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It is widely understood that microplastics (MPs) can induce various biological stresses in macroinvertebrates that are incapable of biodegrading plastics. However, the biodegradation and physiological responses of plastic-degrading macroinvertebrates toward MPs of different degradability levels remain unexplored. In this study, Tenebrio molitor larvae (mealworms) were selected as a model of plastics-degrading macroinvertebrate, and were tested against three common plastics of different degradability rankings: polyvinyl chloride (PVC), polystyrene (PS), and polylactic acid (PLA) MPs (size <300 μm). These three MPs were biodegraded with the rate sequence of PLA > PS > PVC, resulting in a reversed order of negative physiological responses (body weight loss, decreased survival, and biomass depletion) of mealworms. Simultaneously, the levels of reactive oxygen species (ROS), antioxidant enzyme activities, and lipid peroxidation were uniformly increased as polymer degradability decreased and intermediate toxicity increased. PVC MPs exhibited higher toxicity than the other two polymers. The oxidative stresses were effectively alleviated by supplementing co-diet bran. The T. molitor larvae fed with PLA plus bran showed sustainable growth without an increase in oxidative stress. The results provide new insights into the biotoxicity of MPs on macroinvertebrates and offer comprehensive information on the physiological stress responses of plastic-degrading macroinvertebrates during the biodegradation of plastics with different degradability levels. [Display omitted] • Microplastics (MPs) degradation rate by mealworms with sequence of PLA > PS > PVC. • Physiological responses were dependent on polymer degradability and intermediate toxicity. • Supplementing nutrient-rich co-diets alleviated oxidative stress by MPs. • Nanoplastics were not detected in frass after microplastics biodegradation. [ABSTRACT FROM AUTHOR]

Biodegradation of petroleum-derived plastics in mealworms has been identified with different plastic products. To date, however, the responses of mealworm larvae to plastic biodegradation have received comparatively little attention. Herein, we tested the physiological responses of mealworm larvae after biodegradation of polyethylene (PE) microplastics and evaluated the impact of supplementing co-diet bran on the PE biodegradation and mealworm homeostasis. Significant weight loss, biomass reduction, and decreased survival rates were found for the PE-fed larvae after the six-week test. The efficient removal of PE, molecular weight reduction of residual polymers, and thermal decomposition signals demonstrated the depolymerization and biodegradation. Supplementing bran significantly enhanced the biomass levels and the survival of larvae but disturbed the PE removal in the intestinal tracts. The frass (excrement) from the larvae fed with PE and PE + bran feedstocks both contained undigested residual microplastics of different sizes with no accumulation of nanoplastics. Additionally, higher levels of reactive oxygen species, antioxidant enzyme activities, and lipid peroxidation were detected for larvae fed with pure PE, whereas the oxidative stress levels could be mitigated by supplementing co-diet bran. These results provide methodological insights into assessing the physiological responses of invertebrates after plastic ingestion or under micro(nano)plastic exposure. [Display omitted] • The physiological function and growth of mealworms were negatively influenced by PE degradation. • Oxidative stress and damage were induced in mealworms after PE degradation. • Supplementation of co-diet bran can improve the physiological performance of mealworms. • The levels of oxidative stress and damage in mealworms could be mitigated by supplementing bran. • Nanoplastics could not be accumulated in the frass of mealworms after PE degradation. [ABSTRACT FROM AUTHOR]