Phototrophic periphyton tolerance reflects in situ exposure to pharmaceuticals concentration overtime – a translocation study. PharmaTOX

Rivers worldwide are exposed to contamination by pharmaceuticals, which affect the phototrophic microorganisms that mainly live in the form of periphytic biofilms. This in situ study aimed to assess the causal relationship between in situ exposure to pharmaceuticals and effects on phototrophic periphyton. We performed a translocation experiment in a small French river at an upstream site and a downstream site exposed to different levels of pharmaceutical contamination confirmed using polar organic chemical integrative samplers (POCIS). 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. Overtime, acute toxicity tests of 3 pharmaceuticals (atenolol, diclofenac, paracetamol) on photosynthesis were performed to assess the evolution of PICT (pollution induced community tolerance). Diatom community structure and diversity were assessed using DNA metabarcoding on the rbcL gene. Downstream communities were more tolerant to atenolol and diclofenac than upstream communities. Following translocation, tolerance levels increased in communities transferred downstream and decreased in communities transferred upstream. Higher diatom diversity was observed downstream throughout the period of study, and both the diversity and structure of diatom communities changed substantially when they were moved from one site to the other. This evidence 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.