Microplastic exposure across trophic levels: effects on the host-microbiota of freshwater organisms.

BACKGROUND: Microplastics are a pervasive pollutant widespread in the sea and freshwater from anthropogenic sources, and together with the presence of pesticides, they can have physical and chemical effects on aquatic organisms and on their microbiota. Few studies have explored the combined effects of microplastics and pesticides on the host-microbiome, and more importantly, the effects across multiple trophic levels. In this work, we studied the effects of exposure to microplastics and the pesticide deltamethrin on the diversity and abundance of the host-microbiome across a three-level food chain: daphnids-damselfly-dragonflies. Daphnids were the only organism exposed to 1 µm microplastic beads, and they were fed to damselfly larvae. Those damselfly larvae were exposed to deltamethrin and then fed to the dragonfly larvae. The microbiotas of the daphnids, damselflies, and dragonflies were analyzed. RESULTS: Exposure to microplastics and deltamethrin had a direct effect on the microbiome of the species exposed to these pollutants. An indirect effect was also found since exposure to the pollutants at lower trophic levels showed carry over effects on the diversity and abundance of the microbiome on higher trophic levels, even though the organisms at these levels where not directly exposed to the pollutants. Moreover, the exposure to deltamethrin on the damselflies negatively affected their survival rate in the presence of the dragonfly predator, but no such effects were found on damselflies fed with daphnids that had been exposed to microplastics. CONCLUSIONS: Our study highlights the importance of evaluating ecotoxicological effects at the community level. Importantly, the indirect exposure to microplastics and pesticides through diet can potentially have bottom-up effects on the trophic webs.

Single and combined effects of microplastics, pyrethroid and food resources on the life-history traits and microbiome of Chironomus riparius.

There is growing evidence of widespread contamination of freshwater ecosystems with microplastics. However, the effects of chronic microplastic ingestion and its interaction with other pollutants and stress factors on the life-history traits and the host-microbiome of aquatic invertebrates are not well understood. This study investigates the effects of exposure to sediment spiked with 1 µm polystyrene-based latex microplastic spheres, an environmentally realistic concentration of a pyrethroid pesticide (esfenvalerate), and a combination of both treatments on the life-history traits of the benthic-dwelling invertebrate, Chironomus riparius and its microbial community. The chironomid larvae were also exposed to two food conditions: abundant or limited food in the sediment, monitored for 28 and 34 days respectively. The microplastics and esfenvalerate had negative effects on adult emergence and survival, and these effects differed between the food level treatments. The microbiome diversity was negatively affected by the exposure to microplastics, while the relative abundances of the four top phyla were significantly affected only in the high food level treatment. Although the combined exposure to microplastics and esfenvalerate showed some negative effects on survival and emergence, there was little evidence for synergistic effects when compared to the single exposure. The food level affected all life-history traits and the microbiota, and lower food levels intensified the negative effects of the exposure to microplastics, esfenvalerate and their combination. We argue that these pollutants can affect crucial life-history traits such as successful metamorphosis and the host-microbiome. Therefore, it should be taken into consideration for toxicological assessment of pollutant acceptability. Our study highlights the importance of investigating possible additive and synergic activities between stressors to understand the effects of pollutants in the life story traits and host-microbiome.

Single and mixture impacts of two pyrethroids on damselfly predatory behavior and physiological biomarkers.

Direct mortality due to toxicity of single pesticide exposure along a concentration gradient, while the most common, is only one important parameter for assessing the effects of pesticide contamination on aquatic ecosystems. Sub-lethal toxicity can induce changes in an organism's behavior and physiology that may have population-level ramifications and consequences for ecosystem health. Additionally, the simultaneous detection of multiple contaminants in monitored watersheds stresses the importance of gaining a greater understanding of the toxicities of combined exposures, particularly at low, environmentally relevant concentrations. Using larvae of the Azure Damselfly (Coenagrion puella), we conducted a combined exposure investigation of two widely-used pyrethroid insecticides presumed to share the same neurotoxic mechanism of action, and estimated their effect on predatory ability, mobility and three physiological biomarkers (Glutathione S-transferase; GST, respiratory electron transport system; ETS, and malondialdehyde; MDA). Deltamethrin exposure (0.065µg/L and 0.13µg/L) was found to reduce the predatory ability, but it did not affect the larvae's mobility. Esfenvalerate exposure (0.069µg/L and 0.13µg/L), on the other hand, induced no significant changes in predatory ability or mobility. The decrease in predatory ability after the combination exposure (0.067µg/L deltamethrin and 0.12µg/L esfenvalerate) did not significantly differ from the impact of the single deltamethrin exposures. Glutathione-S-transferase was induced after single esfenvalerate exposure and the lower deltamethrin concentration exposure, but seemingly inhibited after exposure to the higher concentration of deltamethrin as well as the combination of both pyrethroids. Our data indicate that sub-lethal exposure to deltamethrin reduces predatory ability and suggest that sub-lethal combined exposure to deltamethrin and esfenvalerate inhibits the GST detoxification pathway. These effects can eventually result in a lower emergence of adults from contaminated ponds.

Combination effects of pyrethroids and neonicotinoids on development and survival of Chironomus riparius.

Standard ecotoxicological risk assessments are conducted on individual substances, however monitoring of streams in agricultural areas has shown that pesticides are rarely present alone. In fact, brief but intense pulse events such as storm water runoff and spray drift during application subject freshwater environments to complex mixtures of pesticides at high concentrations. This study investigates the potential risks to non-target aquatic organisms exposed to a brief but intense mixture of the neonicotinoid pesticides imidacloprid and thiacloprid and the pyrethroid pesticides deltamethrin and esfenvalerate, compared to single substance exposure. All four of these pesticides have been detected in surface waters at concentrations higher than benchmark values and both classes of pesticides are known to exert adverse effects on non-target aquatic organisms under single substance exposure scenarios. First instar midge larvae of the non-target aquatic organism, Chironomus riparius, were exposed to combinations of these four pesticides at 50% of their LC50 (96 h) values in a 1h pulse. They were then reared to adulthood in uncontaminated conditions and assessed for survival, development time and fecundity. Our results show that the risk of disruption to survival and development of non-target aquatic organisms under this scenario is not negligible on account of the significant increases in mortality of C. riparius found in the majority of the pesticide exposures and the delays in development after pyrethroid exposure. While none of the deleterious effects appear to be amplified by combination of the pesticides, there is some evidence for antagonism. No effects on fecundity by any of the pesticide treatments were observed.