in-volved in nitrogen biotransformation within chloraminated drinking water reservoirs. Spatial changes in the nitrogen species included an increase in nitrate concentra-tions 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., comam-mox) Nitrospira-like bacteria. Functional annotation was used to evaluate genes asso-ciated 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 ge-nomes (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 analy-ses of these MAGs in relation to nitrogen metabolism suggest that under ammonia-limited 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. IMPORTANCE Chloramines are often used as a secondary disinfectant when free chlorine residuals are difficult to maintain. However, chloramination is often associ-ated with the undesirable effect of nitrification, which results in operational prob-lems for many drinking water utilities. The introduction of ammonia during chloram-ination provides a potential source of nitrogen either through the addition of excess ammonia or through chloramine decay. This promotes the growth of nitrifying mi-croorganisms and provides a nitrogen source (i.e., nitrate) for the growth for other organi