Light and temperature influence on diuron bioaccumulation and toxicity in biofilms.

Variations of temperature and photoperiod throughout different seasons can affect aquatic communities such as biofilms. Biofilms, generally present at the base of trophic chains in freshwaters, are also subject to organic contamination, and are especially affected by herbicides. Many studies have investigated the effect and interactions of herbicides and environmental factors on biofilms, but never with a toxicokinetic point of view. The objective of this study was to assess structural and functional changes in biofilms exposed to diuron, and to link them with contaminant accumulation, under the influence of temperature and light variations. To this aim, biofilms were exposed to all possible combinations of three concentrations (0, 5 and 50 µg L-1) of diuron, two temperatures (10 and 26 °C), and two light/dark photoperiods (16/8, 10/14), for durations of 0, 1 and 3 days. Diuron accumulation in biofilms was quantified and structural descriptors (protein and polysaccharide contents, dry weight) and functional endpoints (photosynthetic and enzymatic activities) were analyzed. The results obtained mainly highlighted the influence of temperature on diuron bioaccumulation and the associated toxic impact on biofilms. Bioaccumulation in biofilms exposed during three days at 10 °C, at the highest diuron concentration, was in average 1.4 times higher than bioaccumulation on biofilms exposed to 26 °C. Accordingly, the photosynthetic yield was more inhibited at lower than at higher temperatures. Temperature was also the highest impacting factor for metabolism regulation; for example, at 26 °C after three days of exposure, polysaccharide production was boosted under both photoperiods tested.

Aquatic worms: relevant model organisms to investigate pollution of microplastics throughout the freshwater-marine continuum.

Plastic pollution has become a global and emergency concern. Degradation processes of plastic macrowaste, either at the millimetre- and micrometre-size scales (microplastics, MP) or a nanometre one (nanoplastic, NP), is now well documented in all environmental compartments. It is hence necessary to study the environmental dynamic of MNP (micro(nano)plastic) on aquatic macrofauna considering their dispersion in different compartments. In this context, worms, having a large habitat in natural environments (soil, sediment, water) represent a relevant model organism for MNP investigations. In aquatic systems, worms could be used to compare MNP contamination between freshwater and seawater. The aim of this review was to discuss the relevance of using worms as model species for investigating MNP pollution in freshwater, estuarine, and marine systems. In this context, studies conducted in the field and in laboratory, using diverse classes of aquatic worms (polychaete and clitellate, i.e. oligochaete and hirudinea) to assess plastic contamination, were analysed. In addition, the reliability between laboratory exposure conditions and the investigation in the field was discussed. Finally, in a context of plastic use regulation, based on the literature, some recommendations about model species, environmental relevance, and experimental needs related to MNP are given for future studies.

Benthic diatom growth kinetics under combined pressures of microalgal competition, predation and chemical stressors.

Pesticides are increasingly used worldwide to protect crops. However, only a small fraction of pesticides really hit their target organisms, with the remaining fraction reaching the environment by several phenomena such as leaching, and ending up in aquatic ecosystems: the final receptor of micropollutants. Chemical stressors induce changes in taxonomic composition of fauna and flora which are now the focus of many biomonitoring studies. Interspecific competition and predation are structuring factors of community composition. But the combined effects of biotic relationships (competition, predation) and pesticides are rarely accounted for. We tested four factors (Predation, Competition, Diuron (Herbicide) and Imidacloprid (insecticide)) separately on three distinct morphotypes of two diatoms species: Planothidium lanceolatum and Gomphonema gracile (normal and teratogen forms), to quantify the daily growth kinetics of each under varied pressures. The predator used was a nematode, cosmopolitan in soils and aquatic ecosystems (Aphelenchoides bicaudatus). We reproduced experiments combining the factors in binary and ternary combinations. Diuron had lower toxicity than expected, while imidacloprid affected the growth of non-target diatoms. Interalgal competition had marked negative effects on diatom growth kinetics, which increased as supplementary pressures (nematodes and/or pesticides) were added. These results demonstrate that ecological relationships in freshwater biofilms (competition, predation) have a non negligible effect on community composition, population behavior and impacts usually observed. Multistress conditions including the presence of pesticides in freshwaters are expected to affect biodiversity in ways that are hard to predict from simple toxicity assays.

Selective grazing behaviour of chironomids on microalgae under pesticide pressure.

The herbicide diuron and the insecticide imidacloprid are amongst the most frequently detected pesticides in French rivers, and each is known to affect many aquatic organisms. However, the question of whether and how both pesticides together might induce multi-stress conditions, which could induce indirect effects such as the modification of biological interactions within freshwater microbial communities has not received much attention. This study was undertaken to determine the effect of diuron and imidacloprid alone and in combination on the feeding behaviour of chironomid larvae. An initial experiment measured the impact of the different contamination conditions at environmental concentrations (5 µg L-1 for each pesticide) on the grazing rate of chironomids on three microalgae species, independently. Two diatom species, Gomphonema gracile (two different morphotypes: normal and teratogen) and Planothidium lanceolatum, and one green alga Desmodesmus sp. were offered as food, during 24 h. Chironomids grazing rates varied according to the pesticide and algae species. Indeed, diuron impacted algae more strongly and probably affected their palatability, leading chironomids to increase grazing pressure on less nutritionally interesting algae. Imidacloprid, by targeting insect larvae, increased or inhibited their grazing capacity depending on the food source. In a second experiment (cafeteria design), the food selectivity of chironomids on previous algae was determined under similar contamination conditions during 4 h: under diuron, larvae switched equally between the microalgae and were as mobile as in the control without pesticide. However, imidacloprid and the pesticide mixture condition altered chironomid movements and grazing behaviour. By investigating the impact of an herbicide and an insecticide, alone and in combination, on the responses of food (algae growth rate) and biological (mortality) and behavioural (mobility, food selection) responses of chironomid larvae, this study provided new insights on the direct and indirect effects of pesticide contamination on a simplified trophic web.