Molecular mechanisms of shape- and size-dependent toxicity of microplastics in the freshwater mussel Dreissena bugensis – A transcriptomic and proteomic approach

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Abstract
There is increasing evidence that microplastic (MP) particles cannot be viewed as a single, specific stressor. Instead, MPs have been shown to cause various, mainly sublethal effects that depend on their physical and chemical properties, including those related to size and shape. However, the molecular mechanisms explaining how MP shapes and sizes lead to different effects are not well understood. Therefore, we examined transcriptomic and proteomic changes in the freshwater mussel Dreissena bugensis after six days of exposure to two size classes of monodispersed PS spheres (6 μm; 20 μm) and fragments (d90: 5.7 μm; d90:18 μm) as well as fibers (Ø 3 μm, mean length 75.5 μm), at a concentration of 1000 particles/ml. Our findings suggest that for smaller MPs, the molecular responses were mainly driven by particle size, whereas particle shape played a larger role for larger MPs. Overall, our results show that MP-induced molecular responses are highly specific to shape and size, emphasizing that environmentally relevant mixtures of microplastic types may lead to complex biological effects. The involvement of MP particle size and shape-dependent pathways related to immune regulation, cellular maintenance, and reproduction suggests that even mainly sublethal effects could impact organismal fitness and, in the long term, populations and ecosystems.

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