Département Toxicologie de l’environnement

The growing proliferation of silver nanoparticles (AgNPs) calls for detailed information on ecotoxicological effects, particularly on diverse communities and key ecosystem processes where impacts remain poorly known. This includes the decomposition of plant litter by fungi and bacteria in streams. Impacts are likely to depend on community composition, because species vary in their sensitivities to stressors. Therefore, our goal was to determine if shifts in microbial communities triggered by chronic exposure to low concentrations of nano (<200 μg L⁻¹) and ionic (20 μg L⁻¹) silver increase community tolerance to these contaminants, as described in the pollution-induced community tolerance (PICT) concept. We used stream microbial decomposers associated with leaf litter in microcosms to assess the applicability of this concept by determining tolerance acquisition towards AgNP and ionic Ag in short-term inhibition assays. Endpoints included fungal sporulation, bacterial production, microbial respiration and the potential activity of a protein-degrading enzyme, leucine aminopeptidase. Analyses of microbial communities showed that chronic exposure to the highest AgNP concentrations led to similar communities, and that these were distinct from the control communities. Most important, chronic exposure of fungi and bacteria to both AgNP and ionic Ag also increased tolerance of the microbes, as revealed by notably reduced adverse effects on bacterial production. Overall, our results demonstrate the usefulness of applying the PICT concept to litter decomposers and decomposition as an approach to assess the risks posed by nano and ionic silver to freshwater ecosystems.

Preventing and remedying fresh waters from chemical pollution is a fundamental societal and scientific challenge. With other nonchemical stressors potentially co-occurring, assessing the ecological consequences of reducing chemical loads in the environment is arduous. In this case study, we comparatively assessed the community structure, functions, and tolerance of stream biofilms to micropollutant mixtures extracted from deployed passive samplers at wastewater treatment plant effluents. These biofilms were growing up- and downstream of one upgraded and two nonupgraded wastewater treatment plants before being sampled for analyses. Our results showed a substantial decrease in micropollutant concentrations by 85%, as the result of upgrading the wastewater treatment plant at one of the sampling sites with activated carbon filtration. This decrease was positively correlated with a loss of community tolerance to micropollutants and the recovery of the community structure downstream of the effluent. On the other hand, downstream biofilms at the nonupgraded sites displayed higher tolerance to the extracts than the upstream biofilms. The observed higher tolerance was positively linked to micropollutant levels both in stream water and in biofilm samples, and to shifts in the community structure. Although more investigations of upgraded sites are needed, our findings point toward the suitability of using community tolerance for the retrospective assessment of the risks posed by micropollutants, to assess community recovery, and to relate effects to causes in complex environmental conditions.

Zebrafish (Danio rerio) early life stages offer a versatile model system to study the efficacy and safety of drugs or other chemicals with regard to human and environmental health. This is because, aside from the well-characterized genome of zebrafish and the availability of a broad range of experimental and computational research tools, they are exceptionally well suited for high-throughput approaches. Yet, one important pharmacokinetic aspect is thus far only poorly understood in zebrafish embryo and early larvae: their biotransformation capacity. Especially biotransformation of electrophilic compounds is a critical pathway because they easily react with nucleophile molecules, such as DNA or proteins, potentially inducing adverse health effects. To combat such adverse effects, conjugation reactions with glutathione and further processing within the mercapturic acid pathway have evolved. We here explore the functionality of this pathway in zebrafish early life stages using a reference substrate (1-chloro-2,4-dinitrobenzene, CDNB).

With this work we show that zebrafish embryos can biotransform CDNB to the respective glutathione conjugate as early as 4 hours post fertilization. At all examined life stages, the glutathione conjugate is further biotransformed to the last metabolite of the mercapturic acid pathway, the mercapturate, which is slowly excreted.

Being able to biotransform electrophiles within the mercapturic acid pathway shows that zebrafish early life stages possess the potential to process xenobiotic compounds through glutathione conjugation and the formation of mercapturates. The presence of this chemical biotransformation and clearance route in zebrafish early life stages supports the application of this model in toxicology and chemical hazard assessment.

1-Chloro-2,4-dinitrobenzene (CDNB) is widely used as a model substrate for measuring enzyme activity of glutathione S-transferases in toxicity studies and in studies focusing on the metabol-ic capacity of different test systems. To allow the quantification of CDNB at low, non-toxic concentra-tions, we developed a sensitive liquid chromatography-mass spectrometry (LC-MS) technique, which is based on electron capture ionization using atmospheric pressure chemical ionization (APCI) in negative ion mode. Gas phase reactions occurring under atmospheric pressure produce specific ions that allow direct CDNB quantification down to 17 ng/ml in water. Using the new technique, we were able to verify CDNB exposure concentrations applied in two typical toxicity studies with early life stages of the com-mon model organisms, zebrafish (Danio rerio) and a zebrafish embryonic cell line (Pac2).