Pollution-induced community tolerance of freshwater biofilms: measuring heterotrophic tolerance to Pb using an enzymatic toxicity test.

This study aims at investigating the impacts of Pb on freshwater biofilms with a pollution-induced community tolerance (PICT) approach using a recently developed short-term toxicity test based on β-glucosidase activity to measure biofilms' tolerance to Pb. We first investigated more closely the influence of the total suspended solids (TSS) concentrations of biofilm suspensions used for short-term toxicity tests performed to assess Pb tolerance. The Pb EC(50) values of four dilutions of the same biofilm suspension increased with their TSS concentrations. TSS-normalization allowed to obtain a unique measure of Pb tolerance, thus confirming that TSS-normalization of EC(50) values is a good means to estimate biofilm tolerance to Pb. The experiment was repeated with three different biofilm samples collected at different sites and dates. Second, biofilms were exposed to Pb (0, 1, 10 and 100 μg/L) for 3 weeks in microcosms to assess the impacts of Pb exposure on the communities. An increase in Pb tolerance was observed for the biofilm exposed to 100 μg/L. Automated Ribosomal Intergenic Spacer Analysis revealed modifications of bacterial and eukaryotic community structure with Pb exposure. Moreover, exposure to 100 μg/L Pb also led to an increase in Zn tolerance but not Cu tolerance. This study shows that tolerance acquisition to Pb can be detected after exposure to environmental concentrations of Pb using a PICT methodology and normalized EC(50) values as measures of Pb tolerance.