Co-exposure of microplastics and 2-methoxy-1,4-naphthoquinone affects Daphnia magna depending on the developmental stage

Organisms in freshwater environments are threatened by an interplay of various stressors. Amongst these, adverse effects of microplastic pollution and the impacts of invasive species are emerging threats to the environment and associated organisms. However, the impact on organisms in co-exposure scenarios is poorly understood. As microplastics can increase the bioavailability and thus toxicity of chemical stressors through adsorption, we hypothesized that microplastics might enhance the effect of 2-MNQ, an allelochemical substance released by the invasive plant Impatiens glandulifera, on freshwater organisms. Hence, we tested via a chronic exposure scenario on Daphnia magna the interaction between two stressors of emerging concern, microplastic fragments (PA 66 & PS) and 2-MNQ as a proxy for the introduction of invasive species. We tested the impact of each stressor alone and, in combination, on the life history and morphology of D. magna. Additionally, we used natural control particles to rule out effects caused by the presence of particles per se. We found that exposure to both microplastic polymer types in two different concentrations led to reduced body length at primiparity. At the same time, treatments with 2-MNQ did not cause adverse effects at this developmental stage. After 21 days, co-exposure reduced the daphnids' body length, while adverse effects of microplastic per se were no longer detectable. We further found polymer-type-dependent adverse effects on the clutch size, with the combination of PA 66 and 2-MNQ being more harmful than the combination of PS and 2-MNQ. Hence, our results demonstrate that the interaction between microplastic and 2-MNQ acts highly variable on D. magna and changes depending on the development stage and the polymer type. In the long term, the observed sublethal effects caused by the co-exposure of microplastic and 2-MNQ may change freshwater species compositions, as D. magna acts as a keystone species in these ecosystems. Also see: https://micro2024.sciencesconf.org/559672/document