Nitrifying niche differentiation in biofilms from full-scale chloraminated drinking water distribution system

Tropical conditions favour the auto-decomposition of monochloramine (MCA) leading to disinfectant decay and free ammonia in drinking water distribution systems (DWDS); thus, they promote the growth of nitrifiers and the development of biofilms on the inner-pipe surface. Biofilms can adversely impact the provision of safe and biologically stable water. Moreover, there is a general lack of understanding of the role of microbial communities in DWDS in regions with warm temperatures and no distinct seasons. Here, we report a survey on biofilms from full-scale monochloraminated DWDS in a highly urbanised metropolis using next generation sequencing tools. The monitoring campaign consisted of sampling biofilms and bulk waters from 21 in-service pipes. We characterized the microbial community with emphasis on nitrifying bacteria and archaea using 16S rRNA gene amplicon sequencing and potential nitrification activity. Samples grouped into two clusters, characterized by their low (Cluster LD) and high (Cluster HD) α-diversity. Both clusters harbour microorganisms related to nitrification: i) Nitrosomonas (24.9-68.8%), an ammonia oxidising bacterium (AOB) that dominated Cluster LD, and ii) a co-aggregation of genus Nitrospira (9.8-32.5%), a nitrite oxidising bacterium (NOB), and Thaumarchaeota (1.4-10.9%), chemolithotrophic ammonia oxidising (AOA) archaea that were among the most abundant OTUs in Cluster HD. Activity tests performed with fresh biofilm samples confirmed that these two clusters represent distinctive biofilm niches performing different stages of the nitrification process. Cluster LD correlated with a high concentration of MCA, which caused dysbiosis and resulted in high unevenness of the cluster. In cluster HD, with more biomass, chemical reactions involving nitrite increased the MCA demand, releasing ammonia and allowing more nitrifiers to grow, like AOA and NOB. From this study, we conclude that an MCA residual gradient along the DWDS drives and shapes the microbial community assembly and should be considered when designing effective disinfection strategies.