Aging-Induced Physicochemical Changes in Petroleum- and Biobased Microplastics Influence Depolymerization and Gut Microbiota in Tenebrio molitor Larvae

In this study, we evaluated the influence of physicochemical aging on the biological processing and depolymerization performance of polyethylene (PE) and polylactic acid (PLA) by Tenebrio molitor larvae, with the goal of improving insect-based plastic treatment strategies. PE and PLA subjected to a sequential freezing–ultraviolet aging protocol showed modest increases in total larval consumption (approximately 11% for PE and 10% for PLA) compared with pristine materials. Aging also accelerated the processes related to chemical depolymerization, as evidenced by Fourier transform infrared spectroscopy and scanning electron microscopy showing the formation of oxidized functional groups and surface structural deterioration, respectively. Gel permeation chromatography indicated significant reductions in molecular weight. In addition, thermogravimetric analysis was used to evaluate the changes in thermal stability associated with polymer degradation. Gut microbiome analysis revealed that plastic diets and aging collectively shaped microbial structure and compositional shifts, with deterministic ecological processes dominating community assembly. PE diets enriched Proteobacteria, while PLA diets enriched Firmicutes and Desulfobacterota. Notably, aging strengthened microbial cooperation and enriched key genera, such as Spiroplasma sp. and Lactobacillus sp., which are potentially associated with plastic-associated metabolic adaptation. Overall, aging modestly facilitated larval processing and partial depolymerization of both fossil-based and bio-based plastics, as reflected by increased plastic consumption, polymer chain scission, and surface oxidation. It also enhanced the functional robustness of the larval gut microbiome. These findings provide mechanistic insights into insect-mediated plastic processing systems, offering mechanistic guidance for future, combined plastic treatment strategies rather than an immediately scalable stand-alone solution.