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Background  Eukaryotes are ubiquitous in natural environments such as soil and freshwater. Little is known of their presence in drinking water distribution systems (DWDSs) or of the environmental conditions that affect their activity and survival.    Methods  Eukaryotes were characterized by Illumina high-throughput sequencing targeting 18S rRNA gene (DNA) that estimates the total community and the 18S rRNA gene transcript (RNA) that is more representative of the active part of the community. DWDS cold water ( N  = 124), hot water ( N  = 40), and biofilm ( N  = 16) samples were collected from four cities in Finland. The sampled DWDSs were from two waterworks A–B with non-disinfected, recharged groundwater as source water and from three waterworks utilizing chlorinated water (two DWDSs of surface waterworks C–D and one of ground waterworks E). In each DWDS, samples were collected from three locations during four seasons of 1 year.    Results  A beta-diversity analysis revealed that the main driver shaping the eukaryotic communities was the DWDS (A–E) ( R  = 0.73,  P  < 0.001, ANOSIM). The kingdoms  Chloroplastida  (green plants and algae),  Metazoa  (animals: rotifers, nematodes),  Fungi  (e.g.,  Cryptomycota ),  Alveolata  (ciliates, dinoflagellates), and  Stramenopiles  (algae  Ochrophyta ) were well represented and active—judging based on the rRNA gene transcripts—depending on the surrounding conditions. The unchlorinated cold water of systems (A–B) contained a higher estimated total number of taxa (Chao1, average 380–480) than chlorinated cold water in systems C–E (Chao1 ≤ 210). Within each DWDS, unique eukaryotic communities were identified at different locations as was the case also for cold water, hot water, and biofilms. A season did not have a consistent impact on the eukaryotic community among DWDSs.    Conclusions  This study comprehensively characterized the eukaryotic community members within the DWDS of well-maintained ground and surface waterworks providing good quality water. The study gives an indication that each DWDS houses a unique eukaryotic community, mainly dependent on the raw water source and water treatment processes in place at the corresponding waterworks. In particular, disinfection as well as hot water temperature seemed to represent a strong selection pressure that controlled the number of active eukaryotic species.    Electronic supplementary material  The online version of this article (10.1186/s40168-019-0715-5) contains supplementary material, which is available to authorized users.

Active eukaryotes in drinking water distribution systems of ground and surface waterworks