Characterizing Fragmentation of Polystyrene Foam Debris by Isopods Oniscus asellus (Isopoda: Oniscidae) and Trachelipus rathkii (Isopoda: Trachelipodidae).

Microplastics present a novel and potentially unique threat to soil ecosystems, one whose effects may be mediated by soil organisms themselves. We investigated fragmentation of polystyrene (PS) foam into microplastic particles by two isopods, Oniscus asellus L. and Trachelipus rathkii Brandt, in laboratory arena experiments. First, we examined the temporal dynamics of fragmentation across a time span of 96 h. O. asellus produced more fragments than T. rathkii, and neither species significantly fragmented the PS foam until 48 h had passed. Second, we asked whether O. asellus would still fragment PS foam in the presence of an alternate, more natural substrate like wood. Wood did not significantly affect fragmentation rates, in line with the few other studies examining the effect of alternate food on soil invertebrates' propensity to consume and/or fragment plastics. Our results provide additional characterization of PS foam fragmentation by isopods and indicate that laboratory experiments involving soil invertebrates and plastic debris can take place over relatively short timespans of four or fewer days, but do not necessarily need to provide alternate food to prove that plastic consumption would still occur in its presence.

Soil Invertebrates Generate Microplastics From Polystyrene Foam Debris.

To fully understand microplastics' impact on soil ecosystems, one must recognize soil organisms as not just passively enduring their negative effects, but potentially contributing to microplastics' formation, distribution, and dynamics in soil. We investigated the ability of four soil invertebrates, the cricket Acheta domesticus L. (Orthoptera: Gryllidae), the isopod Oniscus asellus L. (Isopoda: Oniscidae), larvae of the beetle Zophobas morio Fabricius (Coleoptera: Tenebrionidae), and the snail Cornu aspersum Müller (Stylommatophora: Helicidae) to fragment macroscopic pieces of weathered or pristine polystyrene (PS) foam. We placed invertebrates into arenas with single PS foam pieces for 24 h, then collected and assessed the microplastic content of each invertebrate's fecal material, its cadaver, and the sand substrate of its arena via hydrogen peroxide digestion, filtration, and fluorescent staining. All taxa excreted PS particles, though snails only to a tiny extent. Beetle larvae produced significantly more microplastics than snails, and crickets and isopods fragmented the weathered PS foam pieces more than the pristine pieces, which they left untouched. A follow-up experiment with pristine PS foam assessed the effect of different treatments mimicking exposure to the elements on fragmentation by isopods. PS foam pieces soaked in a soil suspension were significantly more fragmented than untreated pieces or pieces exposed to UV light alone. These findings indicate that soil invertebrates may represent a source of microplastics to the environment in places polluted with PS foam trash, and that the condition of macroplastic debris likely affects its palatability to these organisms.