Phototrophic periphyton tolerance reflects in situ exposure to pharmaceuticals overtime – A translocation study using a pollution-induced community approach (PICT)

Rivers worldwide are contaminated by pharmaceuticals. This in situ study investigated the association relationship between in situ exposure to pharmaceuticals and effects on phototrophic periphyton. We performed a translocation experiment in a French river, considering two sites exposed to different levels of pharmaceutical contamination, as confirmed using polar organic chemical integrative samplers. After a 4-week growth period, periphytic biofilms were moved, or not, between the sites, to simulate a 6-week period of restoration (downstream to upstream) or deterioration (upstream to downstream) of chemical water quality. Acute toxicity tests on photosynthesis were performed to assess the evolution of community tolerance to three pharmaceuticals (atenolol, diclofenac, paracetamol), using a pollution-induced community tolerance (PICT) approach. Diatom community structure and diversity were assessed using DNA metabarcoding on the rbcL gene. After 28 days, tolerance to atenolol and diclofenac (based on EC₅₀ values for photosynthesis) was approximately three and two times higher, respectively, in the downstream communities compared to the upstream ones. Downstream diatom communities also showed consistently higher diversity, with Shannon index values 1.5-3 times greater than upstream. Following translocation, tolerance levels increased in communities transferred downstream and decreased in communities transferred upstream and both the diversity and structure of diatom communities were modified. It suggests that changes in tolerance, reflecting community adaptation or resilience, were at least partly related to changes in diatom assemblages. Combining PICT with DNA metabarcoding is a relevant strategy for in situ assessment of the effects of pharmaceuticals on aquatic microbial communities.