The Microbiota of Evaporative Cooling Systems and the Impact of Rainwater as an Alternative Water Source.

Climate change and an increasing global population are supposed to cause progressive water shortages in the future. Using rainwater in water tanks for evaporative cooling systems as a substitute might help reduce water consumption needed for cooling. Therefore, our study identified the present microbiota in evaporative cooling systems (ECS) and used an ex situ approach to identify effects of the substitution of tap water with rainwater. Culturable cell counts (CCC) based on colony-forming units for total counts or selected pathogens, total biofilm masses based on crystal violet staining, and the bacterial composition (16S rRNA sequencing) of biofilms sampled in different ECS were considered. Data suggest that rainwater does not enhance biofilm growth, since both culturable cells and total biofilm mass tended to decrease when using rainwater instead of tap water. Interestingly, biocide treatment slightly enhanced the total biofilm mass and CCC. Sequencing data revealed differences in the microbiota when using the different water types; yet, the community shift did not include an enrichment of typical pathogens in both cases. Overall, our results suggest that a substitution of tap water with rainwater for ECS creates no disadvantage with regard to the microbial safety and might allow a reduction of financial and resource investments.