Environmental concentrations of the fungicide tebuconazole alter microbial biodiversity and trigger biofilm-released transformation products.

Freshwater microbial communities are integral components of riverine biodiversity. The ecological effects of toxic chemical pollutants, such as fungicides (e.g., tebuconazole), on microbial abundance and diversity are needed for risk assessment and regulation. The emergence of RNA metabarcoding approaches allow us to describe at unprecedented resolution the microbial diversity of the active part of a microbial community. Our study assesses the ecotoxicological impact of chronic and acute tebuconazole exposures on fungal, bacterial, and algal biomass and biodiversity of aquatic fungi and bacteria in stream biofilms using an RNA metabarcoding approach. In addition, the study uses HPLC-MS to evaluate the capability of biofilms to metabolize tebuconazole. Natural biofilm communities from a Swedish stream were exposed chronically (24 days) and acutely (96 h) to environmental concentrations of tebuconazole (10 and 100 μg/L) in microcosms conditions. The diversity and community structure of fungi and bacteria was assessed by ITS2 and 16S cDNA amplicon-sequencing, respectively. Biofilms chronically exposed to tebuconazole produced and released unidentified transformation products into the water column, suggesting a biotransformation capability following 24 days of uninterrupted exposure. The fungal biomass markedly decreased by a biomass loss of 40% when chronically exposed to 10 μg/L, and 60% when chronically exposed to 100 μg/L. Bacterial and algal biomass remained comparable with the controls in all tebuconazole treatments. Fungal and bacterial alpha diversity metrics were not significantly impacted, although a decreasing trend in fungal richness was observed with the treatments. However, beta diversity was significantly impacted in both fungal and bacterial compartments. Chronic exposures resulted in a shift in community composition, where taxa potentially more tolerant to tebuconazole (i.e. Lecanoromycetes) replaced more sensitive taxa (i.e. Malasseziomycetes). This study indicates that tebuconazole at environmental concentrations might pose a risk to freshwater systems, mainly due to its high toxicity to fungi.