Hypoactivity following chronic exposure to tire, polylactic acid, and mixtures of micro and nanoplastics in an estuarine zooplankton species

Micro and nanoplastics (MNPs) are ubiquitous across environmental matrices and have been found within many organisms. Many studies thus far continue to focus on the effects of uniform polystyrene spheres, which are not representative of environmental samples. To assess the effects of environmentally relevant MNPs, we conducted a chronic toxicity test (28 days) on the estuarine indicator species, Americamysis bahia, using various MNP types, concentrations, and introduced particle weathering. A. bahia were exposed to the most common environmental MNPs, tire particles (TPs), the bio-based plastic polylactic acid (PLA), and a polyester fiber/tire particle mixture at 3 concentrations (5, 50, 500 particles/mL for TP and PLA or 10, 60, 530 particles/mL for the particle mixture) each as both pristine and weathered plastics. We also tested leachates from micro-TPs and PLA. After a 28-day exposure, we measured growth and behavioral endpoints. While we only saw significant differences in growth in the pristine mixture treatment, A. bahia demonstrated hypoactivity when exposed to MNPs across treatments, as single types or in a mixture. Behaviors such as total distance moved and velocity decreased compared to controls while freezing and turn angle increased, and these trends held across MNP types and in leachate treatments. We found that the bio-based plastic treatment, PLA, had the most instances of hypoactivity compared to controls and overall, further emphasizing the significance of testing plastic alternatives. However, the most environmentally relevant treatment, the weathered mixture, showed the fewest effects, highlighting the need for more research investigating the interactions between different environmentally relevant MNPs. Any changes in behavior, especially in response to an external stimulus, may have implications for A. bahia and similar species in the wild. Hypoactivity may result in decreased prey response, the ability to forage for food, and an impact on reproduction. Understanding how MNPs impact these key indicator organisms is vital to understanding MNPs in the larger context of estuarine and marine ecosystems, especially considering the importance of mysid shrimp globally as prey for critical megafauna and fisheries. These findings underscore the need to include MNP mixtures, bio-based plastic alternatives, and weathering in chronic ecotoxicological assessments.