Adapting an enzymatic toxicity test to allow comparative evaluation of natural freshwater biofilms' tolerance to metals

A simple, low-cost and non-radioactive short-term toxicity test was developed to study the effects of urban metals on natural freshwater periphytic communities. beta-glucosidase activity of natural freshwater biofilms collected in situ was chosen as an endpoint. Metals (Cd, Cu, Ni, Pb, and Zn) successfully inhibited bacterial enzymatic activity after a 1-h exposure enabling the calculation of EC(50). The EC(50) value of a biofilm sample varied with the Total Suspended Solids concentration (TSS) of the biofilm suspension, showing that EC(50) values (expressed as total added metal concentrations) are not representative of the bioavailable metal concentration during the toxicity test. For Cu, Cd, Ni, Zn and Pb, the EC(50) values increased linearly with the TSS concentration leading us to define a normalized EC(50): the value of the EC(50) divided by the corresponding TSS concentration. Normalized EC(50) proved to be a robust, reliable way to assess metal tolerance of a biofilm for Cd, Cu, Ni, Zn and Pb. Normalized EC(50) obtained, expressed as kg(metal)/g(TSS), varied between 0.2 to 7.6 for Cu, 1 to 8 for Cd, 1.8 to 92.3 for Ni, 1.8 to 76.6 for Zn and 25 to 189 for Pb.