Microplastic impacts on soil and sediment bioturbation: insights from microcosm experiments across diverse ecosystems

Ecosystem engineering by bioturbation shapes ecosystems by physically modifying soil and sediment habitats, affecting microbial communities and biogeochemical processes. Although bioturbation is a key process for ecosystem health and functioning, its response to contaminants like microplastics (MPs) remains poorly explored. To address this gap, we conducted three microcosm experiments across terrestrial, freshwater, and marine ecosystems, using three species of ecosystem engineers: Aporrectodea caliginosa (earthworm), Tubifex tubifex (freshwater worm), and Hediste diversicolor (marine worm). Each ecosystem was contaminated by a mixture of polystyrene and polyamide MP fragments and fibers, ranging from 10 to 1000 μm, at two MP concentrations: 0 and 100 mg kg^-1 soil/sediment dry weight. Tracer particles (luminophores) were used to quantify soil/sediment reworking activities of ecosystem engineers. After 21 days of exposure, distinct bioturbation proxies (i.e., maximum penetration depth, surface reworking, particle displacement coefficient, cast production, biodiffusion-like and non-local reworking coefficients) were measured in each microcosm experiment. Our results demonstrated contrasted effects of MPs on bioturbation across ecosystems/ecosystem engineer species. In the terrestrial ecosystem, surface sediment reworking by earthworms remained unaffected by MP contamination although the cast production by A. caliginosa tended to increase (+26 %) with MP contamination. Conversely, in the freshwater ecosystem, the presence of MPs significantly reduced bioturbation activity of tubificid worms, resulting in a nearly fourfold decrease in particle displacement and a substantial reduction in maximum penetration depth of the luminophores. In the marine system, although H. diversicolor maintained effective particle reworking activities with MP contamination, gallery-biodiffusion activity tended to decrease in presence of MPs. Overall, the impact of MP contamination on bioturbation process was context-dependent. It can be hypothesized that the impact of MPs on bioturbation process was dependent on the size of the ecosystem engineering species, with the strongest effect observed on the smallest organism (T. tubifex).