Water and biofilm microbial ecology at the outlet of a storage tank in a drinking water distribution network

Abstract

A qualitative and quantitative analysis of microbial ecology was carried out at the outlet of a drinking water storage tank in the Paris suburbs. The concentrations of planktonic and biofilm bacteria determined by cytometry were higher in December 2023 than in March 2024. Biofilm detachment in March 2024 was confirmed by scanning electron microscopy. Sequencing of 16S rRNA gene amplicons showed that the genus Phreatobacter, corresponding to strictly aerobic, motile Gram-negative bacilli, was the dominant genus in the water in December and in the biofilm in December and March. The dominant genus changed in the water in March to Polycyclovorans, another motile rod-shaped Gram-negative genus. Sequencing of 18S rRNA gene amplicons revealed that Malassezia (Basidiomycota) was the dominant eukaryotic genus in the water in December and March. In December, the dominant genus in the biofilm was Acanthamoeba (Amoebozoa). In March, Neobodo (Excavata) was the most abundant genus in the biofilm. Among all the sequences obtained for the least abundant genera, cyanotoxin-producing cyanobacteria and potentially pathogenic microorganisms (bacteria and eukaryotes) were detected. Alpha and beta diversity analyses showed that bacterial communities may be more influenced by temporal dynamics, while eukaryotic communities appeared to be more strongly structured by biofilm versus planktonic lifestyle. These observations indicate that the outlet of a water storage tank constitutes a risk zone for the development and detachment of biofilm harbouring prokaryotic and eukaryotic microorganisms whose communities evolve over time. This type of area should therefore be subject to constant and specific monitoring.