Nascent nitrifying community response to monochloramine exposure in drinking water

Abstract

Nitrifiers autochthonous to drinking water distribution systems (DWDS) were enriched in separate ammonia-oxidizing Archaea (AOA), ammonia-oxidizing Bacteria (AOB) and nitrite-oxidizing Bacteria (NOB) enrichment cultures. A strategy was developed to selectively enrich AOA from DWDS biofilms containing both AOA and AOB, while new primers were designed to improve the quantification of the amoA and nxrB gene copies of AOA/AOB and NOB, respectively, using quantitative PCR and droplet digital PCR. Over several years, individual cultures containing AOA, AOB and NOB were enriched to 51.6 %, 71.2 % and 73.5 %, respectively. The nitrifiers were closely related to Nitrososphaera viennensis for AOA, Nitrosomonas oligotropha for AOB and Nitrospira moscoviensis for NOB. Under planktonic growth conditions, NOB were more resistant to monochloramine than AOA and AOB, and AOB were more resistant than AOA at lower monochloramine concentrations. Monochloramine also inactivated specific nitrifiers experimentally enriched in biofilms, but a small fraction of AOA or AOB cells persisted, likely due to the low concentration of free ammonia in the chloraminated bulk water. The persistence of AOA and AOB in the nitrifying biofilms after continuous exposure to monochloramine could be due to monochloramine cometabolism, where intracellular monochloramine was degraded by ammonia monooxygenase while also performing ammonia oxidation. This study is the first to address the effect of monochloramine on NOB and the regrowth potential of all three types of nitrifiers under planktonic conditions. By directly comparing monochloramine resistance under planktonic and biofilm conditions in the absence of other nitrifier groups fundamental insights were obtained on their respective roles in DWDS.