Evaluation of single and joint toxic effects of diuron and its main metabolites on natural phototrophic biofilms using a pollution-induced community tolerance (PICT) approach.

This study assessed the single and joint acute toxicity of diuron and two of its metabolites (DCPMU and 3,4-DCA) on natural phototrophic biofilms using a PICT approach with photosynthesis bioassays. River biofilm communities were collected at three sampling stations exhibiting increasing concentrations of diuron, DCPMU and 3,4-DCA from upstream to downstream. Applied individually, the parent compound was more toxic than its metabolites, with DCPMU being more toxic than 3,4-DCA which only inhibited photosynthesis at very high concentrations (EC25 at about 5 mg/l). Sensitivity of biofilm communities to diuron and DCPMU decreased from upstream to downstream, revealing tolerance induction in contaminated sections of the river, as expected from the PICT concept. Nevertheless, PICT was not applicable for 3,4-DCA, which similarly affected upstream, intermediate and downstream biofilm communities. Chemical mixtures of diuron and DCPMU demonstrated additive effects whereas combinations with 3,4-DCA enhanced the observed effects. Our results reveal that the individual and combined presence of diuron and DCPMU in lotic ecosystems can have both short-term effects (as shown with bioassays) and long-term effects (as shown through the PICT approach) on phototrophic biofilms, whereas environmental concentrations of 3,4-DCA may not affect biofilm photosynthetic activity.