Wastewater treatment plant effluent alters particle-associated bacterial assemblages in an intermittent stream.

Wastewater Treatment Plants (WWTPs) can be point-sources of nutrients, organic matter, anthropogenic contaminants, and microbes to lotic environments, and the input of WWTP effluent can alter the activity and composition of stream microbial assemblages in the benthos and water column. Within the stream benthos, fine particulate matter (FPM) supports especially high levels of microbial activity, but the effect of WWTP effluents on the microbial communities specifically associated with benthic FPM in rivers has not been studied. The present work sought to address this knowledge gap by analyzing FPM quantity, nutrient content, microbial activity, and bacterial assemblage composition in FPM samples collected from sites upstream and downstream of a WWTP effluent input into an intermittent Mediterranean stream. Sampling was conducted on two sampling dates (November and July) and the study included both amplicon and metagenomic sequencing in order to assess both the taxonomic and functional gene composition of the particle-associated bacterial assemblages. Effluent input resulted in increased FPM concentration and increased nutrient content of FPM, which were both correlated with an increase in microbial metabolic activity (MMA) immediately downstream of the WWTP. Effluent was a significant source of bacterial taxa to the stream, and there was a significant decrease in the diversity of the bacterial assemblages associated with the fine particles immediately downstream, as well as changes in their taxonomic and functional gene profiles. Taxa associated with wastewater treatment (Rhodocyclaceae and Xanthomonadaceae) and the metabolism of anthropogenic contaminants (Sphingobacteriales), and genes associated with multidrug efflux pumps and denitrification were more abundant immediately downstream. The effect of effluent on the taxonomic composition of the bacterial assemblages was much stronger than the effect on functional gene profiles, highlighting the functional redundancy within these communities. In addition, all the effects of effluent were short-lived and decreased with distance downstream, especially in November when the upstream flow moderated the effects of effluent. This study indicates that WWTP effluent impacts natural microbial communities by altering environmental conditions and sourcing new microbes but also demonstrates the functional redundancy and resilience of these communities.