Herbicide toxicity on river biofilms assessed by pulse amplitude modulated (PAM) fluorometry

• Rapid Light Curves were shown to be early markers of toxicant exposure.
• Diuron and norflurazon effects were significant at environmentally realistic concentrations.
• Toxic effects in intact biofilms seem to be delayed compared to disrupted biofilms.

The use of Rapid light curves (RLCs) as a toxicity endpoint for river biofilms was examined in this study and compared to “classical fluorescence parameters” i.e. minimal fluorescence (F0), optimal and effective quantum yields of photosystem II (Fv/Fm and ФPSII). Measurements were performed after exposure to five concentrations of diuron (from 0.3 to 33.4 μg L−1), its main degradation product (DCPMU) (from 1.0 to 1014 μg L−1) and norflurazon (from 0.6 to 585 μg L−1) with the lowest exposure concentrations corresponding to levels regularly encountered in chronically contaminated sites. Biofilm responses were evaluated after 1, 5, 7 and 14 days of exposure to the different toxicants.Overall, the responses of both “classical fluorescence parameters” and RLC endpoints were highly time dependent and related to the mode of action of the different compounds. Interestingly, parameters calculated from RLCs (α, ETRmax and Ik) were useful early markers of pesticide exposure since they revealed significant effects of all the tested toxicants from the first day of exposure. In comparison, classical fluorescence endpoints (F0 and Fv/Fm) measured at day 1 were only affected in the DCPMU treatment.Our results demonstrated the interest of RLCs as early markers of toxicant exposure particularly when working with toxicants with less specific mode of action than PSII inhibitors.